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+The Project Gutenberg EBook of A Quantitative Study of the Nocturnal
+Migration of Birds., by George H. Lowery.
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Quantitative Study of the Nocturnal Migration of Birds.
+       Vol.3 No.2
+
+Author: George H. Lowery.
+
+Editor: E. Raymond Hall
+
+Release Date: October 31, 2011 [EBook #37894]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A QUANTITATIVE STUDY OF THE ***
+
+
+
+
+Produced by Chris Curnow, Tom Cosmas, Joseph Cooper, The
+Internet Archive for some images and the Online Distributed
+Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+  A Quantitative Study of the Nocturnal
+  Migration of Birds
+
+  BY
+
+  GEORGE H. LOWERY, JR.
+
+  University of Kansas Publications
+  Museum of Natural History
+
+  Volume 3, No. 2, pp. 361-472, 47 figures in text
+  June 29, 1951
+
+  University of Kansas
+  LAWRENCE
+  1951
+
+
+
+
+  UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
+
+  Editors: E. Raymond Hall, Chairman; A. Byron Leonard,
+  Edward H. Taylor, Robert W. Wilson
+
+  UNIVERSITY OF KANSAS
+  Lawrence, Kansas
+
+  PRINTED BY
+  FERD VOILAND, JR., STATE PRINTER
+  TOPEKA, KANSAS
+  1951
+
+  [Union Label]
+
+  23-1020
+
+
+
+
+  A Quantitative Study of the Nocturnal
+  Migration of Birds
+
+  By
+
+  GEORGE H. LOWERY, JR.
+
+
+
+
+CONTENTS
+
+
+                                                                   Page
+
+  INTRODUCTION                                                      365
+
+  ACKNOWLEDGMENTS                                                   367
+
+  PART I. FLIGHT DENSITIES AND THEIR DETERMINATION                  370
+
+     Lunar Observations of Birds and the Flight Density Concept     370
+
+     Observational Procedure and the Processing of Data             390
+
+  PART II. THE NATURE OF NOCTURNAL MIGRATION                        408
+
+     Horizontal Distribution of Birds on Narrow Fronts              409
+
+     Density as a Function of the Hour of the Night                 413
+
+     Migration in Relation to Topography                            424
+
+     Geographical Factors and the Continental Density Pattern       432
+
+     Migration and Meteorological Conditions                        453
+
+  CONCLUSIONS                                                       469
+
+  LITERATURE CITED                                                  470
+
+
+
+
+LIST OF FIGURES
+
+
+   Figure                                                          Page
+
+   1. The field of observation as it appears to the observer        374
+
+   2. Determination of diameter of cone at any point                375
+
+   3. Temporal change in size of the field of observation           376
+
+   4. Migration at Ottumwa, Iowa                                    377
+
+   5. Geographic variation in size of cone of observation           378
+
+   6. The problem of sampling migrating birds                       380
+
+   7. The sampling effect of a square                               381
+
+   8. Rectangular samples of square areas                           382
+
+   9. The effect of vertical components in bird flight              383
+
+  10. The interceptory potential of slanting lines                  384
+
+  11. Theoretical possibilities of vertical distribution            388
+
+  12. Facsimile of form used to record data in the field            391
+
+  13. The identification of co-ordinates                            392
+
+  14. The apparent pathways of birds seen in one hour               393
+
+  15. Standard form for plotting the apparent paths of flight       395
+
+  16. Standard sectors for designating flight trends                398
+
+  17. The meaning of symbols used in the direction formula          399
+
+  18. Form used to compute zenith distance and azimuth of the moon  400
+
+  19. Plotting sector boundaries on diagrammatic plots              402
+
+  20. Form to compute sector densities                              403
+
+  21. Determination of the angle [alpha]                            404
+
+  22. Facsimile of form summarizing sector densities                405
+
+  23. Determination of net trend density                            406
+
+  24. Nightly station density curve at Progreso, Yucatán            407
+
+  25. Positions of the cone of observation at Tampico, Tamps        411
+
+  26. Average hourly station densities in spring of 1948            414
+
+  27. Hourly station densities plotted as a percentage of peak      415
+
+  28. Incidence of maximum peak at the various hours of the
+        night in 1948                                               416
+
+  29. Various types of density-time curves                          418
+
+  30. Density-time curves on various nights at Baton Rouge          422
+
+  31. Directional components in the flight at Tampico, Tamps        428
+
+  32. Hourly station density curve at Tampico, Tamps                429
+
+  33. The nightly net trend of migrations at three stations in 1948 431
+
+  34. Stations at which telescopic observations were made in 1948   437
+
+  35. Positions of the cone of observation at Progreso, Yucatán     443
+
+  36. Hourly station density curve at Progreso, Yucatán             444
+
+  37. Sector density representation on two nights at
+        Rosedale, Miss.                                             451
+
+  38. Over-all sector vectors at major stations in spring of 1948   455
+
+  39. Over-all net trend of flight directions shown in Figure 38    456
+
+  40. Comparison of flight trends and surface weather conditions
+        on April 22-23, 1948                                        460
+
+  41. Winds aloft at 10:00 P. M. on April 22 (CST)                  461
+
+  42. Comparison of flight trends and surface weather conditions
+        on April 23-24, 1948                                        462
+
+ 43. Winds aloft at 10:00 P. M. on April 23 (CST)                   463
+
+  44. Comparison of flight trends and surface weather conditions
+        on April 24-25, 1948                                        464
+
+  45. Winds aloft at 10:00 P. M. on April 24 (CST)                  465
+
+  46. Comparison of flight trends and surface weather conditions
+        on May 21-22, 1948                                          466
+
+  47. Winds aloft at 10:00 P. M. on May 21 (CST)                    467
+
+
+
+
+INTRODUCTION
+
+
+The nocturnal migration of birds is a phenomenon that long has
+intrigued zoologists the world over. Yet, despite this universal
+interest, most of the fundamental aspects of the problem remain
+shrouded in uncertainty and conjecture.
+
+Bird migration for the most part, whether it be by day or by night, is
+an unseen movement. That night migrations occur at all is a conclusion
+derived from evidence that is more often circumstantial than it is
+direct. During one day in the field we may discover hundreds of
+transients, whereas, on the succeeding day, in the same situation, we
+may find few or none of the same species present. On cloudy nights we
+hear the call notes of birds, presumably passing overhead in the
+seasonal direction of migration. And on stormy nights birds strike
+lighthouses, towers, and other tall obstructions. Facts such as these
+are indisputable evidences that migration is taking place, but they
+provide little basis for evaluating the flights in terms of magnitude
+or direction.
+
+Many of the resulting uncertainties surrounding the nocturnal
+migration of birds have a quantitative aspect; their resolution hinges
+on how many birds do one thing and how many do another. If we knew,
+for instance, how many birds are usually flying between 2 and 3 A. M.
+and how this number compares with other one-hour intervals in the
+night, we would be in a position to judge to what extent night flight
+is sustained from dusk to dawn. If we could measure the number of
+birds passing selected points of observation, we could find out
+whether such migration in general proceeds more or less uniformly on a
+broad front or whether it follows certain favored channels or flyways.
+This in turn might give us a clearer insight into the nature of the
+orienting mechanism and the extent to which it depends on visual
+clues. And, if we had some valid way of estimating the number of birds
+on the wing under varying weather conditions, we might be able to
+understand better the nature and development of migration waves so
+familiar to field ornithologists. These are just random examples
+suggesting some of the results that may be achieved in a broad field
+of inquiry that is still virtually untouched--the quantitative study
+of migratory flights.
+
+This paper is a venture into that field. It seeks to evaluate on a
+more factual basis the traditional ideas regarding these and similar
+problems, that have been developed largely from circumstantial
+criteria. It is primarily, therefore, a study of comparative
+quantities or volumes of migration--or what may be conveniently called
+flight densities, if this term be understood to mean simply the number
+of birds passing through a given space in a given interval of time.
+
+In the present study, the basic data permitting the numerical
+expression of such migration rates from many localities under many
+different sets of circumstances were obtained by a simple method. When
+a small telescope, mounted on a tripod, is focused on the moon, the
+birds that pass before the moon's disc may be seen and counted, and
+their apparent pathways recorded in terms of coördinates. In bare
+outline, this approach to the problem is by no means new.
+Ornithologists and astronomers alike have recorded the numbers of
+birds seen against the moon in stated periods of time (Scott, 1881a
+and 1881b; Chapman, 1888; Libby, 1889; West, 1896; Very, 1897;
+Winkenwerder, 1902a and 1902b; Stebbins, 1906; Carpenter, 1906).
+Unfortunately, as interesting as these observations are, they furnish
+almost no basis for important generalizations. Most of them lack
+entirely the standardization of method and the continuity that would
+make meaningful comparisons possible. Of all these men, Winkenwerder
+appears to have been the only one to follow up an initial one or two
+nights of observation with anything approaching an organized program,
+capable of leading to broad conclusions. And even he was content
+merely to reproduce most of his original data without correlation or
+comment and without making clear whether he fully grasped the
+technical difficulties that must be overcome in order to estimate the
+important flight direction factor accurately.
+
+The present study was begun in 1945, and early results obtained were
+used briefly in a paper dealing with the trans-Gulf migration of birds
+(Lowery, 1946). Since that time the volume of field data, as well as
+the methods by which they can be analyzed, has been greatly expanded.
+In the spring of 1948, through the cooperation and collaboration of a
+large number of ornithologists and astronomers, the work was placed on
+a continent-wide basis. At more than thirty stations (Figure 34, page
+437) on the North American continent, from Yucatán to Ontario, and
+from California to South Carolina, observers trained telescopes
+simultaneously on the moon and counted the birds they saw passing
+before its disc.
+
+Most of the stations were in operation for several nights in the full
+moon periods of March, April, and May, keeping the moon under constant
+watch from twilight to dawn when conditions permitted. They have
+provided counts representing more than one thousand hours of
+observation, at many places in an area of more than a million square
+miles. But, as impressive as the figures on the record sheets are,
+they, like the published observations referred to above, have dubious
+meaning as they stand. Were we to compare them directly, station for
+station, or hour for hour, we would be almost certain to fall into
+serious errors. The reasons for this are not simple, and the measures
+that must be taken to obtain true comparisons are even less so. When I
+first presented this problem to my colleague, Professor William A.
+Rense, of the Department of Physics and Astronomy at Louisiana State
+University, I was told that mathematical means exist for reducing the
+data and for ascertaining the desired facts. Rense's scholarly insight
+into the mathematics of the problem resulted in his derivation of
+formulae that have enabled me to analyze on a comparable basis data
+obtained from different stations on the same night, and from the same
+station at different hours and on different nights. Astronomical and
+technical aspects of the problem are covered by Rense in his paper
+(1946), but the underlying principles are discussed at somewhat
+greater length in this paper.
+
+Part I of the present paper, dealing with the means by which the data
+were obtained and processed, will explore the general nature of the
+problem and show by specific example how a set of observations is
+prepared for analysis. Part II will deal with the results obtained and
+their interpretation.
+
+
+
+
+ACKNOWLEDGMENTS
+
+
+In the pursuit of this research I have received a tremendous amount of
+help from my colleagues, students, and other friends. In the first
+place, in order to obtain much of the data on which the study was
+based, it was necessary to enlist the aid of many persons in various
+parts of the country and to draw heavily on their time and patience to
+get all-night telescopic counts of migrating birds. Secondly, the
+processing of the primary data and its subsequent analysis demanded
+that I delve into the fields of astronomy and mathematics. Here, from
+the outset, I have enjoyed the constant and untiring help of Professor
+W. A. Rense of the Department of Physics and Astronomy at Louisiana
+State University. Without his collaboration, I would not have been
+able to do this work, for he not only supplied formulae whereby I was
+able to make desired computations, but time and again he maneuvered me
+through my difficulties in the mathematical procedures. Moreover,
+Professor Rense has manifested a great interest in the ornithological
+aspect of the problem, and his trenchant advice has been of
+inestimable value to me. No less am I indebted to my associate, Robert
+J. Newman, with whom I have spent untold hours discussing the various
+aspects of the problem. Indeed, most of the concepts that have evolved
+in the course of this study have grown out of discussions over a
+four-year period with both Rense and Newman. Whatever merit this work
+may have may be attributable in no small part to the help these two
+men have given me. In the preparation of many of the illustrations, I
+am further obligated to Newman for his excellent creative ideas as
+well as draftsmanship, and to Miss Helen Behrnes and A. Lowell Wood
+for their assistance.
+
+The mathematical computations required in this study have been
+laborious and time-consuming. It is estimated that more than two
+thousand man-hours have gone into this phase of the work alone.
+Whereas I have necessarily done most of this work, I have received a
+tremendous amount of help from A. Lowell Wood. Further assistance in
+this regard came from Herman Fox, Donald Norwood, and Lewis Kelly.
+
+The recording of the original field data in the spring of 1948 from
+the thirty-odd stations in North America involved the participation of
+more than 200 ornithologists and astronomers. This collaboration
+attests to the splendid cooperative spirit that exists among
+scientists. Many of these persons stayed at the telescope, either as
+observer or as recorder, hours on end in order to get sets of data
+extending through a whole night.
+
+The following were responsible for much of the field data herein used:
+J. R. Andrews, S. A. Arny, M. Dale Arvey, H. V. Autrey, Charles C.
+Ayres, Mr. and Mrs. Roy Bailey, Irwin L. Baird, Maurice F. Baker,
+Rollin H. Baker, Bedortha and Edna Baldwin, Mrs. A. Marguerite
+Baumgartner, T. A. Becket, Paul Bellington, Donald Bird, Carl Black,
+Jr., Lea Black, Lytle Blankenship, Mr. and Mrs. J. Stewart Boswell,
+Bruce Boudreaux, Frank Bray, Mr. and Mrs. Leonard Brecher, Homer
+Brewer, Mrs. Harvey Broome, Heyward Brown, Floyd Browning, Cyril
+Broussard, Paul Buress, Ralph M. Burress, Robert Cain, Don Carlos,
+Mrs. Reba Campbell, Mr. and Mrs. E. Burnham Chamberlain, Laura Chaney,
+Van B. Chaney, Jr., Edward Clebsch, Mr. and Mrs. Ben B. Coffey,
+William Cook, Dr. Jack Craven, Hugh C. and William Davis, Katherine
+Davis, Richard Davis, Richard DeArment, Robert E. Delphia, J. C.
+Dickinson, Mr. and Mrs. Otto Dietrich, John Dietrich, Clara Dixon,
+Nina Driven, John J. Duffy, Mr. and Mrs. R. J. Dunbar, Betty Dupre,
+Bernard E. Eble, Jr., Robert G. Eble, Dr. and Mrs. William H. Elder,
+C. C. Emory, Davis Emory, Alice H. Farnsworth, James Fielding, William
+R. Fish, Mr. and Mrs. Myron Ford, W. G. Fuller, Louis Gainey, Dr. Mary
+E. Gaulden, Mr. and Mrs. John J. Giudice, Lt. L. E. Goodnight, Earl R.
+Greene, Max Grilkey, W. W. H. Gunn, Noel Maxwell Hall, Jr., A. J.
+Hanna, Paul Hansen, Harold W. Harry, Joseph Healy, Dorothy Helmer, Mr.
+and Mrs. John H. Helmer, Philip E. Hoberecht, William D. Hogan, Dr.
+and Mrs. Joseph C. Howell, E. J. Huggins, Mrs. Walter Huxford, Hugh
+Iltis, W. S. Jennings, William M. Johnson, William Kasler, Luther F.
+Keeton, Lawrence C. Kent, W. H. Kiel, L. P. Kindler, Mr. and Mrs.
+Joseph E. King, Harriet Kirby, E. J. Koestner, Roy Komarek, Ann
+Knight, Mr. and Mrs. N. B. Langworthy, Mr. and Mrs. C. F. Lard,
+Prentiss D. Lewis, Ernest Liner, Dr. and Mrs. R. W. Lockwood, Dr.
+Harvey B. Lovell, William J. Lueck, Don Luethy, James Major, Mr. and
+Mrs. Russell L. Mannette, Mrs. John B. Mannix, Donald Mary, Dale E.
+McCollum, Stewart McConnell, Mr. and Mrs. M. L. McCroe, Robert L.
+McDaniel, Mr. and Mrs. Frank McGill, Thomas Merimer, Mr. and Mrs. I.
+S. H. Metcalf, Ann Michener, John Michener, T. H. Milby, D. S. Miller,
+Burt Monroe, Jr., Burt Monroe, Sr., Mrs. R. A. Monroe, Gordon
+Montague, Duryea Morton, James Mosimonn, Don L. Moyle, Grant Murphy,
+John T. Murphy, Mrs. H. F. Murphy, Mrs. Hill Myers, Mr. and Mrs.
+Robert J. Newman, William Nichols, R. A. Norris, Floyd Oaks, Eugene P.
+Odum, Mrs. E. E. Overton, Lennie E. Pate, Kenneth Patterson, Ralph
+Paxton, Louis Peiper, Marie Peiper, Mr. and Mrs. Harold S. Peters,
+Mary Peters, Mr. and Mrs. D. W. Pfitzer, Betty Plice, Max Plice,
+Lestar Porter, D. R. Power, Kenneth Price, George Rabb, Marge Reese,
+Wayne L. Reeve, C. L. Riecke, R. D. Ritchie, V. E. Robinson, Beverly
+J. Rose, Mary Jane Runyon, Roger Rusk, Bernd Safinsley, Mr. and Mrs.
+Glen C. Sanderson, Lewis L. Sandidge, John Sather, J. Benton Schaub,
+Evelyn Schneider, Henry W. Setzer, Mr. and Mrs. Walter Shackleton, Mr.
+and Mrs. Francis P. Shannon, Mr. and Mrs. Charles Shaw, Paul H.
+Shepard, Jr., Alan C. Sheppard, Mabel Slack, Alice Smith, R. Demett
+Smith, Jr., Nat Smith, Major and Mrs. Charles H. Snyder, Albert
+Springs, Dr. and Mrs. Fred W. Stamm, J. S. Steiner, Mrs. Paul
+Stephenson, Herbert Stern, Jr., Herbert Stoddard, Mr. and Mrs. F. W.
+Stomm, Charles Strull, Harold P. Strull, Mrs. Fan B. Tabler, Dr. and
+Mrs. James T. Tanner, S. M. H. Tate, David Taylor, Hall Tennin, Scott
+Terry, Mr. and Mrs. S. Charles Thacher, Olive Thomas, G. A. Thompson,
+Jr., Dr. and Mrs. S. R. Tipton, Robert Tucker, Tom Uzzel, Mr. and Mrs.
+M. G. Vaiden, Richard Vaught, Edward Violante, Brother I. Vincent,
+Marilyn L. Walker, Mr. and Mrs. Willis Weaver, Mr. and Mrs. W. L.
+Webb, Margaret M. L. Wehking, W. A. Welshans, Jr., Mrs. J. F.
+Wernicke, Francis M. Weston, Miss G. W. Weston, Dr. James W. White,
+John A. White, A. F. Wicke, Jr., Oren Williams, J. L. Wilson III, W.
+B. Wilson, Dr. and Mrs. Leonard Wing, Sherry Woo, Rodney Wuthnow,
+Grace Wyatt, Mr. and Mrs. Malcom Young, Mr. and Mrs. A. J. Zimmerman.
+To the scores of other people who assisted in making these
+observations I extend my hearty thanks.
+
+Drs. E. R. Hall, Edward H. Taylor, and H. B. Hungerford of the
+University of Kansas have read the manuscript and have made valuable
+suggestions, as have also Dr. W. H. Gates of Louisiana State
+University and Dr. Donald S. Farner of the State College of
+Washington. Dr. Farner has also been of great help, together with Drs.
+Ernst Mayr, J. Van Tyne, and Ernst Schüz, in suggesting source
+material bearing on the subject in foreign literature. Dr. N. Wyaman
+Storer, of the University of Kansas, pointed out a short-cut in the
+method for determining the altitude and azimuth of the moon, which
+resulted in much time being saved. For supplying climatological data
+and for guidance in the interpretation thereof, I am grateful to Dr.
+Richard Joel Russell, Louisiana State University; Commander F. W.
+Reichelderfer, Chief of the U. S. Weather Bureau, Washington, D. C.;
+Mr. Merrill Bernard, Chief of the Climatological and Hydrologic
+Services; and Mr. Ralph Sanders, U. S. Weather Bureau at New Orleans,
+Louisiana.
+
+Acknowledgment is made to Bausch and Lomb Optical Company for the loan
+of six telescopes for use in this project. Messrs. G. V. Cutler and
+George Duff of Smith and Johnson Steamship Company, operators of the
+Yucatán Line, are to be thanked for granting me free passage on the
+"S. S. Bertha Brřvig" to Progreso, Yucatán, where I made observations
+in 1945 and 1948. I am also indebted to the Louisiana State University
+Committee on Faulty Research for a grant-in-aid.
+
+
+
+
+PART I. FLIGHT DENSITIES AND THEIR DETERMINATION
+
+
+A. LUNAR OBSERVATIONS OF BIRDS AND THE FLIGHT DENSITY CONCEPT
+
+The subject matter of this paper is wholly ornithological. It is
+written for the zoologist interested in the activities of birds. But
+its bases, the principles that make it possible, lie in other fields,
+including such rather advanced branches of mathematics as analytical
+geometry, spherical geometry, and differential calculus. No exhaustive
+exposition of the problem is practicable, that does not take for
+granted some previous knowledge of these disciplines on the part of
+all readers.
+
+There are, however, several levels of understanding. It is possible to
+appreciate _what_ is being done without knowing _how_ to do it; and it
+is possible to learn how to carry out the successive steps of a
+procedure without entirely comprehending _why_. Some familiarity with
+the concepts underlying the method is essential to a full
+understanding of the results achieved, and details of procedure must
+be made generally available if the full possibilities of the
+telescopic approach are to be realized. Without going into proof of
+underlying propositions or actual derivation of formulae, I shall
+accordingly present a discussion of the general nature of the problem,
+conveyed as much as possible in terms of physical visualization. The
+development begins with the impressions of the student when he first
+attempts to investigate the movements of birds by means of the moon.
+
+
+_What the Observer Sees_
+
+Watched through a 20-power telescope on a cloudless night, the full
+moon shines like a giant plaster hemisphere caught in the full glare
+of a floodlight. Inequalities of surface, the rims of its craters, the
+tips of its peaks, gleam with an almost incandescent whiteness; and
+even the darker areas, the so-called lunar seas, pale to a clear,
+glowing gray.
+
+Against this brilliant background, most birds passing in focus appear
+as coal-black miniatures, only 1/10 to 1/30 the apparent diameter of
+the moon. Small as these silhouettes are, details of form are often
+beautifully defined--the proportions of the body, the shape of the
+tail, the beat of the wings. Even when the images are so far away that
+they are pin-pointed as mere flecks of black against the illuminated
+area, the normal eye can follow their progress easily. In most cases
+the birds are invisible until the moment they "enter," or pass
+opposite, the rim of the moon and vanish the instant they reach the
+other side. The interval between is likely to be inestimably brief.
+Some birds seem fairly to flash by; others, to drift; yet seldom can
+their passing be counted in seconds, or even in measureable fractions
+of seconds. During these short glimpses, the flight paths tend to lie
+along straight lines, though occasionally a bird may be seen to
+undulate or even to veer off course.
+
+Now and again, in contrast to this typical picture, more eerie effects
+may be noted. Some of them are quite startling--a minute,
+inanimate-looking object drifting passively by like a corpuscle seen
+in the field of a microscope; a gigantic wing brushing across half the
+moon; a ghost-like suggestion of a bird so transparent it seems
+scarcely more than a product of the imagination; a bird that pauses in
+mid-flight to hang suspended in the sky; another that beats its way
+ineffectually forward while it moves steadily to the side; and flight
+paths that sweep across the vision in astonishingly geometric curves.
+All of these things have an explanation. The "corpuscle" is possibly a
+physical entity of some sort floating in the fluid of the observer's
+eye and projected into visibility against the whiteness of the moon.
+The winged transparency may be an insect unconsciously picked up by
+the unemployed eye and transferred by the _camera lucida_ principle to
+the field of the telescope. It may be a bird flying very close, so
+drastically out of focus that the observer sees right through it, as
+he would through a pencil held against his nose. The same cause,
+operating less effectively, gives a characteristic gray appearance
+with hazy edges to silhouettes passing just beneath the limits of
+sharp focus. Focal distortions doubtless also account for the precise
+curvature of some flight paths, for this peculiarity is seldom
+associated with distinct images. Suspended flight and contradictory
+directions of drift may sometimes be attributable to head winds or
+cross winds but more often are simply illusions growing out of a
+two-dimensional impression of a three-dimensional reality.
+
+Somewhat more commonplace are the changes that accompany clouds. The
+moon can be seen through a light haze and at times remains so clearly
+visible that the overcast appears to be behind, instead of in front
+of, it. Under these circumstances, birds can still be readily
+discerned. Light reflected from the clouds may cause the silhouettes
+to fade somewhat, but they retain sufficient definition to distinguish
+them from out-of-focus images. On occasion, when white cloud banks
+lie at a favorable level, they themselves provide a backdrop against
+which birds can be followed all the way across the field of the
+telescope, whether or not they directly traverse the main area of
+illumination.
+
+
+_Types of Data Obtained_
+
+The nature of the observations just described imposes certain
+limitations on the studies that can be made by means of the moon. The
+speed of the birds, for instance, is utterly beyond computation in any
+manner yet devised. Not only is the interval of visibility extremely
+short, but the rapidity with which the birds go by depends less on
+their real rate of motion than on their proximity to the observer. The
+identification of species taking part in the migration might appear to
+offer more promise, especially since some of the early students of the
+problem frequently attempted it, but there are so many deceptive
+elements to contend with that the results cannot be relied upon in any
+significant number of cases. Shorn of their bills by the diminution of
+image, foreshortened into unfamiliar shape by varying angles of
+perspective, and glimpsed for an instant only, large species at
+distant heights may closely resemble small species a few hundred feet
+away. A sandpiper may appear as large as a duck; or a hawk, as small
+as a sparrow. A goatsucker may be confused with a swallow, and a
+swallow may pass as a tern. Bats, however, can be consistently
+recognized, if clearly seen, by their tailless appearance and the
+forward tilt of their wings, as well as by their erratic flight. And
+separations of nocturnal migrants into broad categories, such as
+seabirds and passerine birds, are often both useful and feasible.
+
+It would be a wonderful convenience to be able to clock the speed of
+night-flying birds accurately and to classify them specifically, but
+neither of these things is indispensable to the general study of
+nocturnal migration, nor as important as the three kinds of basic data
+that _are_ provided by telescopes directed at the moon. These
+concern:--(1) the direction in which the birds are traveling; (2)
+their altitude above the earth; (3) the number per unit of space
+passing the observation station.
+
+Unfortunately none of these things can be perceived directly, except
+in a very haphazard manner. Direction is seen by the observer in terms
+of the slant of a bird's pathway across the face of the moon, and may
+be so recorded. But the meaning of every such slant in terms of its
+corresponding compass direction on the plane of the earth constantly
+changes with the position of the moon. Altitude is only vaguely
+revealed through a single telescope by the size and definition of
+images whose identity and consequent real dimensions are subject to
+serious misinterpretation, for reasons already explained. The number
+of birds per unit of space, seemingly the easiest of all the features
+of migration to ascertain, is actually the most difficult, requiring a
+prior knowledge of both direction and altitude. To understand why this
+is so, it will be necessary to consider carefully the true nature of
+the field of observation.
+
+
+_The Changing Field of Observation_
+
+Most of the observations used in this study were made in the week
+centering on the time of the full moon. During this period the lunar
+disc progresses from nearly round to round and back again with little
+change in essential aspect or apparent size. To the man behind the
+telescope, the passage of birds looks like a performance in two
+dimensions taking place in this area of seemingly constant
+diameter--not unlike the movement of insects scooting over a circle of
+paper on the ground. Actually, as an instant's reflection serves to
+show, the two situations are not at all the same. The insects are all
+moving in one plane. The birds only appear to do so. They may be
+flying at elevations of 500, 1000, or 2000 feet; and, though they give
+the illusion of crossing the same illuminated area, the actual breadth
+of the visible space is much greater at the higher, than at the lower,
+level. For this reason, other things being equal, birds nearby cross
+the moon much more swiftly than distant ones. The field of observation
+is not an area in the sky but a volume in space, bounded by the
+diverging field lines of the observer's vision. Specifically, it is an
+inverted cone with its base at the moon and its vertex at the
+telescope.
+
+Since the distance from the moon to the earth does not vary a great
+deal, the full dimensions of the Great Cone determined by the diameter
+of the moon and a point on the earth remain at all times fairly
+constant. Just what they are does not concern us here, except as
+regards the angle of the apex (roughly 1/2°), because obviously the
+effective field of observation is limited to that portion of the Great
+Cone below the maximum ceiling at which birds fly, a much smaller
+cone, which I shall refer to as the Cone of Observation (Figure 1).
+
+    [Illustration: FIG. 1. The field of observation, showing
+       its two-dimensional aspect as it appears to the observer and
+       its three-dimensional actuality. The breadth of the cone is
+       greatly exaggerated.]
+
+    [Illustration: FIG. 2. Method for determining the diameter
+       of the cone at any point. The angular diameter of the moon
+       may be expressed in radians, or, in other words, in terms of
+       lengths of arc equivalent to the radius of a circle. In the
+       diagram, the arc between C and E, being equivalent to the
+       radius CO, represents a radian. If we allow the arc between A
+       and B to be the diameter of the moon, it is by astronomical
+       calculation about .009 radian, or .009 CO. This ratio will
+       hold for any smaller circle inscribed about the center O;
+       that is, the arc between A´B´ equals .009 C´O. Thus the width
+       of the cone of observation at any point, expressed in degrees
+       of arc, is .009 of the axis of the cone up to that point. The
+       cone is so slender that the arc between A and B is
+       essentially equal to the chord AB. Exactly the same
+       consideration holds true for the smaller circle where the
+       chord A´B´ represents part of the flight ceiling.]
+
+The problem of expressing the number of passing birds in terms of a
+definite quantity of space is fundamentally one of finding out the
+critical dimensions of this smaller cone. The diameter at any distance
+from the observer may be determined with enough accuracy for our
+purposes simply by multiplying the distance by .009, a convenient
+approximation of the diameter of the moon, expressed in radians (see
+Figure 2). One hundred feet away, it is approximately 11 inches; 1000
+feet away, nine feet; at one mile, 48 feet; at two miles, 95 feet.
+Estimating the effective length of the field of observation presents
+more formidable difficulties, aggravated by the fact that the lunar
+base of the Great Cone does not remain stationary. The moon rises in
+the general direction of east and sets somewhere in the west, the
+exact points where it appears and disappears on the horizon varying
+somewhat throughout the year. As it drifts across the sky it carries
+the cone of observation with it like the slim beam of an immense
+searchlight slowly probing space. This situation is ideal for the
+purpose of obtaining a random sample of the number of birds flying out
+in the darkness, yet it involves great complications; for the size of
+the sample is never at two consecutive instants the same. The nearer
+the ever-moving great cone of the moon moves toward a vertical
+position, the nearer its intersection with the flight ceiling
+approaches the observer, shortening, therefore, the cone of
+observation (Figure 3). The effect on the number of birds seen is
+profound. In extreme instances it may completely reverse the meaning
+of counts. Under the conditions visualized in Figure 3, the field of
+observation at midnight is only one-fourth as large as the field of
+observation earlier in the evening. Thus the twenty-four birds seen
+from 7 to 8 P. M., represent not twice as many birds actually flying
+per unit of space as the twelve observed from 11:30 to 12:30 A. M.,
+but only half the amount. Figure 4, based on observations at Ottumwa,
+Iowa, on the night of May 22-23, shows a similar effect graphically.
+Curve A represents the actual numbers of birds per hour seen; Curve B
+shows the same figures expressed as flight densities, that is,
+corrected to take into account the changing size of the field of
+observation. It will be noted that the trends are almost exactly
+opposite. While A descends, B rises, and _vice-versa_. In this case,
+inferences drawn from the unprocessed data lead to a complete
+misinterpretation of the real situation.
+
+    [Illustration: FIG. 3. Temporal change in the effective
+       size of the field of observation. The sample sections, A and
+       B, represent the theoretical densities of flight at 8:20 and
+       12:00 P. M., respectively. Though twice as many birds are
+       assumed to be in the air at midnight when the moon is on its
+       zenith (Z) as there were at the earlier hour, only half as
+       many are visible because of the decrease in size of the cone
+       of observation.]
+
+    [Illustration: FIG. 4. Migration at Ottumwa, Iowa, on the
+       night of May 22-23, 1948. Curve A is a graphic representation
+       of the actual numbers of birds seen hourly through the
+       telescope. Curve B represents the same figures corrected for
+       the variation in the size of the cone of observation. The
+       dissimilarity in the two curves illustrates the deceptive
+       nature of untreated telescopic counts.]
+
+Nor does the moon suit our convenience by behaving night after night
+in the same way. On one date we may find it high in the sky between 9
+and 10 P. M.; on another date, during the same interval of time, it
+may be near the horizon. Consequently, the size of the cone is
+different in each case, and the direct comparison of flights in the
+same hour on different dates is no more dependable than the misleading
+comparisons discussed in the preceding paragraph.
+
+The changes in the size of the cone have been illustrated in Figure 3
+as though the moon were traveling in a plane vertical to the earth's
+surface, as though it reached a point directly over the observer's
+head. In practice this least complicated condition seldom obtains in
+the regions concerned in this study. In most of the northern
+hemisphere, the path of the moon lies south of the observer so that
+the cone is tilted away from the vertical plane erected on the
+parallel of latitude where the observer is standing. In other words it
+never reaches the zenith, a point directly overhead. The farther north
+we go, the lower the moon drops toward the horizon and the more,
+therefore, the cone of observation leans away from us. Hence, at the
+same moment, stationed on the same meridian, two observers, one in the
+north and one in the south, will be looking into different effective
+volumes of space (Figure 5).
+
+    [Illustration: FIG. 5. Geographical variation in the size
+       of the cone of observation. The cones A and B represent the
+       effective fields of observation at two stations situated over
+       1,200 miles apart. The portions of the great cones included
+       here appear nearly parallel, but if extended far enough would
+       be found to have a common base on the moon. Because of the
+       continental scale of the drawing, the flight ceiling appears
+       as a curved surface, equidistant above each station. The
+       lines to the zenith appear to diverge, but they are both
+       perpendicular to the earth. Although the cones are shown at
+       the same instant in time, and have their origin on the same
+       meridian, the dimensions of B are less than one-half as great
+       as those of A, thus materially decreasing the opportunity to
+       see birds at the former station. This effect results from the
+       different slants at which the zenith distances cause the
+       cones to intersect the flight ceiling. The diagram
+       illustrates the principle that northern stations, on the
+       average, have a better chance to see birds passing in their
+       vicinity than do southern stations.]
+
+As a further result of its inclination, the cone of observation,
+seldom affords an equal opportunity of recording birds that are flying
+in two different directions. This may be most easily understood by
+considering what happens on a single flight level. The plane parallel
+to the earth representing any such flight level intersects the
+slanting cone, not in a circle, but in an ellipse. The proportions of
+this ellipse are very variable. When the moon is high, the
+intersection on the plane is nearly circular; when the moon is low,
+the ellipse becomes greatly elongated. Often the long axis may be more
+than twice the length of the short axis. It follows that, if the long
+axis happens to lie athwart the northward direction of flight and the
+short axis across the eastward direction, we will get on the average
+over twice as large a sample of birds flying toward the north as of
+birds flying toward the east.
+
+In summary, whether we wish to compare different stations, different
+hours of the night, or different directions during the same hour of
+the night, no conclusions regarding even the relative numbers of birds
+migrating are warranted, unless they take into account the
+ever-varying dimensions of the field of observation. Otherwise we are
+attempting to measure migration with a unit that is constantly
+expanding or contracting. Otherwise we may expect the same kind of
+meaningless results that we might obtain by combining measurements in
+millimeters with measurements in inches. Some method must be found by
+which we can reduce all data to a standard basis for comparison.
+
+
+_The Directional Element in Sampling_
+
+In seeking this end, we must immediately reject the simple logic of
+sampling that may be applied to density studies of animals on land. We
+must not assume that, since the field of observation is a volume in
+space, the number of birds therein can be directly expressed in terms
+of some standard volume--a cubic mile, let us say. Four birds counted
+in a cone of observation computed as 1/500 of a cubic mile are not the
+equivalent of 500 × 4, or 2000, birds per cubic mile. Nor do four
+birds flying over a sample 1/100 of a square mile mathematically
+represent 400 birds passing over the square mile. The reason is that
+we are not dealing with static bodies fixed in space but with moving
+objects, and the objects that pass through a cubic mile are not the
+sum of the objects moving through each of its 500 parts. If this fact
+is not immediately apparent, consider the circumstances in Figures 6
+and 7, illustrating the principle as it applies to areas. The relative
+capacity of the sample and the whole to intercept bodies in motion is
+more closely expressed by the ratio of their perimeters in the case of
+areas and the ratio of their surface areas in the case of volumes. But
+even these ratios lead to inaccurate results unless the objects are
+moving in all directions equally (see Figure 8). Since bird migration
+exhibits strong directional tendencies, I have come to the conclusion
+that no sampling procedure that can be applied to it is sufficiently
+reliable short of handling each directional trend separately.
+
+    [Illustration: FIG. 6. The problem of sampling migrating
+       birds. The large square in the diagram may be thought of as a
+       square mile on the earth's surface, divided into four equal
+       smaller squares. Birds are crossing over the area in three
+       directions, equally spaced, so that each of the subdivisions
+       is traversed by three of them. We might be tempted to
+       conclude that 4 × 3, or 12, would pass over the large square.
+       Actually there are only seven birds involved all told.
+       Obviously, the interceptive potential of a small square and a
+       larger square do not stand in the same ratio as their areas.]
+
+For this reason, the success of the whole quantitative study of
+migration depends upon our ability to make directional analyses of
+primary data. As I have already pointed out, the flight directions of
+birds may be recorded with convenience and a fair degree of
+objectivity by noting the slant of their apparent pathways across the
+disc of the moon. But these apparent pathways are seldom the real
+pathways. Usually they involve the transfer of the flight line from a
+horizontal plane of flight to a tilted plane represented by the face
+of the moon, and so take on the nature of a projection. They are
+clues to directions, but they are not the directions themselves. For
+each compass direction of birds flying horizontally above the earth,
+there is one, and only one, slant of the pathway across the moon at a
+given time. It is possible, therefore, knowing the path of a bird in
+relation to the lunar disc and the time of the observation, to compute
+the direction of its path in relation to the earth. The formula
+employed is not a complicated one, but, since the meaning of the lunar
+coördinates in terms of their corresponding flight paths parallel to
+the earth is constantly changing with the position of the moon, the
+calculation of each bird's flight separately would require a
+tremendous amount of time and effort.
+
+    [Illustration: FIG. 7. The sampling effect of a square. In
+       Diagram A eight evenly distributed birds are flying from
+       south to north, and another four are proceeding from east to
+       west. Three appear in each of the smaller squares. Thus, if
+       we were to treat any of these smaller sections as a directly
+       proportionate sample of the whole, we would be assuming that
+       3 × 16, or 48, birds had traversed the square mile--four
+       times the real total of 12. If we consider the paths
+       separately as in Diagram B, we see quite clearly what is
+       wrong. Every bird crosses four plots the size of the sample
+       and is being computed into the total over and over a
+       corresponding number of times. Patently, just as many
+       south-north birds cross the bottom tier of squares as cross
+       the four tiers comprising the whole area. Just as many
+       west-east birds traverse one side of the large square as
+       cross the whole square. In other words, the inclusion of
+       additional sections _athwart_ the direction of flight
+       involves the inclusion of additional birds proceeding in that
+       direction, while the inclusion of additional sections _along_
+       the direction does not. The correct ratio of the sample to
+       the whole would seem to be the ratio of their perimeters, in
+       this case the ratio of one to four. When this factor of four
+       is applied to the problem it proves correct: 4 × 3 (the
+       number of birds that have been seen in the sample square)
+       equals 12 (the exact number of birds that could be seen in
+       the square mile).]
+
+    [Illustration: FIG. 8. Rectangular samples of square areas.
+       In Diagram A, where as many birds are flying from west to
+       east as are flying from south to north, the perimeter ratio
+       (three to eight) correctly expresses the number of birds that
+       have traversed the whole area relative to the number that
+       have passed through the sample. But in Diagram B, where all
+       thirty-two birds are flying from south to north, the correct
+       ratio is the ratio of the base of the sample to the base of
+       the total area (one to four), and use of the perimeter ratio
+       would lead to an inaccurate result (forty-three instead of
+       thirty-two birds). Perimeter ratios do not correctly express
+       relative interceptory potential, unless the shape of the
+       sample is the same as the shape of the whole, or unless the
+       birds are flying in all directions equally.]
+
+Whatever we do, computed individual flight directions must be frankly
+recognized as approximations. Their anticipated inaccuracies are not
+the result of defects in the mathematical procedure employed. This is
+rigorous. The difficulty lies in the impossibility of reading the
+slants of the pathways on the moon precisely and in the
+three-dimensional nature of movement through space. The observed
+coördinates of birds' pathways across the moon are the projected
+product of two component angles--the compass direction of the flight
+and its slope off the horizontal, or gradient. These two factors
+cannot be dissociated by any technique yet developed. All we can do is
+to compute what a bird's course would be, if it were flying horizontal
+to the earth during the interval it passes before the moon. We cannot
+reasonably assume, of course, that all nocturnal migration takes place
+on level planes, even though the local distractions so often
+associated with sloping flight during the day are minimized in the
+case of migrating birds proceeding toward a distant destination in
+darkness. We may more safely suppose, however, that deviations from
+the horizontal are random in nature, that it is mainly a matter of
+chance whether the observer happens to see an ascending segment of
+flight or a descending one. Over a series of observations, we may
+expect a fairly even distribution of ups and downs. It follows that,
+although departures from the horizontal may distort individual
+directions, they tend to average out in the computed trend of the
+mean. The working of this principle applied to the undulating flight
+of the Goldfinch (_Spinus_) is illustrated in Figure 9.
+
+    [Illustration: FIG. 9. The effect of vertical components in
+       bird flight. The four diagrams illustrate various effects
+       that might result if a bird with an undulating flight, such
+       as a Goldfinch, flew before a moon 45° above the horizon. In
+       each case the original profile of the pathways, illustrated
+       against the dark background, is flattened considerably as a
+       result of projection. In the situation shown in Diagram A,
+       where the high point of the flight line, GHJ, occurs within
+       the field of the telescope, it is not only obvious that a
+       deviation is involved, but the line GJ drawn between the
+       entry and departure points coincides with the normal
+       coördinates of a bird proceeding on a horizontal plane. In
+       Diagrams B and C, one which catches an upward segment of
+       flight, and the other, a downward segment, the nature of the
+       deviation would not be detectable, and an incorrect direction
+       would be computed from the coördinates. Over a series of
+       observations, including many Goldfinches, one would expect a
+       fairly even distribution of ups and downs. Since the average
+       between the coördinate angles in Diagrams B and C, +19° and
+       -19°, is the angle of the true coördinate, we have here a
+       situation where the errors tend to compensate. In Diagram D,
+       where the bird is so far away that several undulations are
+       encompassed within the diameter of the field of view, the
+       coördinate readings do not differ materially from those of a
+       straight line.]
+
+Since _individually_ computed directions are not very reliable in any
+event, little is to be lost by treating the observed pathways in
+groups. Consequently, the courses of all the birds seen in a one-hour
+period may be computed according to the position of the moon at the
+middle of the interval and expressed in terms of their general
+positions on the compass, rather than their exact headings. For this
+latter purpose, the compass has been divided into twelve fixed
+sectors, 22-1/2 degrees wide. The trends of the flight paths are
+identified by the mid-direction of the sector into which they fall.
+The sectoring method is described in detail in the section on
+procedures.
+
+    [Illustration: Fig. 10. The interceptory potential of
+       slanting lines. The diagram deals with one direction of
+       flight and its incidence across lines of six different
+       slants, lines of identical length oriented in six different
+       ways. Obviously, the number of birds that cross a line
+       depends not only on the length of the line, but also on its
+       slant with respect to the flight paths.]
+
+The problem remains of converting the number of birds involved in each
+directional trend to a fixed standard of measurement. Figure 7A
+contains the partial elements of a solution. All of the west-east
+flight paths that cross the large square also cross one of its
+mile-long sides and suggest the practicability of expressing the
+amount of migration in any certain direction in terms of the assumed
+quantity passing over a one-mile line in a given interval of time.
+However, many lines of that length can be included within the same set
+of flight paths (Figure 10); and the number of birds intercepted
+depends in part upon the orientation of the line. The 90° line is the
+only one that fully measures the amount of flight per linear unit of
+front; and so I have chosen as a standard an imaginary mile on the
+earth's surface lying at right angles to the direction in which the
+birds are traveling.
+
+
+_Definitions of Flight Density_
+
+When the count of birds in the cone of observation is used as a sample
+to determine the theoretical number in a sector passing over such a
+mile line, the resulting quantity represents what I shall call a
+Sector Density. It is one of several expressions of the more general
+concept of Flight Density, which may be defined as the passage of
+migration past an observation station stated in terms of the
+theoretical number of birds flying over a one-mile line on the earth's
+surface in a given interval of time. Note that a flight density is
+primarily a theoretical number, a statistical expression, a _rate_ of
+passage. It states merely that birds were moving through the effective
+field of observation at the _rate_ of so many per mile per unit of
+time. It may or may not closely express the amount of migration
+occurring over an actual mile or series of miles. The extent to which
+it does so is to be decided by other general criteria and by the
+circumstances surrounding a given instance. Its basic function is to
+take counts of birds made at different times and at different places,
+in fields of observation of different sizes, and to put them on the
+statistically equal footing that is the first requisite of any sound
+comparison.
+
+The idea of a one-mile line as a standard spacial measurement is an
+integral part of the basic concept, as herein propounded. But, within
+these limitations, flight density may be expressed in many different
+ways, distinguished chiefly by the directions included and the
+orientation of the one-mile line with respect to them. Three such
+kinds of density have been found extremely useful in subsequent
+analyses and are extensively employed in this paper: Sector, Net
+Trend, and Station Density, or Station Magnitude.
+
+Sector Density has already been referred to. It may be defined as the
+flight density within a 22-1/2° directional spread, or sector,
+measured across a one-mile line lying at right angles to the
+mid-direction of the sector. It is the basic type of density from the
+point of view of the computer, the others being derived from it. In
+analysis it provides a means of comparing directional trends at the
+same station and of studying variation in directional fanning.
+
+Net Trend Density represents the maximum net flow of migration over a
+one-mile line. It is found by plotting the sector densities
+directionally as lines of thrust, proportioned according to the
+density in each sector, and using vector analysis to obtain a vector
+resultant, representing the density and direction of the net trend.
+The mile line defining the spacial limits lies at right angles to this
+vector resultant, but the density figure includes all of the birds
+crossing the line, not just those that do so at a specified angle.
+Much of the directional spread exhibited by sector densities
+undoubtedly has no basis in reality but results from inaccuracies in
+coördinate readings and from practical difficulties inherent in the
+method of computation. By reducing all directions to one major trend,
+net trend density has the advantage of balancing errors one against
+the other and may often give the truer index to the way in which the
+birds are actually going. On the other hand, if the basic directions
+are too widely spread or if the major sector vectors are widely
+separated with little or no representation between, the net trend
+density may become an abstraction, expressing the idea of a mean
+direction but pointing down an avenue along which no migrants are
+traveling. In such instances, little of importance can be learned from
+it. In others, it gives an idea of general trends indispensable in
+comparing station with station to test the existence of flyways and in
+mapping the continental distribution of flight on a given night to
+study the influence of weather factors.
+
+Station Density, or Station Magnitude, represents all of the migration
+activity in an hour in the vicinity of the observation point,
+regardless of direction. It expresses the sum of all sector densities.
+It includes, therefore, the birds flying at right angles over several
+one-mile lines. One way of picturing its physical meaning is to
+imagine a circle one-mile in diameter lying on the earth with the
+observation point in the center. Then all of the birds that fly over
+this circle in an hour's time constitute the hourly station density.
+While its visualization thus suggests the idea of an area, it is
+derived from linear expressions of density; and, while it involves no
+limitation with respect to direction, it could not be computed without
+taking every component direction into consideration. Station density
+is adapted to studies involving the total migration activity at
+various stations. So far it has been the most profitable of all the
+density concepts, throwing important light on nocturnal rhythm,
+seasonal increases in migration, and the vexing problem of the
+distribution of migrating birds in the region of the Gulf of Mexico.
+
+Details of procedure in arriving at these three types of flight
+density will be explained in Section B of this discussion. For the
+moment, it will suffice to review and amplify somewhat the general
+idea involved.
+
+
+_Altitude as a Factor in Flight Density_
+
+A flight density, as we have seen, may be defined as the number of
+birds passing over a line one mile long; and it may be calculated from
+the number of birds crossing the segment of that line included in an
+elliptical cross-section of the cone of observation. It may be thought
+of with equal correctness, without in any way contradicting the
+accuracy of the original definition, as the number of birds passing
+through a vertical plane one mile long whose upper limits are its
+intersection with the flight ceiling and whose base coincides with the
+one mile line of the previous visualization. From the second point of
+view, the sample becomes an area bounded by the triangular projection
+of the cone of observation on the density plane. The dimensions of two
+triangles thus determined from any two cones of observation stand in
+the same ratio as the dimensions of their elliptical sections on any
+one plane; so both approaches lead ultimately to the same result. The
+advantage of this alternative way of looking at things is that it
+enables us to consider the vertical aspects of migration--to
+comprehend the relation of altitude to bird density.
+
+If the field of observation were cylindrical in shape, if it had
+parallel sides, if its projection were a rectangle or a parallelogram,
+the height at which birds are flying would not be a factor in finding
+out their number. Then the sample would be of equal breadth
+throughout, with an equally wide representation of the flight at all
+levels. Since the field of observation is actually an inverted cone,
+triangular in section, with diverging sides, the opportunity to detect
+birds increases with their distance from the observer. The chances of
+seeing the birds passing below an elevation midway to the flight
+ceiling are only one-third as great as of seeing those passing above
+that elevation, simply because the area of that part of the triangle
+below the mid-elevation is only one-third as great as the area of that
+part above the mid-elevation. If we assume that the ratio of the
+visible number of birds to the number passing through the density
+plane is the same as the ratio of the triangular section of the cone
+to the total area of the plane, we are in effect assuming that the
+density plane is made up of a series of triangles the size of the
+sample, each intercepting approximately the same number of birds. We
+are assuming that the same number of birds pass through the inverted
+triangular sample as through the erect and uninvestigable triangle
+beside it (as in Figure 11, Diagram II). In reality, the assumption is
+sound only if the altitudinal distribution of migrants is uniform.
+
+    [Illustration: FIG. 11. Theoretical possibilities of
+       vertical distribution. Diagram I shows the effect of a
+       uniform vertical distribution of birds. The figures indicate
+       the number of birds in the respective areas. Here the sample
+       triangle, ABD, contains the same number of birds as the
+       upright triangle, ACD, adjacent to it; the density plane may
+       be conceived of as a series of such alternating triangles,
+       equal in their content of birds. Diagram II portrays, on an
+       exaggerated scale, the situation when many more birds are
+       flying below the median altitude than above it. In contrast
+       to the 152 birds occurring in the triangle A´C´D´, only
+       seventy-two are seen in the triangle A´B´D´. Obviously, the
+       latter triangle does not provide a representative sample of
+       the total number of birds intersecting the density plane.
+       Diagram III illustrates one method by which this difficulty
+       may be overcome. By lowering the line F´G´ to the median
+       altitude of bird density, F´´G´´ (the elevation above which
+       there are just as many birds as below), we are able to
+       determine a rectangular panel, HIJK, whose content of birds
+       provides a representative sample of the vertical
+       distribution.]
+
+The definite data on this subject are meagre. Nearly half a century
+ago, Stebbins worked out a way of measuring the altitude of migrating
+birds by the principle of parallax. In this method, the distance of a
+bird from the observers is calculated from its apparent displacement
+on the moon as seen through two telescopes. Stebbins and his
+colleague, Carpenter, published the results of two nights of
+observation at Urbana, Illinois (Stebbins, 1906; Carpenter, 1906); and
+then the idea was dropped until 1945, when Rense and I briefly applied
+an adaptation of it to migration studies at Baton Rouge. Results have
+been inconclusive. This is partly because sufficient work has not been
+done, partly because of limitations in the method itself. If the two
+telescopes are widely spaced, few birds are seen by both observers,
+and hence few parallaxes are obtained. If the instruments are brought
+close together, the displacement of the images is so reduced that
+extremely fine readings of their positions are required, and the
+margin of error is greatly increased. Neither alternative can provide
+an accurate representative sample of the altitudinal distribution of
+migrants at a station on a single night. New approaches currently
+under consideration have not yet been perfected.
+
+Meanwhile the idea of uniform vertical distribution of migrants must
+be dismissed from serious consideration on logical grounds. We know
+that bird flight cannot extend endlessly upward into the sky, and the
+notion that there might be a point to which bird density extends in
+considerable magnitude and then abruptly drops off to nothing is
+absurd. It is far more likely that the migrants gradually dwindle in
+number through the upper limits at which they fly, and the parallax
+observations we have seem to support this view.
+
+Under these conditions, there would be a lighter incidence of birds in
+the sample triangle than in the upright triangle beside it (Figure 11,
+Diagram III). Compensation can be made by deliberately scaling down
+the computed size of the sample area below its actual size. A
+procedure for doing this is explained in Figure 11. If it were applied
+to present altitudinal data, it would place the computational flight
+ceiling somewhere below 4000 feet. In arriving at the flight densities
+used in this paper, however, I have used an assumed ceiling of one
+mile. When the altitude factor is thus assigned a value of 1, it
+disappears from the formula, simplifying computations. Until the true
+situation with respect to the vertical distribution of flight is
+better understood, it seems hardly worthwhile to sacrifice the
+convenience of this approximation to a rigorous interpretation of
+scanty data. This particular uncertainty, however, does not
+necessarily impair the analytical value of the computations. Provided
+that the vertical pattern of migration is more or less constant,
+flight densities still afford a sound basis for comparisons, wherever
+we assume the upper flight limits to be. Raising or lowering the
+flight ceiling merely increases or reduces all sample cones or
+triangles proportionately.
+
+A more serious possibility is that the altitudinal pattern may vary
+according to time or place. This might upset comparisons. If the
+divergencies were severe enough and frequent enough, they could throw
+the study of flight densities into utter confusion.
+
+This consideration of possible variation in the altitudinal pattern
+combines with accidents of sampling and the concessions to perfect
+accuracy, explained on pages 379-385, to give to small quantities of
+data an equivocal quality. As large-scale as the present survey is
+from one point of view, it is only a beginning. Years of intensive
+work and development leading to a vast accumulation of data must
+elapse before the preliminary indications yet discernible assume the
+status of proved principles. As a result, much of the discussion in
+Part II of this paper is speculative in intent, and most of the
+conclusions suggested are of a provisional nature. Yet, compared with
+similar procedures in its field, flight density study is a highly
+objective method, and a relatively reliable one. In no other type of
+bird census has there ever been so near a certainty of recording _all_
+of the individuals in a specified space, so nearly independently of
+the subjective interpretations of the observer. The best assurance of
+the essential soundness of the flight density computations lies in the
+coherent results and the orderly patterns that already emerge from the
+analyses presented in Part II.
+
+
+B. OBSERVATIONAL PROCEDURE AND THE PROCESSING OF DATA
+
+At least two people are required to operate an observation
+station--one to observe, the other to record the results. They should
+exchange duties every hour to avoid undue eye fatigue. Additional
+personnel are desirable so that the night can be divided into shifts.
+
+Essential materials and equipment include: (1) a small telescope;
+(2) a tripod with pan-tilt or turret head and a mounting cradle;
+(3) data sheets similar to the one illustrated in Figure 12. Bausch
+and Lomb or Argus spotting scopes (19.5 ×) and astronomical telescopes
+up to 30- or 40-power are ideal. Instruments of higher magnification
+are subject to vibration, unless very firmly mounted, and lead to
+difficulties in following the progress of the moon, unless powered by
+clockwork. Cradles usually have to be devised. An adjustable lawn chair
+is an important factor in comfort in latitudes where the moon reaches
+a point high overhead.
+
+    [Illustration: FIG. 12. Facsimile of form used to record
+       data in the field. One sheet of the actual observations
+       obtained at Progreso, Yucatán, on April 24-25, 1948, is
+       reproduced here. The remainder of this set of data, which is
+       to be used throughout the demonstration of procedures, is
+       shown in Table 1.]
+
+  [Transcription of Figure 12's Data]
+
+  ORIGINAL DATA SHEET
+
+  DATE  24-25 April 1948     LOCALITY  Progreso, Yucatán
+
+  OBSERVERS  Harold Harry; George H. Lowery
+
+  WEATHER  Moderate to strong "trade" winds along coast, slightly
+           N of E. Moon emerged above low cloud bank at 8:26.
+
+  INSTRUMENT  B. & L. 19.5 Spotting Scope; image erect
+
+  REMARKS  Observation station located 1 mile from land, over Gulf of
+           Mexico, at end of new Progreso wharf
+
+  -----------+------+-------+----------------------------------------
+     TIME    |  IN  |  OUT  |                   REMARKS
+  -----------+------+-------+----------------------------------------
+    C.S.T    |      |       |
+    8:26     |  --  |  --   | observations begin; H.H. observing
+      50     | 4:30 |  9    | slow; small
+      56     | 3    | 10    | medium size
+    9:00     | 2    | 10:30 | very small
+      11     | 5    |  9:30 | moderately fast
+      25     | 5    | 10    | very small; rather slow
+      26     | 3    | 11    |     "            "
+      36     | 5    | 10    | medium size
+      40     | 3    | 10    |   "     "
+      43     | 5:30 |  9    |   "     "
+      46     | 3:30 | 10    | small
+      56     | 4:30 | 10    | medium size
+  9:58-10:00 |  --  |  --   | time out to change observers; G.L. at
+   10:05     | 4:30 | 11:30 | scope small
+      06     | 3    | 11    |
+      12     | 5    |  8    | very small
+      25     | 5    | 12    | very fast; small
+      30     | 4    | 10    | small
+      32     | 4    | 11    |   "
+      32     | 2    | 11    |   "
+      33     | 5    | 11    |   "
+      33     | 4    |  1    |   "
+      33     | 5:30 | 11    |   "
+      35     | 4:30 | 10    | swallow-like
+      36     | 5    |  1:30 |
+
+
+As much detail as possible should be entered in the space provided at
+the top of the data sheet. Information on the weather should include
+temperature, description of cloud cover, if any, and the direction
+and apparent speed of surface winds. Care should be taken to specify
+whether the telescope used has an erect or inverted image. The entry
+under "Remarks" in the heading should describe the location of the
+observation station with respect to watercourses, habitations, and
+prominent terrain features.
+
+The starting time is noted at the top of the "Time" column, and the
+observer begins the watch for birds. He must keep the disc of the moon
+under unrelenting scrutiny all the while he is at the telescope. When
+interruptions do occur as a result of changing positions with the
+recorder, re-adjustments of the telescope, or the disappearance of the
+moon behind clouds, the exact duration of the "time out" must be set
+down.
+
+    [Illustration: FIG. 13. The identification of coördinates.
+       These diagrams illustrate how the moon may be envisioned as a
+       clockface, constantly oriented with six o'clock nearest the
+       horizon and completely independent of the rotation of the
+       moon's topographic features.]
+
+    [Illustration: FIG. 14. The apparent pathways of the birds
+       seen in one hour. The observations are those recorded in the
+       11:00-12:00 P. M. interval on April 24-25, 1948, at
+       Progreso, Yucatán (see Table 1).]
+
+Whenever a bird is seen, the exact time must be noted, together with
+its apparent pathway on the moon. These apparent pathways can be
+designated in a simple manner. The observer envisions the disc of the
+moon as the face of a clock, with twelve equally spaced points on the
+circumference marking the hours (Figure 13). He calls the bottommost
+point 6 o'clock and the topmost, 12. The intervals in between are
+numbered accordingly. As this lunar clockface moves across the sky, it
+remains oriented in such a way that 6 o'clock continues to be the
+point nearest the horizon, unless the moon reaches a position directly
+overhead. Then, all points along the circumference are equidistant
+from the horizon, and the previous definition of clock values ceases
+to have meaning. This situation is rarely encountered in the northern
+hemisphere during the seasons of migration, except in extreme
+southern latitudes. It is one that has never actually been dealt with
+in the course of this study. But, should the problem arise, it would
+probably be feasible to orient the clock during this interval with
+respect to the points of the compass, calling the south point
+6 o'clock.
+
+When a bird appears in front of the moon, the observer identifies its
+entry and departure points along the rim of the moon with respect to
+the nearest half hour on the imaginary clock and informs the recorder.
+In the case of the bird shown in Figure 13, he would simply call out,
+"5 to 10:30." The recorder would enter "5" in the "In" column on the
+data sheet (see Figure 12) and 10:30 in the "Out" column. Other
+comment, offered by the observer and added in the remarks column, may
+concern the size of the image, its speed, distinctness, and possible
+identity. Any deviation of the pathway from a straight line should be
+described. This information has no bearing on subsequent mathematical
+procedure, except as it helps to eliminate objects other than birds
+from computation.
+
+The first step in processing a set of data so obtained is to
+blue-pencil all entries that, judged by the accompanying remarks,
+relate to extraneous objects such as insects or bats. Next, horizontal
+lines are drawn across the data sheets marking the beginning and the
+end of each even hour of observation, as 8 P. M.-9 P. M., 9 P. M.-10
+P. M., etc. The coördinates of the birds in each one-hour interval may
+now be plotted on separate diagrammatic clockfaces, just as they
+appeared on the moon. Tick marks are added to each line to indicate
+the number of birds occurring along the same coördinate. The slant of
+the tick marks distinguishes the points of departure from the points
+of entry. Figure 14 shows the plot for the 11 P. M.-12 P. M.
+observations reproduced in Table 1. The standard form, illustrated in
+Figure 15, includes four such diagrams.
+
+Applying the self-evident principle that all pathways with the same
+slant represent the same direction, we may further consolidate the
+plots by shifting all coördinates to the corresponding lines passing
+through the center of the circle, as in Figure 15. To illustrate, the
+6 to 8, 5 to 9, 3 to 11, and 2 to 12 pathways all combine on the 4 to
+10 line. Experienced computers eliminate a step by directly plotting
+the pathways through center, using a transparent plastic straightedge
+ruled off in parallel lines.
+
+    [Illustration: FIG. 15. Standard form for plotting the
+       apparent paths of flight. On these diagrams the original
+       coördinates, exemplified by Figure 14, have been moved to
+       center. In practice the sector boundaries are drawn over the
+       circles in red pencil, as shown by the white lines in Figure
+       19, making it possible to count the number of birds falling
+       within each zone. These numbers are then tallied in the
+       columns at the lower right of each hourly diagram.]
+
+
+    TABLE 1.--Continuation of Data in Figure 12, Showing Time
+                and Readings of Observations on 24-25 April 1948,
+                Progreso, Yucatán
+
+  ==============================+==============================
+  Time          In       Out    |  Time          In       Out
+  ------------------------------+------------------------------
+  10:37-10:41  Time out         | 11:15        8          9:30
+  10:45        5:30      10     | 11:16        4         11
+               6          9     |              5          9
+               5:30      10     | 11:17        5         11:30
+  10:46        6          8     | 11:18        5         12
+               3:30      11     |              6         11:30
+               5         12     | 11:19        5:30      11:30
+  10:47        3:15       1     | 11:20        6         10
+               6          8:30  |              3         12
+               5:45      11:45  |              5         12
+               5         10     | 11:21        5:45      11
+  10:48        6          9:45  |              5         11
+  10:50        5:30      11     | 11:23        5         12
+  10:51        4         11     | 11:25        5         10:30
+  10:52        4          2     |              6         11
+               5:30      11     |              6         12
+  10:53        5:30      11:30  | 11:27        6         10
+               5         11     | 11:28        6         11:30
+  10:55        5         12     |              5:30      12:30
+               5         11     | 11:29        6         11:30
+  10:56        6         10     |              4         12
+  10:58        4:30      11:30  |              6:30      10:30
+               5:45      11:45  |              6         11
+  10:59        6:30      10:30  | 11:30        3         10
+  11:00        3:30      12     |       (2 birds at once)
+               6:30      11     | 11:31        5         10:30
+        (2 birds at once)       |              5:30      10:30
+  11:03        6         11     | 11:32        6         11:30
+  11:04        3         12     | 11:33        7:30       9:30
+               5         12     |              4         10:30
+  11:05        6         10     |              6         11:30
+               5         11     |              8          9:30
+  11:06        6         10:30  | 11:35        7         10
+  11:07        3         10     |              4:30       1
+  11:08        6         11     | 11:38        6:30      11
+  11:10        7          9:30  | 11:40        5:30      12
+  11:11        5          9:15  | 11:42        4          2
+  11:13        5         12     |              5         12
+  11:14        6:30      10     |              6         10
+               5:30       1     |              4          2
+               4         12     |              5         12
+  ------------------------------+------------------------------
+
+               Table 1.--_Concluded_
+  ==============================+==============================
+  Time          In       Out    |  Time          In       Out
+  ------------------------------+------------------------------
+  11:44        8          9:30  |               8         10:15
+               7         11     |  12:16        3:30       1:30
+               6         10     |               8         11
+  11:45        5         12     |  12:23        7          1:30
+               6         10:30  |               6         12:30
+               5:45      11     |  12:36        8         11
+               4         12     |  12:37        7:30       1
+  11:46        7         11     |  12:38        7         12:30
+               6         12     |  12:40        8          1
+  11:47        8         10     |  12:45        7:30       1
+  11:48        6         10     |  12:47        5:30       1
+  11:49        6:30      10:30  |  12:48        7          1
+  11:51        8         10     |  12:52        5:30       1:30
+               8         10     |  12:54-12:55  Time out
+               8         10     |  12:56        8         10:45
+               8         10     |  12:58        5:30       1:30
+               6         10     |               7          1:30
+               8         10     |               7          2
+               6         11     |  12:59        5          3
+               7         12     |   1:00-1:30   Time out
+  11:52        5          1     |   1:37        8         12
+  11:54        7         11     |   1:38        8         12
+               6         12:30  |   1:48        7          1
+  11:55        5         12     |               7          1
+  11:56        7         10     |   1:51        5:30      11
+               5         12     |   1:57        8          1
+  11:58        8         11     |   2:07        7          2
+  11:59        5:30      12     |   2:09        9         12
+  12:00-12:03  Time out         |   2:10        8          1
+  12:03        5:30      11:30  |   2:17        9         12
+  12:04        8         11     |   2:21        6          2
+  12:07        6         12:30  |   2:30        5:30       3:15
+               7:30       1     |   2:32        8          2
+  12:08        5         10:30  |   2:46        7          1
+  12:09        5:30       1     |   3:36        9          2
+               7:30       2     |   3:39        8:30       2
+  12:10        6:30      12:45  |   3:45        6          4
+  12:13        8         11     |   3:55        9          2
+  12:14        7          1     |   4:00        8          3
+  12:15        7         12:30  |   4:03        9          2
+               7:15       1:30  |   4:30        Closed station
+  ------------------------------+------------------------------
+
+We now have a concise picture of the apparent pathways of all the
+birds recorded in each hour of observation. But the coördinates do not
+have the same meaning as readings of a horizontal clock on the earth's
+surface, placed in relation to the points of the compass. They are
+merely projections of the birds' courses. An equation is available for
+reversing the effect of projection and discovering the true directions
+of flight. This formula, requiring thirty-five separate computations
+for the pathways reproduced in Figure 12 alone, is far too-consuming
+for the handling of large quantities of data. A simpler procedure is
+to divide the compass into sectors and, with the aid of a reverse
+equation, to draw in the projected boundaries of these divisions on
+the circular diagrams of the moon. A standardized set of sectors, each
+22-1/2° wide and bounded by points of the compass, has been evolved
+for this purpose. They are identified as shown in Figure 16. The zones
+north of the east-west line are known as the North, or N, Sectors, as
+N_{1}, N_{2}, N_{3}, etc. Each zone south of the east-west line bears
+the same number as the sector opposite, but is distinguished by the
+designation S.
+
+    [Illustration: FIG. 16. Standard sectors for designating
+       flight trends. Each zone covers a span of 22-1/2°. The N_{6}
+       and N_{8}, the N_{5} and N_{7}, and their south complements,
+       where usually few birds are represented, can be combined and
+       identified as N_{6-8} and N_{5-7}, etc.]
+
+Several methods may be used to find the projection of the sector
+boundaries on the plot diagrams of Figure 15. Time may be saved by
+reference to graphic tables, too lengthy for reproduction here,
+showing the projected reading in degrees for every boundary, at every
+position of the moon; and a mechanical device, designed by C. M.
+Arney, duplicating the conditions of the original projection, speeds
+up the work even further. Both methods are based on the principle of
+the following formula:
+
+    tan [theta] = tan ([eta] - [psi]) / cos Z_{0}    (1)
+
+    [Illustration: FIG. 17. The meaning of symbols used in the
+       direction formula.]
+
+The symbols have these meanings:
+
+[theta] is the position angle of the sector boundary on the lunar
+clock, with positive values measured counterclockwise from 12 o'clock,
+negative angles clockwise (Figure 17A).
+
+[eta] is the compass direction of the sector boundary expressed in
+degrees reckoned west from the south point (Figure 17B).
+
+Z_{0} is the zenith distance of the moon's center midway through the
+hour of observation, that is, at the half hour. It represents the
+number of degrees of arc between the center of the moon and a
+point directly over the observer's head (Figure 17C).
+
+[psi] is the azimuth of the moon midway through the hour of
+observation, measured from the south point, positive values to the
+west, negative values to the east (Figure 17D).
+
+    [Illustration: FIG. 18. Form used in the computation of the
+       zenith distance and azimuth of the moon.]
+
+The angle [eta] for any sector boundary can be found immediately by
+measuring its position in the diagram (Figure 16). The form (Figure 18)
+for the "Computation of Zenith Distance and Azimuth of the Moon"
+illustrates the steps in calculating the values of Z_{0} and [psi]_{0}.
+From the American Air Almanac (Anonymous, 1945-1948), issued annually
+by the U. S. Naval Observatory in three volumes, each covering four
+months of the year, the Greenwich Hour Angle (GHA) and the declination
+of the moon may be obtained for any ten-minute interval of the date in
+question. The Local Hour Angle (LHA) of the observation station is
+determined by subtracting the longitude of the station from the GHA.
+Reference is then made to the "Tables of Computed Altitude and Azimuth,"
+published by the U. S. Navy Department, Hydrographic Office (Anonymous,
+1936-1941), and better known as the "H.O. 214," to locate the altitude
+and azimuth of the moon at the particular station for the middle of the
+hour during which the observations were made. The tables employ three
+variables--the latitude of the locality measured to the nearest degree,
+the LHA as determined above, and the declination of the moon measured
+to the nearest 30 minutes of arc. Interpolations can be made, but this
+exactness is not required. When the latitude of the observation
+station is in the northern hemisphere, the H.O. 214 tables entitled
+"Declinations Contrary Name to Latitude" are used with south
+declinations of the moon, and the tables "Declinations Same Name as
+Latitude," with north declinations. In the sample shown in Figure 15,
+the declination of the moon at 11:30 P. M., midway through the 11 to
+12 o'clock interval, was S 20° 22´. Since the latitude of Progreso,
+Yucatán is N 21° 17´, the "Contrary Name" tables apply to this hour.
+
+Because the H.O. 214 expresses the vertical position of the moon in
+terms of its altitude, instead of its zenith distance, a conversion is
+required. The former is the number of arc degrees from the horizon to
+the moon's center; therefore Z_{0} is readily obtained by subtracting
+the altitude from 90°. Moreover, the azimuth given in the H.O. 214 is
+measured on a 360° scale from the north point, whereas the azimuth
+used here ([psi]_{0}) is measured 180° in either direction from the south
+point, negative values to the east, positive values to the west. I
+have designated the azimuth of the tables as Az_{n} and obtained the
+desired azimuth ([psi]_{0}) by subtracting 180° from Az_{n}. The sign
+of [psi]_{0} may be either positive or negative, depending on whether
+or not the moon has reached its zenith and hence the meridian of the
+observer. When the GHA is greater than the local longitude (that is,
+the longitude of the observation station), the azimuth is positive.
+When the GHA is less than the local longitude, the azimuth is
+negative.
+
+Locating the position of a particular sector boundary now becomes a
+mere matter of substituting the values in the equation (1) and
+reducing. The computation of the north point for 11 to 12 P. M. in
+the sample set of data will serve as an example. Since the north point
+reckoned west from the south point is 180°, its [eta] has a value of
+180°.
+
+    [Illustration: FIG. 19. Method of plotting sector
+       boundaries on the diagrammatic plots. The example employed is
+       the 11:00 to 12:00 P. M. diagram of Figure 15.]
+
+
+    tan [theta]_{Npt.} = tan (180° - [psi]_{0}) / cos Z_{0}
+
+Substituting values of [psi]_{0} found on the form (Figure 18):
+
+    tan [theta]_{Npt.} = tan [180° - (-35°)] / cos 50°
+                       = tan 215° / cos 50° = .700 / .643 = 1.09
+
+    [theta]_{Npt.} = 47°28´
+
+
+    [Illustration: FIG. 20. Form for computing sector
+       densities.]
+
+Four angles, one in each quadrant, have the same tangent value.
+Since, in processing spring data, we are dealing mainly with north
+sectors, it is convenient to choose the acute angle, in this instance
+47° 28´. In doubtful cases, the value of the numerator of the equation
+(here 215°) applied as an angular measure from 6 o'clock will tell in
+which quadrant the projected boundary must fall. The fact that
+projection always draws the boundary closer to the 3-9 line serves as
+a further check on the computation.
+
+    [Illustration: FIG. 21. Determinationn of the angle [alpha]]
+
+In the same manner, the projected position angles of all the pertinent
+sector boundaries for a given hour may be calculated and plotted in
+red pencil with a protractor on the circular diagrams of Figure 15. To
+avoid confusion in lines, the zones are not portrayed in the black and
+white reproduction of the sample plot form. They are shown, however,
+in the shaded enlargement (Figure 19) of the 11 to 12 P. M. diagram.
+The number of birds recorded for each sector may be ascertained by
+counting the number of tally marks between each pair of boundary lines
+and the information may be entered in the columns provided in the plot
+form (Figure 15). We are now prepared to turn to the form for
+"Computations of Sector Densities" (Figure 20), which systematizes the
+solution of the following equation:
+
+         (220) 60/T (No. of Birds) (cos^2 Z_{0})
+    D =  ---------------------------------------                   (2)
+         (1 - sin^2 Z_{0} cos^2 [alpha])^0.5
+
+
+    [Illustration: FIG. 22. Facsimile of form summarizing
+       sector densities. The totals at the bottom of each column
+       give the station densities.]
+
+
+    [Illustration: FIG. 23. Determination of Net Trend Density.]
+
+
+Some of the symbols and factors, appearing here for the first time,
+require brief explanation. D stands for Sector Density. The constant,
+220, is the reciprocal of the quotient of the angular diameter of the
+moon divided by 2. T is Time In, arrived at by subtracting the total
+number of minutes of time out, as noted for each hour on the original
+data sheets, from 60. "No. of Birds" is the number for the sector and
+hour in question as just determined on the plot form. The symbol
+[alpha] represents the angle between the mid-line of the sector and
+the azimuth line of the moon. The quantity is found by the equation:
+
+    [alpha] = 180° - [eta] + [psi]_{0}                             (3)
+
+The symbol [eta] here represents the position of the mid-line of the
+sector expressed in terms of its 360° compass reading. This equation
+is illustrated in Figure 21. The values of [eta] for various zones are
+given in the upper right-hand corner of the form (Figure 20). The
+subsequent reductions of the equations, as they appear in the figure
+for four zones, are self-explanatory. The end result, representing the
+sector density, is entered in the rectangular box provided.
+
+After all the sector densities have been computed, they are tabulated
+on a form for the "Summary of Sector Densities" (Figure 22). By
+totaling each vertical column, sums are obtained, expressing the
+Station Density or Station Magnitude for each hour.
+
+An informative way of depicting the densities in each zone is to plot
+them as lines of thrust, as in Figure 23. Each sector is represented
+by the directional slant of its mid-line drawn to a length expressing
+the flight density per zone on some chosen scale, such as 100 birds
+per millimeter. Standard methods of vector analysis are then applied
+to find the vector resultant. This is done by considering the first
+two thrust lines as two sides of an imaginary parallelogram and using
+a drawing compass to draw intersecting arcs locating the position of
+the missing corner. In the same way, the third vector is combined
+with the invisible resultant whose distal end is represented by the
+intersection of the first two arcs. The process is repeated
+successively with each vector until all have been taken into
+consideration. The final intersection of arcs defines the length and
+slant of the Vector Resultant, whose magnitude expresses the Net Trend
+Density in terms of the original scale.
+
+The final step in the processing of a set of observations is to plot
+on graph paper the nightly station density curve as illustrated by
+Figure 24.
+
+    [Illustration: FIG. 24. Nightly station density curve at
+       Progreso, Yucatán, on April 24-25, 1948.]
+
+
+
+
+PART II. THE NATURE OF NOCTURNAL MIGRATION
+
+
+Present day concepts of the whole broad problem of bird migration are
+made up of a few facts and many guesses. The evolutionary origin of
+migration, the modern necessities that preserve its biologic utility,
+the physiological processes associated with it, the sensory mechanisms
+that make it possible, the speed at which it is achieved, and the
+routes followed, all have been the subject of some investigation and
+much conjecture. All, to a greater or less extent, remain matters of
+current controversy. All must be considered unknowns in every logical
+equation into which they enter. Since all aspects of the subject are
+intimately interrelated, since all have a bearing on the probabilities
+relating to any one, and since new conjectures must be judged largely
+in the light of old conjectures rather than against a background of
+ample facts, the whole field is one in which many alternative
+explanations of the established phenomena remain equally tenable.
+Projected into this uncertain atmosphere, any statistical approach
+such as determinations of flight density will require the accumulation
+of great masses of data before it is capable of yielding truly
+definitive answers to those questions that it is suited to solve. Yet,
+even in their initial applications, density analyses can do much to
+bring old hypotheses regarding nocturnal migration into sharper
+definition and to suggest new ones.
+
+The number of birds recorded through the telescope at a particular
+station at a particular time is the product of many potential
+variables. Some of these--like the changing size of the field of
+observation and the elevation of flight--pertain solely to the
+capacity of the observer to see what is taking place. It is the
+function of the density and direction formulae to eliminate the
+influences of these two variables insofar as is possible, so that the
+realities of the situation take shape in a nearly statistically true
+form. There remain to be considered those influences potentially
+responsible for variations in the real volume of migration at
+different times and places--things like the advance of season,
+geographic location, disposition of terrain features, hourly activity
+rhythm, wind currents, and other climatological causes. The situation
+represented by any set of observations probably is the end result of
+the interaction of several such factors. It is the task of the
+discussions that follow to analyze flight densities in the light of
+the circumstances surrounding them and by statistical insight to
+isolate the effects of single factors. When this has been done, we
+shall be brought closer to an understanding of these influences
+themselves as they apply to the seasonal movements of birds. Out of
+data that is essentially quantitative, conclusions of a qualitative
+nature will begin to take form. It should be constantly borne in mind,
+however, that such conclusions relate to the movement of birds _en
+masse+ and that caution must be used in applying these conclusions to
+any one species.
+
+Since the dispersal of migrants in the night sky has a fundamental
+bearing on the sampling procedure itself, and therefore on the
+reliability of figures on flight density, consideration can well be
+given first to the horizontal distribution of birds on narrow fronts.
+
+
+A. HORIZONTAL DISTRIBUTION OF BIRDS ON NARROW FRONTS
+
+Bird migration, as we know it in daytime, is characterized by spurts
+and uneven spatial patterns. Widely separated V's of geese go honking
+by. Blackbirds pass in dense recurrent clouds, now on one side of the
+observer, now on the other. Hawks ride along in narrow file down the
+thermal currents of the ridges. Herons, in companies of five to fifty,
+beat their way slowly along the line of the surf. And an unending
+stream of swallows courses low along the levees. Everywhere the
+impression is one of birds in bunches, with vast spaces of empty sky
+between.
+
+Such a situation is ill-suited to the sort of sampling procedure on
+which flight density computations are based. If birds always traveled
+in widely separated flocks, many such flocks might pass near the cone
+of observation and still, by simple chance, fail to enter the sliver
+of space where they could be seen. Chance would be the dominating
+factor in the number of birds recorded, obscuring the effects of other
+influences. Birds would seldom be seen, but, when they did appear, a
+great many would be observed simultaneously or in rapid succession.
+
+When these telescopic studies were first undertaken at Baton Rouge in
+1945, some assurance already existed, however, that night migrants might
+be so generally dispersed horizontally in the darkness above that the
+number passing through the small segment of sky where they could be
+counted would furnish a nearly proportionate sample of the total number
+passing in the neighborhood of the observation station. This assurance
+was provided by the very interesting account of Stone (1906: 249-252),
+who enjoyed the unique experience of viewing a nocturnal flight as a
+whole. On the night of March 27, 1906, a great conflagration occurred in
+Philadelphia, illuminating the sky for a great distance and causing the
+birds overhead to stand out clearly as their bodies reflected the light.
+Early in the night few birds were seen in the sky, but thereafter they
+began to come in numbers, passing steadily from the southwest to the
+northeast. At ten o'clock the flight was at its height. The observer
+stated that two hundred birds were in sight at any given moment as he
+faced the direction from which they came. This unparalleled observation
+is of such great importance that I quote it in part, as follows: "They
+[the birds] flew in a great scattered, wide-spread host, never in
+clusters, each bird advancing in a somewhat zigzag manner.... Far off in
+front of me I could see them coming as mere specks...gradually growing
+larger as they approached.... Over the illuminated area, and doubtless
+for great distances beyond, they seemed about evenly distributed.... I
+am inclined to think that the migrants were not influenced by the fire,
+so far as their flight was concerned, as those far to the right were not
+coming toward the blaze but keeping steadily on their way.... Up to
+eleven o'clock, when my observations ceased, it [the flight] continued
+apparently without abatement, and I am informed that it was still in
+progress at midnight."
+
+Similarly, in rather rare instances in the course of the present
+study, the combination of special cloud formations and certain
+atmospheric conditions has made it possible to see birds across the
+entire field of the telescope, whether they actually passed before the
+moon or not. In such cases the area of the sky under observation is
+greatly increased, and a large segment of the migratory movement can
+be studied. In my own experience of this sort, I have been forcibly
+impressed by the apparent uniformity and evenness of the procession of
+migrants passing in review and the infrequence with which birds
+appeared in close proximity.
+
+As striking as these broader optical views of nocturnal migration are,
+they have been too few to provide an incontestable basis for
+generalizations. A better test of the prevailing horizontal
+distribution of night migrants lies in the analysis of the telescopic
+data themselves.
+
+    [Illustration: FIG. 25. Positions of the cone of observation
+       at Tampico, Tamps., on April 21-22, 1948. Essential features
+       of this diagrammatic map are drawn to scale, the triangular
+       white lines representing the projections of the cone of
+       observation on the actual terrain at the mid-point of each
+       hour of observation. If the distal ends of the position lines
+       were connected, the portion of the map encompassed would
+       represent the area over which all the birds seen between
+       8:30 P. M. and 3:30 A. M. must have flown.]
+
+The distribution in time of birds seen by a single observer may be
+studied profitably in this connection. Since the cone of observation
+is in constant motion, swinging across the front of birds migrating
+from south to north, each interval of time actually represents a
+different position in space. This is evident from the map of the
+progress of the field of observation across the terrain at Tampico,
+Tamaulipas, on April 21-22, 1948 (Figure 25). At this station on this
+night, a total of 259 birds were counted between 7:45 P. M. and
+3:45 A. M. The number seen in a single hour ranged from three to
+seventy-three, as the density overhead mounted to a peak and then
+declined. The number of birds seen per minute was not kept with stop
+watch accuracy; consequently, analysis of the number of birds that
+passed before the moon in short intervals of time is not justified. It
+appears significant, however, that in the ninety minutes of heaviest
+flight, birds were counted at a remarkably uniform rate per fifteen
+minute interval, notwithstanding the fact that early in the period the
+flight rate overhead had reached a peak and had begun to decline. The
+number of birds seen in successive fifteen-minute periods was
+twenty-six, twenty-five, nineteen, eighteen, fifteen, and fifteen.
+
+Also, despite the heavy volume of migration at this station on this
+particular night, the flight was sufficiently dispersed horizontally
+so that only twice in the course of eight hours of continuous
+observation did more than one bird simultaneously appear before the
+moon. These were "a flock of six birds in formation" seen at 12:09 A. M.
+and "a flock of seven, medium-sized and distant," seen at 2:07 A. M.
+In the latter instance, as generally is the case when more than one
+bird is seen at a time, the moon had reached a rather low altitude,
+and consequently the cone of observation was approaching its maximum
+dimensions.
+
+The comparative frequency with which two or more birds simultaneously
+cross before the moon would appear to indicate whether or not there is
+a tendency for migrants to fly in flocks. It is significant, therefore,
+that in the spring of 1948, when no less than 7,432 observations were
+made of birds passing before the moon, in only seventy-nine instances,
+or 1.1 percent of the cases, was more than one seen at a time. In
+sixty percent of these instances, only two birds were involved. In one
+instance, however, again when the moon was low and the cone of
+observation near its maximum size, a flock estimated at twenty-five
+was recorded.
+
+The soundest approach of all to the study of horizontal distribution at
+night, and one which may be employed any month, anywhere, permitting the
+accumulation of statistically significant quantities of data, is to set
+up two telescopes in close proximity. Provided the flight overhead is
+evenly dispersed, each observer should count approximately the same
+number of birds in a given interval of time. Some data of this type are
+already available. On May 19-20, at Urbana, Illinois, while stationed
+twenty feet apart making parallax studies with two telescopes to
+determine the height above the earth of the migratory birds, Carpenter
+and Stebbins (_loci cit._) saw seventy-eight birds in two and one-half
+hours. Eleven were seen by both observers, thirty-three by Stebbins
+only, and thirty-four by Carpenter only. On October 10, 1905, at the
+same place, in two hours, fifty-seven birds were counted, eleven being
+visible through both telescopes. Of the remainder, Stebbins saw
+seventeen and Carpenter, twenty-nine. On September 12, 1945, at Baton
+Rouge, Louisiana, in an interval of one hour and forty minutes, two
+independent observers each counted six birds. Again, on October 17,
+1945, two observers each saw eleven birds in twenty-two minutes. On
+April 10, 1946, in one hour and five minutes, twenty-four birds were
+seen through one scope and twenty-six through the other. Likewise on May
+12, 1946, in a single hour, seventy-three birds were counted by each of
+two observers. The Baton Rouge observations were made with telescopes
+six to twelve feet apart. These results show a remarkable conformity,
+though the exceptional October observation of Carpenter and Stebbins
+indicates the desirability of continuing these studies, particularly in
+the fall.
+
+On the whole, the available evidence points to the conclusion that night
+migration differs materially from the kind of daytime migration with
+which we are generally familiar. Birds are apparently evenly spread
+throughout the sky, with little tendency to fly in flocks. It must be
+remembered, however, that only in the case of night migration have
+objective and truly quantitative studies been made of horizontal
+distribution. There is a possibility that our impressions of diurnal
+migration are unduly influenced by the fact that the species accustomed
+to flying in flocks are the ones that attract the most attention.
+
+These conclusions relate to the uniformity of migration in terms of
+short distances only, in the immediate vicinity of an observation
+station. The extent to which they may be applied to broader fronts is a
+question that may be more appropriately considered later, in connection
+with continental aspects of the problem.
+
+
+B. DENSITY AS FUNCTION OF THE HOUR OF THE NIGHT
+
+There are few aspects of nocturnal migration about which there is less
+understanding than the matter of when the night flight begins, at what
+rate it progresses, and for what duration it continues. One would think,
+however, that this aspect of the problem, above most others, would have
+been thoroughly explored by some means of objective study. Yet, this is
+not the case. Indeed, I find not a single paper in the American
+literature wherein the subject is discussed, although some attention has
+been given the matter by European ornithologists. Siivonen (1936)
+recorded in Finland the frequency of call notes of night migrating
+species of _Turdus_ and from these data plotted a time curve showing a
+peak near midnight. Bergman (1941) and Putkonen (1942), also in Finland,
+studied the night flights of certain ducks (_Clangula hyemalis_ and
+_Oidemia fusca_ and _O. nigra_) and a goose (_Branta bernicla_) and
+likewise demonstrated a peak near midnight. However, these studies were
+made at northern latitudes and in seasons characterized by evenings of
+long twilight, with complete darkness limited to a period of short
+duration around midnight. Van Oordt (1943: 34) states that in many cases
+migration lasts all night; yet, according to him, most European
+investigators are of the opinion that, in general, only a part of the
+night is used, that is, the evening and early morning hours. The
+consensus of American ornithologists seems to be that migratory birds
+begin their flights in twilight or soon thereafter and that they remain
+on the wing until dawn. Where this idea has been challenged at all, the
+implication seems to have been that the flights are sustained even
+longer, often being a continuation far into the night of movements begun
+in the daytime. The telescopic method fails to support either of these
+latter concepts.
+
+    [Illustration: FIG. 26. Average hourly station densities in
+       spring of 1948. This curve represents the arithmetic mean
+       obtained by adding all the station densities for each hour,
+       regardless of date, and dividing the sum by the number of
+       sets of observations at that hour (CST).]
+
+
+_The Time Pattern_
+
+When the nightly curves of density at the various stations are plotted
+as a function of time, a salient fact emerges--that the flow of birds is
+in no instance sustained throughout the night. The majority of the
+curves rise smoothly from near zero at the time of twilight to a single
+peak and then decline more or less symmetrically to near the base line
+before dawn. The high point is reached in or around the eleven to twelve
+o'clock interval more often than at any other time.
+
+    [Illustration: FIG. 27. Hourly station densities plotted as
+       a percentage of peak. The curve is based only on those sets
+       of data where observations were continued long enough to
+       include the nightly peak. In each set of data the station
+       density for each hour has been expressed as a percentage of
+       the peak for the night at the station in question. All
+       percentages for the same hour on all dates have been averaged
+       to obtain the percentile value of the combined station
+       density at each hour (CST).]
+
+Figure 26, representing the average hourly densities for all stations on
+all nights of observation, demonstrates the over-all effect of these
+tendencies. Here the highest density is reached in the hour before
+midnight with indications of flights of great magnitude also in the hour
+preceding and the hour following the peak interval. The curve ascends
+somewhat more rapidly than it declines, which fact may or may not be
+significant. Since there is a great disproportion in the total volume of
+migration at different localities, the thought might be entertained that
+a few high magnitude stations, such as Tampico and Progreso, have
+imposed their own characteristics on the final graph. Fortunately, this
+idea may be tested by subjecting the data to a second treatment. If
+hourly densities are expressed as a percentage of the nightly peak, each
+set of observations, regardless of the number of birds involved, carries
+an equal weight in determining the character of the over-all curve.
+Figure 27 shows that percentage analysis produces a curve almost
+identical with the preceding one. To be sure, all of the individual
+curves do not conform with the composite, either in shape or incidence
+of peak. The extent of this departure in the latter respect is evident
+from Figure 28, showing the number of individual nightly station curves
+reaching a maximum peak in each hour interval. Even this graph
+demonstrates that maximum densities near midnight represent the typical
+condition.
+
+    [Illustration: FIG. 28. Incidence of maximum peak at the
+       various hours of the night in 1948. "Number of stations"
+       represents the total for all nights of the numbers of station
+       peaks falling within a given hour.]
+
+The remarkable smoothness and consistency of the curves shown in Figures
+26 and 27 seem to lead directly to the conclusion that the volume of
+night migration varies as a function of time. Admittedly other factors
+are potentially capable of influencing the number of birds passing a
+given station in a given hour. Among these are weather conditions,
+ecological patterns, and specific topographical features that might
+conceivably serve as preferred avenues of flight. However, if any of
+these considerations were alone responsible for changes in the numbers
+of birds seen in successive intervals, the distribution of the peak in
+time could be expected to be haphazard. For example, there is no reason
+to suppose that the cone of observation would come to lie over favored
+terrain at precisely the hour between eleven and twelve o'clock at so
+many widely separated stations. Neither could the topographical
+hypothesis explain the consistently ascending and descending pattern of
+the ordinates in Figure 28. This is not to say that other factors are
+without effect; they no doubt explain the divergencies in the time
+pattern exhibited by Figure 28. Nevertheless, the underlying
+circumstances are such that when many sets of data are merged these
+other influences are subordinated to the rise and fall of an evident
+time pattern.
+
+Stated in concrete terms, the time frequencies shown in the graphs
+suggest the following conclusions: first, nocturnal migrations are not a
+continuation of daytime flights; second, nearly all night migrants come
+to earth well before dawn; and, third, in each hour of the night up
+until eleven or twelve o'clock there is typically a progressive increase
+in the number of birds that have taken wing and in each of the hours
+thereafter there is a gradual decrease. Taken at its face value, the
+evidence seems to indicate that birds do not begin their night
+migrations _en masse_ and remain on the wing until dawn and that in all
+probability most of them utilize less than half of the night.
+
+Interestingly enough, the fact that the plot points in Figure 26 lie
+nearly in line tempts one to a further conclusion. The curve behaves as
+an arithmetic progression, indicating that approximately the same number
+of birds are leaving the ground in each hour interval up to a point and
+that afterwards approximately the same number are descending within
+each hour. However, some of the components making up this curve, as
+later shown, are so aberrant in this regard that serious doubt is cast
+on the validity of this generalization.
+
+Because the results of these time studies are unexpected and
+startling, I have sought to explore other alternative explanations and
+none appears to be tenable. For example, the notion that the varying
+flight speeds of birds might operate in some way to produce a
+cumulative effect as the night progresses must be rejected on close
+analysis. If birds of varying flight speeds are continuously and
+evenly distributed in space, a continuous and even flow would result
+all along their line of flight. If they are haphazardly distributed in
+space, a correspondingly haphazard density pattern would be expected.
+
+Another explanation might be sought in the purely mathematical effects
+of the method itself. The computational procedure assumes that the
+effective area of the sample is extremely large when the moon is low,
+a condition that usually obtains in the early hours of the evening in
+the days surrounding the full moon. Actually no tests have yet been
+conducted to ascertain how far away a silhouette of a small bird can
+be seen as it passes before the moon. Consequently, it is possible
+that some birds are missed under these conditions and that the
+effective field of visibility is considerably smaller than the
+computed field of visibility. The tendency, therefore, may be to
+minimize the densities in such situations more than is justified.
+However, in many, if not most, cases, the plotting of the actual
+number of birds seen, devoid of any mathematical procedures, results
+in an ascending and descending curve.
+
+    [Illustration: FIG. 29. Various types of density-time curves.
+       (A) Near typical, Ottumwa, April 22-23; (B) random fluctuation,
+       Stillwater, April 23-24; (C) bimodal, Knoxville, April 22-23;
+       (D) sustained peak, Ottumwa, April 21-22; (E) early peak, Oak
+       Grove, May 21-22; (F) late peak, Memphis, April 23-24.]
+
+A third hypothesis proposes that all birds take wing at nearly the
+same time, gradually increase altitude until they reach the mid-point
+of their night's journey, and then begin a similarly slow descent.
+Since the field of observation of the telescope is conical, it is
+assumed that the higher the birds arise into the sky the more they
+increase their chances of being seen. According to this view, the
+changes in the density curve represent changes in the opportunity to
+see birds rather than an increase or decrease in the actual number of
+migrants in the air. Although measurements of flight altitude at
+various hours of the night have not been made in sufficient number to
+subject this idea to direct test, it is hardly worthy of serious
+consideration. The fallacy in the hypothesis is that the cone of
+observation itself would be rising with the rising birds so that
+actually the greatest proportion of birds flying would still be seen
+when the field of observation is in the supine position of early
+evening.
+
+It cannot be too strongly emphasized that the over-all time curves
+just discussed have been derived from a series of individual curves,
+some of which differ radically from the composite pattern. In Figure
+29, six dissimilar types are shown. This variation is not surprising
+in view of the fact that many other causative factors aside from time
+operate on the flow of birds from hour to hour. Figure 29A illustrates
+how closely some individual patterns conform with the average. Figure
+29B is an example of a random type of fluctuation with no pronounced
+time character. It is an effect rarely observed, occurring only in the
+cases where the number of birds observed is so small that pure chance
+has a pronounced effect on the computed densities; its vacillations
+are explicable on that account alone. Errors of sampling may similarly
+account for some, though not all, of the curves of the bimodal type
+shown in Figure 29C. Some variation in the curves might be ascribed to
+the variations in kinds of species comprising the individual flights
+at different times at different places, provided that it could be
+demonstrated that different species of birds show dissimilar temporal
+patterns. The other atypical patterns are not so easily dismissed and
+will be the subject of inquiry in the discussions that follow. It is
+significant that in spite of the variety of the curves depicted, which
+represent every condition encountered, in not a single instance is the
+density sustained at a high level throughout the night. Moreover,
+these dissident patterns merge into a remarkably harmonious, almost
+normal, average curve.
+
+When, at some future date, suitable data are available, it would be
+highly desirable to study the average monthly time patterns to
+ascertain to what extent they may deviate from the over-all average.
+At present this is not justifiable because there are not yet enough
+sets of data in any two months representing the same selection of
+stations.
+
+_Correlations with Other Data_
+
+It is especially interesting to note that the data pertaining to this
+problem derived from other methods of inquiry fit the conclusions
+adduced by the telescopic method. Overing (1938), who for several
+years kept records of birds striking the Washington Monument, stated
+that the record number of 576 individuals killed on the night of
+September 12, 1937, all came down between 10:30 P. M. and midnight.
+His report of the mortality on other nights fails to mention the time
+factor, but I am recently informed by Frederick C. Lincoln (_in
+litt._) that it is typical for birds to strike the monument in
+greatest numbers between ten and twelve o'clock at night. At the
+latter time the lights illuminating the shaft are extinguished, thus
+resulting in few or no casualties after midnight. The recent report by
+Spofford (1949) of over 300 birds killed or incapacitated at the
+Nashville airport on the night of September 9-10, 1948, after flying
+into the light beam from a ceilometer, is of interest in this
+connection even though the cause of the fatality is shrouded in
+mystery. It may be noted, however, that "most of the birds fell in the
+first hour," which, according to the account, was between 12:30 A. M.
+and 1:30 A. M. Furthermore, birds killed at the Empire State
+Building in New York on the night of September 10-11, 1948, began to
+strike the tower "shortly after midnight" (Pough, 1948). Also it will
+be recalled that the observations of Stone (_loc. cit._), already
+referred to in this paper (page 410), show a situation where the
+flight in the early part of the night was negligible but mounted to a
+peak between ten and eleven o'clock, with continuing activity at least
+until midnight.
+
+All of these observations are of significance in connection with the
+conclusions herein advanced, but by far the most striking correlation
+between these present results and other evidences is found in the
+highly important work of various European investigators studying the
+activity of caged migratory birds. This work was recently reviewed and
+extended by Palmgren (1944) in the most comprehensive treatise on the
+subject yet published. Palmgren recorded, by an electrically operated
+apparatus, the seasonal, daily, and hourly activity patterns in caged
+examples of two typical European migrants, _Turdus ericetorum
+philomelas_ Brehm and _Erithacus rubecula_ (Linnaeus). Four rather
+distinct seasonal phases in activity of the birds were discerned:
+_winter non-migratory_, _spring migratory_, _summer non-migratory_,
+and _autumn migratory_. The first of these is distinguished by morning
+and evening maxima of activity, the latter being better developed but
+the former being more prolonged. Toward the beginning of migration,
+these two periods of activity decline somewhat. The second, or spring
+migratory phase, which is of special interest in connection with the
+present problem, is characterized by what Palmgren describes as
+nightly migratory restlessness (_Zugunruhe_). The morning maximum,
+when present, is weaker and the evening maximum often disappears
+altogether. Although variations are described, the migratory
+restlessness begins ordinarily after a period of sleep ("sleeping
+pause") in the evening and reaches a maximum and declines before
+midnight.
+
+This pattern agrees closely with the rhythm of activity indicated by
+the time curves emerging from the present research. Combining the two
+studies, we may postulate that most migrants go to sleep for a period
+following twilight, thereby accounting for the low densities in the
+early part of the night. On awakening later, they begin to exhibit
+migratory restlessness. The first hour finds a certain number of birds
+sufficiently stimulated so that they rise forthwith into the air. In
+the next hour still others respond to this urge and they too mount
+into the air. This continues until the "restlessness" begins to abate,
+after which fewer and fewer birds take wing. By this time, the birds
+that began to fly early are commencing to descend, and since their
+place is not being filled by others leaving the ground, the density
+curve starts its decline. Farner (1947) has called attention to the
+basic importance of the work by Palmgren and the many experimental
+problems it suggests. Of particular interest would be studies
+comparing the activity of caged American migrant species and the
+nightly variations in the flight rates.
+
+_The Baton Rouge Drop-off_
+
+As already stated, the present study was initiated at Baton Rouge,
+Louisiana, in 1945, and from the outset a very peculiar density time
+pattern was manifest. I soon found that birds virtually disappeared
+from the sky after midnight. Within an hour after the termination of
+twilight, the density would start to ascend toward a peak which was
+usually reached before ten o'clock, and then would begin, surprisingly
+enough, a rapid decline, reaching a point where the migratory flow was
+negligible. In Figure 30 the density curves are shown for five nights
+that demonstrate this characteristically early decline in the volume
+of migration at this station. Since, in the early stages of the work,
+coördinates of apparent pathways of all the birds seen were not
+recorded, I am unable now to ascertain the direction of flight and
+thereby arrive at a density figure based on the dimension of the cone
+and the length of the front presented to birds flying in certain
+directions. It is feasible, nevertheless, to compute what I have
+termed a "plus or minus" flight density figure stating the rate of
+passage of birds in terms of the maximum and minimum corrections which
+all possible directions of flight would impose. In other words,
+density is here computed, first, as if all the birds were flying
+perpendicular to the long axis of the ellipse, and, secondly, as if
+all the birds were flying across the short axis of the ellipse. Since
+the actual directions of flight were somewhere between these two
+extremes, the "plus or minus" density figure is highly useful.
+
+    [Illustration: FIG. 30. Density-time curves on various nights
+       at Baton Rouge. (A) April 25, 1945; (B) April 15, 1946;
+       (C) May 10, 1946; (D) May 15, 1946; (E) April 22-23, 1948.
+       These curves are plotted on a "plus or minus" basis as
+       described in the text, with the bottom of the curve
+       representing the minimum density and the top of the curve
+       the maximum.]
+
+The well-marked decline before midnight in the migration rates at
+Baton Rouge may be regarded as one of the outstanding results emerging
+from this study. Many years of ornithological investigation in this
+general region failed to suggest even remotely that a situation of
+this sort obtained. Now, in the light of this new fact, it is possible
+for the first time to rationalize certain previously incongruous data.
+Ornithologists in this area long have noted that local storms and
+cold-front phenomena at night in spring sometimes precipitate great
+numbers of birds, whereupon the woods are filled the following day
+with migrants. On other occasions, sudden storms at night have
+produced no visible results in terms of bird densities the following
+day. For every situation such as described by Gates (1933) in which
+hordes of birds were forced down at night by inclement weather, there
+are just as many instances, even at the height of spring migration,
+when similar weather conditions yielded no birds on the ground.
+However, the explanation of these facts is simple; for we discover
+that storms that produced birds occurred before midnight and those
+that failed to produce birds occurred after that time (the storm
+described by Gates occurred between 8:30 and 9:00 P. M.).
+
+The early hour decline in density at Baton Rouge at first did not seem
+surprising in view of the small amount of land area between this
+station and the Gulf of Mexico. Since the majority of the birds
+destined to pass Baton Rouge on a certain night come in general from
+the area to the south of that place, and since the distances to
+various points on the coast are slight, we inferred that a three-hour
+flight from even the more remote points would probably take the bulk
+of the birds northward past Baton Rouge. In short, the coastal plain
+would be emptied well before midnight of its migrant bird life, or at
+least that part of the population destined to migrate on any
+particular night in question. Although data in quantity are not
+available from stations on the coastal plain other than Baton Rouge,
+it may be pointed out that such observations as we do have, from
+Lafayette and New Orleans, Louisiana, and from Thomasville, Georgia,
+are in agreement with this hypothesis.
+
+A hundred and seventy miles northward in the Mississippi Valley, at
+Oak Grove, Louisiana, a somewhat more normal density pattern is
+manifested. There, in four nights of careful observation, a pronounced
+early peak resulted on the night of May 21-22 (Figure 29E), but on the
+other three nights significant densities held up until near twelve
+o'clock, thereby demonstrating the probable effect of the increased
+amount of land to the south of the station.
+
+Subsequent studies, revealing the evident existence of an underlying
+density time pattern, cast serious doubt on the explanations just
+advanced of the early decline in the volume of migration at Baton
+Rouge. It has as yet been impossible to reconcile the early drop-off
+at this station with the idea that birds are still mounting into the
+air at eleven o'clock, as is implied by the ideal time curves.
+
+
+C. MIGRATION IN RELATION TO TOPOGRAPHY
+
+To this point we have considered the horizontal distribution of birds
+in the sky only on a very narrow scale and mainly in terms of the
+chance element in observations. Various considerations have supported
+the premise that the spread of nocturnal migration is rather even, at
+least within restricted spacial limits and short intervals of time.
+This means that in general the flow of birds from hour to hour at a
+single station exhibits a smooth continuity. It does not mean that it
+is a uniform flow in the sense that approximately the same numbers of
+birds are passing at all hours, or at all localities, or even on all
+one-mile fronts in the same locality. On the contrary, there is
+evidence of a pronounced but orderly change through the night in the
+intensity of the flight, corresponding to a basic and definitely timed
+cycle of activity. Other influences may interfere with the direct
+expression of this temporal rhythm as it is exhibited by observations
+at a particular geographical location. Among these, as we have just
+seen, is the disposition of the areas that offer suitable resting
+places for transient birds and hence contribute directly and
+immediately to the flight overhead. A second possible geographical
+effect is linked with the question of the tendency of night migrants
+to follow topographical features.
+
+_General Aspects of the Topographical Problem_
+
+That many diurnal migrants tend to fly along shorelines, rivers, and
+mountain ridges is well known, but this fact provides no assurance
+that night migrants do the same thing. Many of the obvious advantages
+of specialized routes in daylight, such as feeding opportunities, the
+lift provided by thermal updrafts, and the possible aid of certain
+landmarks in navigation, assume less importance after night falls.
+Therefore, it would not be safe to conclude that _all_ nocturnal
+migrants operate as do _some_ diurnal migrants. For instance, the
+passage of great numbers of certain species of birds along the Texas
+coast in daylight hours cannot be regarded as certain proof that the
+larger part of the nocturnal flight uses the same route. Neither can
+we assume that birds follow the Mississippi River at night simply
+because we frequently find migrants concentrated along its course in
+the day. Fortunately we shall not need to speculate indefinitely on
+this problem; for the telescopic method offers a means of study based
+on what night migrants are doing _at night_. Two lines of attack may
+be pursued. First we may compare flight densities obtained when the
+field of the telescope lies over some outstanding topographical
+feature, such as a river, with the recorded volume of flight when the
+cone of observation is directed away from that feature. Secondly, we
+may inquire how the major flight directions at a certain station are
+oriented with respect to the terrain. If the flight is concentrated
+along a river, for instance, the flight density curve should climb
+upward as the cone of observation swings over the river, _regardless
+of the hour at which it does so_. The effect should be most pronounced
+if the observer were situated on the river bank, so that the cone
+would eventually come to a position directly along the watercourse.
+Though in that event birds coming up the river route would be flying
+across the short axis of an elliptical section of the cone, the fact
+that the whole field of observation would be in their path should
+insure their being seen in maximum proportions. If, on the other hand,
+the telescope were set up some distance away from the river so that
+the cone merely moved _across_ its course, only a section of the
+observation field would be interposed on the main flight lane.
+
+The interaction of these possibilities with the activity rhythm should
+have a variety of effects on the flight density curves. If the cone
+comes to lie over the favored topographical feature in the hour of
+greatest migrational activity, the results would be a simple sharp
+peak of doubtful meaning. However, since the moon rises at a different
+time each evening, the cone likewise would reach the immediate
+vicinity of the terrain feature at a different time each night. As a
+result, the terrain peak would move away from its position of
+coincidence with the time peak on successive dates, producing first,
+perhaps, a sustention of peak and later a definitely bimodal curve.
+Since other hypotheses explain double peaks equally well, their mere
+existence does not necessarily imply that migrants actually do travel
+along narrow topographical lanes. Real proof requires that we
+demonstrate a moving peak, based on properly corrected density
+computations, corresponding always with the position of the cone over
+the most favored terrain, and that the flight vectors be consistent
+with the picture thus engendered.
+
+_The Work of Winkenwerder_
+
+To date, none of the evidence in favor of the topographical hypothesis
+completely fills these requirements. Winkenwerder (_loc. cit._), in
+analyzing the results of telescopic counts of birds at Madison and
+Beloit, Wisconsin, Detroit and Ann Arbor, Michigan, and at Lake
+Forest, Illinois, between 1898 and 1900, plotted the number of birds
+seen at fifteen-minute intervals as a function of the time of the
+night. He believed that the high points in the resulting frequency
+histograms represented intervals when the field of the telescope was
+moving over certain topographically determined flight lanes, though he
+did not specify in all cases just what he assumed the critical
+physiographic features to be. Especially convincing to him were
+results obtained at Beloit, where the telescope was situated on the
+east bank of the Rock River, on the south side of the city.
+Immediately below Beloit the river turns southwestward and continues
+in this direction about five miles before turning again to flow in a
+southeastward course for approximately another five miles. In this
+setting, on two consecutive nights of observation in May, the number
+of birds observed increased tremendously in the 2 to 3 A. M. interval,
+when, according to Winkenwerder's interpretation of the data (he did
+not make the original observations at Beloit himself), the telescope
+was pointing directly down the course of the river. This conclusion is
+weakened, however, by notable inconsistencies. Since the moon rises
+later each evening, it could not have reached the same position over
+the Rock River at the same time on both May 12-13 and May 13-14, and
+therefore, if the peaks in the graph were really due to a greater
+volume of migration along the watercourse, they should not have so
+nearly coincided. As a matter of fact the incidence of the peak on
+May 12-13 should have preceded that of the peak on May 13-14; whereas
+his figure shows the reverse to have been true. Singularly enough,
+Winkenwerder recognized this difficulty in his treatment of the data
+from Madison, Wisconsin. Unable to correlate the peak period with the
+Madison terrain by the approach used for Beloit, he plotted the
+observations in terms of hours after moonrise instead of standard
+time. This procedure was entirely correct; the moon does reach
+approximately the same position at each hour after its rise on
+successive nights. The surprising thing is that Winkenwerder did not
+seem to realize the incompatibility of his two approaches or to
+realize that he was simply choosing the method to suit the desired
+results.
+
+Furthermore, as shown in Part I of this paper, the number of birds
+seen through the telescope often has only an indirect connection with
+the actual number of birds passing over. My computations reveal that
+the highest counts of birds at Beloit on May 12-13 were recorded when
+the moon was at an altitude of only 8° to 15° and, that when
+appropriate allowance is made for the immense size of the field of
+observation at this time, the partially corrected flight density for
+the period is not materially greater than at some other intervals in
+the night when the telescope was not directed over the course of the
+Rock River. These allowances do not take the direction factor into
+consideration. Had the birds been flying at right angles to the short
+axis of an elliptical section of the cone throughout the night, the
+flight density in the period Winkenwerder considered the peak would
+have been about twice as high as in any previous interval. On the
+other hand, if they had been flying across the long axis at all times,
+the supposed peak would be decidedly inferior to the flight density at
+10 to 11:00 P. M., before the cone came near the river.
+
+Admittedly, these considerations contain a tremendous element of
+uncertainty. They are of value only because they expose the equal
+uncertainty in Winkenwerder's basic evidence. Since the coördinates of
+the birds' apparent pathways at Beloit were given, I at first
+entertained the hope of computing the flight densities rigorously, by
+the method herein employed. Unfortunately, Winkenwerder was apparently
+dealing with telescopes that gave inverted images, and he used a
+system for recording coördinates so ambiguously described that I am
+not certain I have deciphered its true meaning. When, however, his
+birds are plotted according to the instructions as he stated them, the
+prevailing direction of flight indicated by the projection formula
+falls close to west-northwest, not along the course of the Rock River,
+but _at direct right angles to it_.
+
+    [Illustration: FIG. 31. Directional components in the flight
+       at Tampico on three nights in 1948. The lengths of the
+       sector vectors are determined by their respective densities
+       expressed as a percentage of the station density for that
+       night; the vector resultants are plotted from them by
+       standard procedure. Thus, the nightly diagrams are not on the
+       same scale with respect to the actual number of birds involved.]
+
+
+    [Illustration: FIG. 32. Hourly station density curve at
+       Tampico, Tamaulipas, on the night of April 21-22, 1948 (CST).]
+
+_Interpretation of Recent Data_
+
+I am in a position to establish more exact correlations between flight
+density and terrain features in the case of current sets of
+observations. Some of these data seem at first glance to fit the idea
+of narrow topographically-oriented flight lanes rather nicely. At
+Tampico, where six excellent sets of observations were made in March
+and April, 1948, the telescope was set up on the beach within a few
+yards of the Gulf of Mexico. As can be seen from Figure 25 (_ante_),
+the slant of the coastline at this point is definitely west of north,
+as is also the general trend of the entire coast from southern
+Veracruz to southern Tamaulipas (see Figure 34, beyond). The over-all
+vector resultant of all bird flights at this station was N 11° W, and,
+as will be seen from Figure 31, none of the nightly vector resultants
+in April deviates more than one degree from this average. Thus the
+prevailing direction of flight, as computed, agrees with the trend of
+the coast at the precise point of the observations, at least to the
+extent that both are west of north. To be sure, the individual sector
+vectors indicate that not all birds were following this course;
+indeed, some appear to have been flying east of north, heading for a
+landfall in the region of Brownsville, Texas, and a very few to have
+been traveling northeastward toward the central Gulf coast. But it
+must be remembered that a certain amount of computational deviation
+and of localized zigzagging in flight must be anticipated. Perhaps
+none of these eastward vectors represents an actual extended flight
+path. The nightly vector resultants, on the other hand, are so
+consistent that they have the appearance of remarkable accuracy and
+tempt one to draw close correlations with the terrain. When this is
+done, it is found that, while the prevailing flight direction is 11°
+west of north, the exact slant of the coastline at the location of the
+station is about 30° west of north, a difference of around 19°. It
+appears, therefore, that the birds were not following the shoreline
+precisely but cutting a chord about ten miles long across an
+indentation of the coast. If it be argued that the method of
+calculation is not accurate enough to make a 19° difference
+significant, and that most of the birds might have been traveling
+along the beach after all, it can be pointed out with equal
+justification that, if this be so, the 11° divergence from north does
+not mean anything either and that perhaps the majority of the birds
+were going due north. We are obliged to conclude either that the main
+avenue of flight paralleled the disposition of the major topographical
+features only in a general way or that the angle between the line of
+the coast and true north is not great enough to warrant any inference
+at all.
+
+Consideration of the Tampico density curves leads to similarly
+ambiguous results. On the night of April 21-22, as is evident from a
+comparison of Figures 25 and 32, the highest flight density occurred
+when the projection of the cone on the terrain was wholly included
+within the beach. This is very nearly the case on the night of April
+23-24 also, the positions of the cone during the peak period of
+density being only about 16° apart. (On the intervening date, clouds
+prevented continuous observation during the critical part of the
+night.) These correlations would seem to be good evidence that most of
+these night migrants were following the coastline of the Gulf of
+Mexico. However, the problem is much more complicated. The estimated
+point of maximum flight density fell at 10:45 P. M. on April 21-22
+and 11:00 P. M. on April 23-24, both less than an hour from the peak
+in the ideal time curve (Figure 26, _ante_). We cannot be sure,
+therefore, that the increase in density coinciding with the position
+of the moon over the beach is not an increase which would have
+occurred anyway. Observations conducted several nights before or after
+the second quarter, when the moon is not on or near its zenith at the
+time of the predictable peak in the density curve, would be of
+considerable value in the study of this particular problem.
+
+The situation at Tampico has been dealt with at length because, among
+all the locations for which data are available, it is the one that
+most strongly supports the topographical hypothesis. In none of the
+other cases have I been able to find a definite relation between the
+direction of migration and the features of the terrain. Studies of
+data from some of these stations disclose directional patterns that
+vary from night to night only slightly more than does the flight at
+Tampico. In three nights of observation at Lawrence, Kansas, marked by
+very high densities, the directional trend was north by
+north-northeast with a variation of less than 8°, yet Lawrence is so
+situated that there seems to be no feature of the landscape locally or
+in the whole of eastern Kansas or of western Missouri that coincides
+with this heading. At Mansfield, Louisiana, in twelve nights of
+observation, the strong east by northeast trend varied less than 15°,
+but again there appears to be no correlation over a wide area between
+this direction and any landmarks. And, at Progreso, Yucatán, where the
+vector resultants were 21° and 27° on successive nights, most of the
+birds seen had left the land and were beginning their flight northward
+over the trackless waters of the Gulf of Mexico. Furthermore, as I
+have elsewhere pointed out (1946: 205), the whole northern part of the
+Yucatán Peninsula itself is a flat terrain, unmarked by rivers,
+mountains, or any other strong physiographic features that conceivably
+might be followed by birds.
+
+    [Illustration: FIG. 33. The nightly net trend of migrations
+       at three stations in 1948. Each arrow is the vector resultant
+       for a particular night, its length expressing the nightly
+       density as a percentage of the total station density for the
+       nights represented. Thus, the various station diagrams are
+       not to the same scale.]
+
+In Figure 33 I have shown the directional patterns at certain stations
+where, unlike the cases noted above, there is considerable change on
+successive nights. Each vector shown is the vector resultant for one
+particular night. The lengths of the vectors have been determined by
+their respective percentages of the total computed density, or total
+station magnitude, for all the nights in question. In other words, the
+lengths of the individual vectors denote the percentile rôle that each
+night played in the total density. From the directional spread at
+these stations it becomes apparent that if most of the birds were
+traveling along a certain topographic feature on one night, they
+could not have been traveling along the same feature on other nights.
+
+The possibility should be borne in mind, however, that there may be
+more than one potential topographic feature for birds to follow at
+some stations. Moreover, it is conceivable that certain species might
+follow one feature that would lead them in the direction of their
+ultimate goal, whereas other species, wishing to go in an entirely
+different direction, might follow another feature that would lead them
+toward their respective destination. It would seem unlikely, however,
+that the species composition of the nocturnal flights would change
+materially from night to night, although there is a strong likelihood
+that it might do so from week to week and certainly from month to
+month.
+
+By amassing such data as records of flight direction along the same
+coast from points where the local slant of the shoreline is materially
+different, and comparisons of the volume of migration at night along
+specialized routes favored during the day with the flight densities at
+progressive distances from the critical terrain feature involved, we
+shall eventually be able to decide definitely the rôle topography
+plays in bird migration. We cannot say on the basis of the present
+ambiguous evidence that it is not a factor in determining which way
+birds fly, but, if I had to hazard a guess one way or the other, I
+would be inclined to discount the likelihood of its proving a major
+factor.
+
+
+D. GEOGRAPHICAL FACTORS AND THE CONTINENTAL DENSITY PATTERN
+
+A study of the total nightly or seasonal densities at the various
+stations brings forth some extremely interesting factors, many of
+which, however, cannot be fully interpreted at this time. A complete
+picture of the magnitude of migration at a given station cannot be
+obtained from the number of birds that pass the station on only a few
+nights in one spring. Many years of study may be required before hard
+and fast principles are justifiable. Nevertheless, certain salient
+features stand out in the continental density pattern in the spring of
+1948. (The general results are summarized in Tables 2-5; the location
+of the stations is shown in Figure 34.) These features will be
+discussed now on a geographical basis.
+
+    TABLE 2.--Extent of Observations and Seasonal Station
+                Densities at Major Stations in 1948
+
+  ==========================================================================
+                       |Nights of observation| Hours of observation|
+  OBSERVATION STATION  |---------------------+---------------------|Season
+                       |March|April|May|Total|March|April|May|Total|density
+  ---------------------+-----+-----+---+-----+-----+-----+---+-----+--------
+  CANADA               |     |     |   |     |     |     |   |     |
+    Pt. Pelee          |     |     | 1 |  1  |     |     | 6 |   6 |   2,500
+                       |     |     |   |     |     |     |   |     |
+  MEXICO               |     |     |   |     |     |     |   |     |
+    S. L. P.: Ebano    |  1  |     |   |  1  |  3  |     |   |   3 |   1,300
+    Tamps.: Tampico    |  3  |  3  |   |  6  | 20  | 20  |   |  40 | 140,300
+    Yuc.: Progreso     |     |  3  |   |  3  |     | 18  |   |  18 |  60,500
+                       |     |     |   |     |     |     |   |     |
+  UNITED STATES        |     |     |   |     |     |     |   |     |
+    Fla.: Pensacola    |     |  2  | 2 |  4  |     |  8  | 7 |  15 |   1,500
+      Winter Park      |     |  5  | 6 | 11  |     | 39  |38 |  77 |  21,700
+    Ga.: Athens        |     |  2  |   |  2  |     | 10  |   |  10 |   4,000
+      Thomasville      |     |  1  | 1 |  2  |     |  8  | 8 |  16 |   4,700
+    Iowa: Ottumwa      |     |  5  | 5 | 10  |     | 16  |28 |  44 | 134,400
+    Kans.: Lawrence    |  2  |  1  |   |  3  | 16  |  4  |   |  20 |  68,700
+    Ky.: Louisville    |     |  3  | 2 |  5  |     | 20  |14 |  34 |  49,300
+      Murray           |     |  2  |   |  2  |     | 13  |   |  13 |  26,200
+    La.: Baton Rouge   |     |  3  |   |  3  |     | 15  |   |  15 |  11,000
+      Lafayette        |     |  1  |   |  1  |     |  5  |   |   5 |   2,800
+      Mansfield        |  1  |  5  | 4 | 10  |  2  | 16  |22 |  40 |  22,400
+      New Orleans      |  1  |  1  |   |  2  |  5  |  2  |   |   7 |   1,900
+      Oak Grove        |     |  2  | 2 |  4  |     | 16  |15 |  31 |  33,900
+    Mich.: Albion      |     |  1  |   |  1  |     |  3  |   |   3 |   1,100
+    Minn.: Hopkins     |     |     | 1 |  1  |     |     | 4 |   4 |   2,000
+    Miss.: Rosedale    |     |  1  | 1 |  2  |     |  6  | 8 |  14 |  12,600
+    Mo.: Columbia      |     |  2  | 1 |  3  |     |  8  | 6 |  14 |  13,100
+    Liberty            |     |  1  | 1 |  2  |     |  7  | 7 |  14 |   4,800
+    Okla.: Stillwater  |  1  |  2  | 1 |  4  |  5  | 11  | 3 |  19 |   8,400
+    S. Car.: Charleston|  1  |  1  | 1 |  3  |  5  |  8  | 9 |  22 |   3,000
+    Tenn.: Knoxville   |     |  2  | 2 |  4  |     | 18  |14 |  32 |  35,400
+      Memphis          |  2  |  3  | 2 |  7  | 13  | 20  |12 |  45 |  29,700
+    Tex.: College      |     |  3  | 1 |  4  |     | 19  | 8 |  27 |  32,200
+      Station Rockport |     |  1  |   |  1  |     |  4  |   |   4 |   6,200
+  --------------------------------------------------------------------------
+
+    TABLE 3.--Average Hourly Station Densities in 1948
+
+  ========================================================
+  OBSERVATION STATION     | March | April |  May  | Season
+  ------------------------+-------+-------+-------+-------
+  CANADA                  |       |       |       |
+    Pt. Pelee             |       |       |   400 |   400
+                          |       |       |       |
+  MEXICO                  |       |       |       |
+    S. L. P.: Ebano       |   400 |       |       |   400
+    Tamps.: Tampico       |   700 | 6,300 |       | 3,500
+    Yuc.: Progreso        |       | 2,800 |       | 2,800
+                          |       |       |       |
+  UNITED STATES           |       |       |       |
+    Fla.: Pensacola       |       |     0+|   200 |   100
+      Winter Park         |       |   300 |   200 |   300
+    Ga.: Athens           |       |   400 |       |   400
+      Thomasville         |       |   500 |   100 |   300
+    Iowa: Ottumwa         |       | 1,700 | 3,800 | 3,100
+    Kans.: Lawrence       | 4,000 | 1,400 |       | 3,400
+    Ky.: Louisville       |       | 2,000 |   700 | 1,500
+      Murray              |       | 2,000 |       | 2,000
+    La.: Baton Rouge      |       |   700 |       |   700
+      Lafayette           |       |   600 |       |   600
+      Mansfield           |     0 |   700 |   800 |   600
+      New Orleans         |    60 |   800 |       |   300
+      Oak Grove           |       | 1,400 |   800 | 1,100
+    Mich.: Albion         |       |   400 |       |   400
+    Minn.: Hopkins        |       |       |   500 |   500
+    Miss.: Rosedale       |       | 1,100 |   700 |   900
+    Mo.: Columbia         |       |   400 | 1,700 |   900
+      Liberty             |       |   500 |   200 |   300
+    Okla.: Stillwater     |   500 |   200 | 1,000 |   400
+    S. Car.: Charleston   |   200 |   200 |     0+|   100
+    Tenn.: Knoxville      |       | 1,300 |   800 | 1,100
+      Memphis             |   300 |   800 |   900 |   700
+    Tex.: College Station |       | 1,100 | 1,500 | 1,200
+      Rockport            |       | 1,600 |       | 1,600
+  --------------------------------------------------------
+
+    TABLE 4.--Maximum Hourly Station Densities in 1948
+
+  ======================================================
+  OBSERVATION STATION     |  March  |  April  |   May
+  ------------------------+---------+---------+---------
+  CANADA                  |         |         |
+    Pt. Pelee             |         |         |  1,400
+                          |         |         |
+  MEXICO                  |         |         |
+    S. L. P.: Ebano       |    600  |         |
+    Tamps.: Tampico       |  3,100  | 21,200  |
+    Yuc.: Progreso        |         | 11,900  |
+                          |         |         |
+  UNITED STATES           |         |         |
+    Fla.: Pensacola       |         |    100  |    700
+      Winter Park         |         |  2,300  |  1,000
+    Ga.: Athens           |         |    900  |
+      Thomasville         |         |  1,500  |    200
+    Iowa: Ottumwa         |         |  3,800  | 12,500
+    Kans.: Lawrence       | 14,500  |  2,200  |
+    Ky.: Louisville       |         |  5,000  |  1,400
+      Murray              |         |  3,700  |
+    La.: Baton Rouge      |         |  3,400  |
+      Lafayette           |         |  1,800  |
+      Mansfield           |         |  2,100  |  1,600
+      New Orleans         |    200  |  1,100  |
+      Oak Grove           |         |  2,700  |  2,500
+    Mich.: Albion         |         |    700  |
+    Minn.: Hopkins        |         |         |  1,100
+    Miss.: Rosedale       |         |  2,200  |  1,400
+    Mo.: Columbia         |         |    800  |  3,400
+      Liberty             |         |    800  |    800
+    Okla.: Stillwater     |    900  |    700  |  1,400
+    S. Car.: Charleston   |    400  |    600  |    200
+    Tenn.: Knoxville      |         |  5,800  |  1,900
+      Memphis             |  1,200  |  3,400  |  2,100
+    Tex.: College Station |         |  3,400  |  3,100
+      Rockport            |         |  2,400  |
+  ------------------------------------------------------
+
+    TABLE 5.--Maximum Nightly Densities at Stations with More
+                Than One Night of Observation
+
+  ======================================================
+  OBSERVATION STATION     |  March  |  April  |   May
+  ------------------------+---------+---------+---------
+                          |         |         |
+  MEXICO                  |         |         |
+    Tamps.: Tampico       |  5,500  | 63,600  |
+    Yuc.: Progreso        |         | 31,600  |
+                          |         |         |
+  UNITED STATES           |         |         |
+    Fla.: Winter Park     |         |  6,200  |
+    Ga.: Athens           |         |  2,600  |
+      Thomasville         |         |  3,900  |
+    Iowa: Ottumwa         |         | 15,300  | 54,600
+    Kans.: Lawrence       | 51,600  |  5,400  |
+    Ky.: Louisville       |         | 17,000  |  8,400
+      Murray              |         | 16,400  |
+    La.: Baton Rouge      |         |  6,200  |
+      Mansfield           |         |  4,900  |  5,200
+      Oak Grove           |         | 13,600  |  5,800
+    Miss.: Rosedale       |         |  6,800  |  5,800
+    Mo.: Columbia         |         |  1,400  | 10,300
+    Okla.: Stillwater     |  2,700  |  1,900  |  3,000
+    Tenn.: Knoxville      |         | 15,200  |  9,000
+      Memphis             |  3,600  |  7,900  |  7,000
+    Tex.: College Station |         |  6,200  | 13,200
+  ------------------------------------------------------
+
+    [Illustration: FIG. 34. Stations at which telescopic
+       observations were made in 1948.]
+
+_Gulf Migration: A Review of the Problem_
+
+In view of the controversy in recent years pertaining to migration
+routes in the region of the Gulf of Mexico (Williams, 1945 and 1947;
+Lowery, 1945 and 1946), the bearing of the new data on the problem is
+of especial interest. While recent investigations have lent further
+support to many of the ideas expressed in my previous papers on the
+subject, they have suggested alternative explanations in the case of
+others. In the three years that have elapsed since my last paper
+dealing with Gulf migration, some confusion seems to have arisen
+regarding the concepts therein set forth. Therefore, I shall briefly
+re-state them.
+
+It was my opinion that evidence then available proved conclusively
+that birds traverse the Gulf frequently and intentionally; that the
+same evidence suggested trans-Gulf flights of sufficient magnitude to
+come within the meaning of migration; that great numbers of birds move
+overland around the eastern and western edges of the Gulf; that it was
+too early to say whether the coastal or trans-Gulf route was the more
+important, but that enough birds cross the water from Yucatán to
+account for transient migration in the extreme lower Mississippi
+Valley; and, that, in fair weather, most trans-Gulf migrants continue
+on inland for some distance before coming to land, creating an area of
+"hiatus" that is usually devoid of transient species. I tried to make
+it emphatically clear that I realized that many birds come into Texas
+from Mexico overland, that I did not think the hordes of migrants
+normally seen on the Texas coast in spring were by any means all
+trans-Gulf migrants. I stated (1946: 206): "Proving that birds migrate
+in numbers across the Gulf does not prove that others do not make the
+journey by the coastal routes. But that is exactly the point. No one
+has ever pretended that it does." Although some ornithologists seem to
+have gained the impression that I endorse only the trans-Gulf route,
+this is far from the truth. I have long held that the migrations
+overland through eastern Mexico and southern Texas on one hand, and
+the over-water flights on the other, are each part of the broad
+movement of transients northward into the United States. There are
+three avenues of approach by which birds making up the tremendous
+concentrations on the Texas coast may have reached there: by a
+continental pathway from a wintering ground in eastern and southern
+Mexico; by the over-water route from Yucatán and points to the
+southward; and, finally, by an overland route from Central America via
+the western edge of the Gulf. As a result of Louisiana State
+University's four-year study of the avifauna in eastern Mexico, I
+know that migrants reach Texas from the first source. As a consequence
+of my studies in Yucatán of nocturnal flight densities and their
+directional trends, I strongly believe that migrants reach Texas from
+this second source. As for the third source, I have never expressed an
+opinion. I am not prepared to do so now, for the reason that today, as
+three years ago, there is no dependable evidence on which to base a
+judgment one way or another.
+
+    TABLE 6.--Computed Hourly Densities at Tampico, Tamps.,
+                in Spring of 1948
+
+  =========================================================================
+             |                    Average hour of observation
+    DATE     |-----+------+-------+-------+------+------+------+------+----
+             | 8:30| 9:30 | 10:30 | 11:30 |12:30 | 1:30 | 2:30 | 3:30 |4:30
+  -----------|-----+------+-------+-------+------+------+------+------+----
+  22-23 March|  600|  700 | 1,000 |   800 |  100 |  100 |    0 |  100 | ..
+  23-24 March|    0|  400 | 1,200 | 3,100 |  800 |  ..  |  ..  |  ..  | ..
+  24-25 March|  300|  700 |   800 | 1,600 |1,100 |  ..  |  ..  |  ..  | ..
+  21-22 April|1,100|7,000 |14,900 |12,900 |8,100 |3,800 |3,500 |  200 | ..
+  22-23 April|  700|2,900 | 7,500 |   ..  |  ..  |  ..  |  ..  |  ..  | ..
+  23-24 April|  600|4,700 |19,100 |21,200 |5,500 |5,900 |4,000 |2,000 |200
+  -------------------------------------------------------------------------
+
+
+_Western Gulf Area_
+
+Among the present flight density data bearing on the above issues, are
+the six sets of observations from the vicinity of Tampico, Tamaulipas,
+already referred to. These were secured in the spring of 1948 by a
+telescope set up on the Gulf beach just north of the Miramar pavilion
+and only a hundred feet from the surf (see Figure 25, _ante_). The
+beach here is approximately 400 feet wide and is backed by
+scrub-covered dunes, which rapidly give way toward the west to a
+rather dense growth of low shrubs and trees. One might have expected
+that station densities at Tampico in March would be rather high.
+Actually, though they are the second highest recorded for the month,
+they are not impressive and afford a striking contrast with the record
+flights there in April (Table 6). Unfortunately, only a few stations
+were operating in March and thus adequate comparisons are impossible;
+but the indications are that, in March, migration activity on the
+western edges of the Gulf is slight. It fails even to approach the
+volume that may be observed elsewhere at the same time, as for
+example, in eastern Kansas where, however, the migration is not
+necessarily correlated with the migration in the lower Gulf area.
+Strangely enough, on the night of March 22-23, at Tampico,
+approximately 85 per cent of the birds were flying from north of an
+east-west line to south of it, opposite to the normal trend of spring
+migration. This phenomenon, inexplicable in the present instance, will
+be discussed below. On the other two nights in March, the directional
+trend at Tampico was northward with few or no aberrant components.
+Observations made approximately thirty-five miles inland from the
+Gulf, at Ebano, San Luis Potosí, on the night of March 25-26, show
+lower station densities than the poorest night at Tampico, but since
+they cover only a three-hour watch, they reveal little or nothing
+concerning the breadth of the so-called coastal flyway.
+
+April flight densities at Tampico are the highest recorded in the
+course of this study. The maximum hourly density of 21,200 birds is 46
+per cent higher than the maximum hourly density anywhere else. The
+average hourly density of 6,300 in April is more than twice as great
+as the next highest average for that month. These figures would seem
+to satisfy certain hypotheses regarding a coastwise flight of birds
+around the western edge of the Gulf. Other aspects of the observations
+made at that time do not satisfy these hypotheses. Texas
+ornithologists have found that in periods of heavy spring migration,
+great numbers of birds are invariably precipitated by rainy weather.
+On April 23, in the midst of the record-breaking telescopic studies at
+Tampico, Mr. Robert J. Newman made a daytime census immediately
+following four hours of rain. He made an intensive search of a small
+area of brush and low growth back of the beach for traces of North
+American migrants. In his best hour, only thirteen individual birds
+out of seventy-five seen were of species that do not breed there. The
+transient species were the Ruby-throated Hummingbird (1),
+Scissor-tailed Flycatcher (1), Western Wood Pewee (1), Black-throated
+Green Warbler (2) Orchard Oriole (7), and Baltimore Oriole (1), all of
+which winter extensively in southern Mexico. Perhaps, however, the
+apparent scarcity of transients on this occasion is not surprising in
+the light of the analysis of flight density in terms of bird density
+on the ground which I shall develop beyond. My only point here is to
+demonstrate that rain along the coast does not always produce birds.
+
+As large as the nocturnal flights at Tampico have so far proved to be,
+they are not commensurate with the idea that nearly all birds follow a
+narrow coastwise route around the Gulf. To establish the latter idea,
+one must be prepared to show that the migrant species returning to the
+United States pass along two flyways a few miles wide in the immense
+volume necessary to account for their later abundance on a 1500-mile
+front extending across eastern North America. One might expect at
+least ten to twenty fold the number observable at any point in the
+interior of the United States. In actuality, the highest nightly
+density of 63,600 birds at Tampico is barely sufficient to account for
+the highest nightly density of 54,600 at Ottumwa, Iowa, alone.
+
+Of course, there is no way of knowing how closely a ratio of anywhere
+from ten to one through twenty to one, employed in this comparison,
+expresses the true situation. It may be too high. It could be too low,
+particularly considering that preliminary studies of flight density in
+Florida indicate that the western shores of the Gulf of Mexico must
+carry the major part of the traffic if migratory flights back to the
+United States in spring take place only along coastwise routes.
+Consideration of the data obtained in Florida in 1948 will serve to
+emphasize the point.
+
+_Eastern Gulf Area_
+
+At Winter Park, Florida, seventy-seven hours were spent at the
+telescope in April and May. This was 71 per cent more hours of actual
+observation than at the next highest station. Nevertheless, the total
+seasonal density amounted to only 21,700 birds. The average hourly
+density was only 300 birds, with the maximum for any one hour being
+2,300 birds. In contrast, forty-five hours of observation at Tampico,
+Tamaulipas, in March and April, yielded a total station density of
+140,300 birds. At the latter place, on the night of April 23-24,
+almost as many birds passed _in a single hour_ as passed Winter Park
+in all of its seventy-seven hours of observation.
+
+Should future telescopic studies at Florida stations fail to produce
+densities appreciably higher than did Winter Park in 1948, the
+currently-held ideas that the Florida Peninsula is a major flyway will
+be seriously shaken. But one consideration must be kept in mind
+regarding the present picture. No observations were made at Winter
+Park in March, when it is conceivable that densities may have been
+materially higher. We know, for instance, that many of the early
+migrants to the southern United States are species whose winter homes
+are in the West Indies. Numbers of Vireonidae and Parulidae (notably
+the genera _Vireo_, _Parula_, _Protonotaria_, _Mniotilta_, _Seiurus_,
+_Geothlypis_, _Setophaga_, and certain _Dendroica_ and _Vermivora_)
+winter extensively in this region and are among the first birds to
+return to the southern states in the spring. Many of them often reach
+Louisiana and other states on the Gulf coastal plain by mid-March. In
+the same connection, it may be mentioned that many of the outstanding
+instances of birds striking lighthouses in southern Florida occurred
+in March and early April (Howell, 1932).
+
+_Yucatán Area_
+
+I have long felt that the answers to many of the questions which beset
+us in our study of Gulf migration are to be found on the open waters
+of the Gulf of Mexico itself or on the northern tip of the Yucatán
+Peninsula. Accordingly, in the spring of 1945 I crossed the Gulf by
+slow freighter for the purpose of determining how many and what kinds
+of birds might be seen between the mouth of the Mississippi River and
+the Yucatán Peninsula in fair weather, when it could not be argued
+that the birds had been blown there by inclement weather. To my own
+observations I was able to add those of other ornithologists who
+likewise had been aboard ship in the Gulf.
+
+The summary of results proved that birds of many species cross the
+Gulf and do so frequently. It failed to demonstrate beyond all doubt
+that they do so in large numbers. Nor had I expected it to do so. The
+consensus of Gulf coast ornithologists seemed to be that transient
+migration in their respective regions is often performed at too high
+an elevation to be detected unless the birds are forced to earth by
+bad weather. I saw no reason to anticipate that the results would be
+otherwise over the waters of the Gulf of Mexico.
+
+The application of the telescopic method held promise of supplying
+definite data on the numbers of trans-Gulf migrants, however high
+their flight levels. The roll and vibration of the ship had prevented
+me in 1945 from making telescopic observations at sea. Since no
+immediate solution to the technical difficulties involved presented
+itself, I undertook to reach one of the small cays in Alacrán Reef,
+lying seventy-five miles north of Yucatán and in line with the coast
+of southern Louisiana. Because of transportation difficulties, my
+plans to place a telescopic station in this strategic location failed.
+Consequently, I returned in 1948 by freighter to Progreso, Yucatán,
+where telescopic counts were made for three nights, one of which was
+rendered almost valueless by the cloud cover.
+
+    [Illustration: FIG. 35. Positions of the cone of
+       observation at Progreso, Yucatán, on the night of April
+       23-24, 1948, from 8:53 P. M. to 3:53 A. M. Essential
+       features of this map are drawn to scale. The telescope was
+       set up on the end of a one-mile long wharf that extends
+       northward from the shore over the waters of the Gulf of
+       Mexico. The triangular (white) lines represent the
+       projections of the cone of visibility on the earth at the
+       mid-point of each hour of observation. Only briefly, in the
+       first two hours, did the cone lie even in part over the
+       adjacent mainland. Hence, nearly all of the birds seen in the
+       course of the night had actually left the land behind.]
+
+The observation station at Progreso was situated on the northern
+end of the new wharf which projects northward from the beach to
+a point one mile over the Gulf. As will be seen from Figure 35, the
+entire cone of observation lay at nearly all times over the intervening
+water between the telescope on the end of the wharf and the
+beach. Therefore, nearly all of the birds seen were actually observed
+leaving the coast and passing out over the open waters of the
+Gulf. The hourly station densities are shown in Table 7 and Figures
+24 and 36. In the seventeen hours of observation on the nights of
+April 23-24 and April 24-25, a total computed density of 59,200 birds
+passed within one-half mile of each side of Progreso. This is the
+third highest density recorded in the course of this study. The
+maximum for one hour was a computed density of 11,900 birds. This
+is the fourth highest hourly density recorded in 1948.
+
+    [Illustration: FIG. 36. Hourly station density curve for
+       night of April 23-24, 1948, at Progreso, Yucatán.]
+
+    TABLE 7.--Computed Hourly Densities at Progreso, Yuc.,
+                in Spring of 1948
+
+  ===========+============================================================
+             |               Average hour of observation
+     DATE    +-----+------+------+-------+------+------+------+-----+-----
+             |8:30 | 9:30 |10:30 | 11:30 |12:30 | 1:30 | 2:30 |3:30 |4:30
+  -----------+-----+------+------+-------+------+------+------+-----+-----
+  23-24 April| 400 |3,000 |5,100 |10,000 |9,000 |2,800 |  900 | 400 |....
+  24-25 April|   0 |  500 |3,700 |11,900 |7,900 |1,900 |1,100 | 400 | 200
+  -----------+-----+------+------+-------+------+------+------+-----+-----
+
+
+It is not my contention that this many birds leave the northern coast
+of Yucatán every night in spring. Indeed, further studies may show
+negligible flight densities on some nights and even greater densities
+on others. As a matter of fact several hours of observation on the
+night of April 25-26, at Mérida, Yucatán, approximately twenty-five
+miles inland from Progreso, indicated that on this night the density
+overhead was notably low, a condition possibly accounted for by a
+north wind of 10 mph blowing at 2,000 feet. I merely submit that on
+the nights of April 23-24 and 24-25, birds were leaving the coast of
+Yucatán _at Progreso_ at the rate indicated. But, as I have emphasized
+in this paper and elsewhere (1946: 205-206), the northern part of the
+Yucatán Peninsula is notably unmarked by streams or any other
+physiographic features which birds might follow. The uniformity of the
+topography for many miles on either side of Progreso, if not indeed
+for the entire breadth of the Peninsula, makes it probable that
+Progreso is not a particularly favored spot for observing migration,
+and that it is not the only point along the northern coast of Yucatán
+where high flight densities can be recorded. This probability must be
+considered when comparisons are made between Progreso densities and
+those at Tampico. The argument could be advanced that the present
+densities from Tampico do not sufficiently exceed those at Progreso to
+establish the coastal route as the main avenue of traffic in spring,
+since there is every reason to suspect topography of exerting some
+influence to produce a channeling effect in eastern Mexico. Here the
+coast parallels the directional trend of the migratory movement for
+more than 600 miles. Likewise the Sierra Madre Oriental of eastern
+Mexico, situated approximately 100 miles inland (sometimes less), lies
+roughly parallel to the coast. Because of the slant of the Mexican
+land mass, many winter residents in southern Mexico, by short
+northward movements, would sooner or later filter into the coastal
+plain. Once birds are shunted into this lowland area, it would seem
+unlikely that they would again ascend to the top of the Sierra Madre
+to the west. In this way the great north-south cordillera of mountains
+may act as a western barrier to the horizontal dispersion of
+transients bound for eastern North America. Similarly, the Gulf itself
+may serve as an eastern barrier; for, as long as migrants may progress
+northward in the seasonal direction of migration and still remain over
+land, I believe they would do so.
+
+To put the matter in a slightly different way, the idea of a very
+narrow flight lane is inherent in the idea of coastwise migration.
+For, as soon as we begin to visualize flights of great volume over
+fronts extending back more than fifty miles from the shore line, we
+are approaching, if indeed we have not already passed, the point where
+the phenomenon is no longer coastwise in essence, but merely overland
+(as indeed my own unprocessed, telescopic data for 1949 indicate may
+be the case). In actuality, those who have reported on the migration
+along the western edge of the Gulf of Mexico have never estimated the
+width of the main flight at more than fifty miles and have intimated
+that under some circumstances it may be as narrow as two miles. No
+evidence of such restrictions can be discerned in the case of the
+trans-Gulf flights. If it cannot be said that they may be assumed to
+be as wide as the Gulf itself, they at least have the potential
+breadth of the whole 260-mile northern coast of the Yucatán Peninsula.
+On these premises, to be merely equal in total magnitude, the
+coastwise flights must exhibit, depending on the particular situation,
+from five to 130 times the concentrations observable among trans-Gulf
+migrants. This point seems almost too elementary to mention, but I
+have yet to find anyone who, in comparing the two situations, takes it
+into consideration.
+
+Judged in this light, the average hourly density of 2,800 birds at
+Progreso in April would appear to be indicative of many more migrants
+on the entire potential front than the 6,300 birds representing the
+average hourly density for the same month at Tampico.
+
+That the Progreso birds were actually beginning a trans-Gulf flight
+seems inevitable. The Yucatán Peninsula projects 200 miles or more
+northward into the vast open expanses of the Gulf of Mexico and the
+Caribbean Sea, with wide stretches of water on either side. The great
+majority of the birds were observed _after_ they had proceeded beyond
+the northern edge of this land mass. Had they later veered either to
+the east or the west, they would have been obliged to travel several
+hundred miles before again reaching land, almost as far as the
+distance straight across the Gulf. Had they turned southward, some
+individuals should have been detected flying in that direction. As can
+be seen from Figures 23, 42, and 44, not one bird observed was heading
+south of east or south of west on either night. No other single piece
+of evidence so conclusively demonstrates that birds cross the Gulf of
+Mexico in spring in considerable numbers as do flight density data
+recorded from Progreso in 1948.
+
+_Northern Gulf Area_
+
+Unfortunately only a few data on flight density are available from
+critical localities on the northern shores of the Gulf in spring. As
+the density curves in Figure 30 demonstrate, several sets of
+observation, including some phenomenal flights, have been recorded at
+Baton Rouge. This locality, however, lies sixty-four miles from the
+closest point on the Gulf coast, and the point due southward on the
+coast is eighty-four miles distant. Since all of the birds seen at
+Baton Rouge on any one night may have come from the heavily forested
+area between Baton Rouge and the coast of the Gulf, we cannot use data
+from Baton Rouge as certainly representative of incoming trans-Gulf
+flights. Data from repeated observations at stations on the coast
+itself are needed to judge the degree of trans-Gulf migration
+northward. On the few nights of observation at such localities
+(Cameron and Grand Isle, Louisiana, and Pensacola, Florida), flight
+densities have been zero or negligible. To be sure, negative results
+have been obtained at stations in the interior of the United States,
+and flights of low density have been recorded on occasion at stations
+where the flight densities are otherwise high. Nevertheless, in view
+of the volume of migration departing from Progreso, Yucatán, it would
+appear, upon first consideration, that we should at times record on
+the coast of Louisiana enough birds arriving in a night of continuous
+observation to yield a high density figure.
+
+Upon further consideration, however, there are factors mitigating
+against heavy densities of birds in northern flight on the northern
+coast of the Gulf. In the first place, presuming the main trans-Gulf
+flight to originate from northern Yucatán, and that there is a
+directional fanning to the northward, the birds leave on a 260-mile
+front, and arrive on a front 400 miles or more wide. Consequently,
+other factors remaining the same, there would be only approximately
+half the number of birds on the coast of arrival, at a given time and
+place, as there was on the coast of departure. Secondly, we may now
+presume on the basis of the telescopic studies at Progreso, that most
+migrants leaving northern Yucatán do so in the few hours centering
+about midnight. The varying speeds of the birds making the 580-mile
+flight across the Gulf distribute them still more sparsely on the
+north coast of the Gulf both in time and in space. Also we can see
+only that segment of the flight, which arrives in that part of a
+twenty-four hour period when the moon is up. This circumstance further
+reduces the interceptive potential because the hours after dark, to
+which the present telescopic studies have been restricted, comprise
+the period in which the fewest migrants arrive from over the water. To
+illustrate: it is a mathematical certainty that _none_ of the birds
+leaving Yucatán in the hours of heaviest flight, before 12 P. M.,
+and flying on a straight course at a speed of approximately 33 mph
+will reach the northern Gulf coast after nightfall; they arrive in the
+daytime. It will be useful to devise a technique for employing the sun
+as a background for telescopic observation of birds, thereby making
+observations possible on a twenty-four hour basis, so as to test these
+inferences by objective data.
+
+When a whole night's observation (1949 data not yet processed) at Port
+Aransas, on the southern coast of Texas, on the great overland route
+from eastern Mexico, yields in one night in April only seven birds,
+the recording of no birds at a station near the mouth of the
+Mississippi River becomes less significant.
+
+As I have previously remarked in this paper, the new data obtained
+since 1946, when I last wrote on the subject of migration in the
+region of Gulf of Mexico, requires that I alter materially some of my
+previously held views. As more and more facts come to light, I may be
+compelled to alter them still further. For one thing, I have come to
+doubt seriously the rigidity of the coastal hiatus as I envisioned it
+in 1945. I believe instead that the scarcity of records of transient
+migrants on the Gulf coastal plain in fair weather is to a very large
+extent the result of a wide dispersion of birds in the dense cover
+that characterizes this general region. I now question if appreciable
+bird densities on the ground ever materialize anywhere except when the
+sparseness of suitable habitat for resting or feeding tends to
+concentrate birds in one place, or when certain meteorological
+conditions erect a barrier in the path of an oncoming migratory
+flight, precipitating many birds in one place.
+
+This retrenchment of ideas is a direct consequence of the present
+study, for time and again, as discussed in the case of Tampico
+densities, maximal nightly flights have failed to produce a visible
+abundance of transients on land the following day. A simple example
+may serve to illustrate why. The highest one-hour density recorded in
+the course of this study is 21,200 birds. That means that this many
+birds crossed a line one mile long on the earth's surface and at right
+angles to the direction of flight. Let us further assume that the
+average flight speed of all birds comprising this flight was 30 mph.
+Had the entire flight descended simultaneously, it would have been
+dispersed over an area one mile wide and thirty miles long, and the
+precipitated density on the ground would have been only 1.1 birds per
+acre. Moreover, if as many as ten species had been involved in the
+flight, this would have meant an average per species of less than one
+bird per nine acres. This would have failed, of course, to show
+appreciable concentrations to the observer in the field the following
+day. If, however, on the other hand, the same flight of 21,200 birds
+had encountered at one point a weather barrier, such as a cold-front
+storm, all 21,200 birds might have been precipitated in one place and
+the field observer would have recorded an "inundation of migrants."
+This would be especially true if the locality were one with a high
+percentage of open fields or prairies and if the flight were mainly of
+woodland dwelling species, or conversely, if the locality were densely
+forested with few open situations and the flight consisted mainly of
+open-country birds. As explained on page 389, the density formula may
+be too conservative in its expression of actual bird densities. Even
+if the densities computed for birds in the air are only half as high
+as the actual densities in the air, the corresponding ground density
+of 2.2 birds per acre that results if all the birds descended
+simultaneously would hardly be any more impressive than the 1.1 bird
+per acre.
+
+This consideration is doubtless highly modified by local
+circumstances, but, in general, it seems to suggest a working
+hypothesis that provides an explanation for many of the facts that we
+now have. For example, on the coast of Texas there are great expanses
+of terrain unattractive to such birds as warblers, vireos, tanagers,
+and thrushes. The precipitation there by bad weather of even a
+mediocre nightly flight composed of birds of the kinds mentioned would
+surely produce an overwhelming concentration of birds in the scattered
+woods and shrubs.
+
+In spite of all that has been written about the great concentrations
+of transient migrants on the coast of Texas in spring, I am not convinced
+that they are of a different order of magnitude than those concentrations
+that sometimes occur along the cheniers and coastal islands
+of Louisiana and Mississippi. I have read over and over the
+highly informative accounts of Professor Williams (_loci cit._) and the
+seasonal summaries by Davis (1936-1940) and Williams (1941-1945).
+I have conversed at length with Mrs. Jack Hagar, whom I
+regard as one of the leading authorities on the bird life of the
+Texas coast, and she has even permitted me access to her voluminous
+records covering a period of fifteen years residence at Rockport.
+Finally, I have spent a limited amount of time myself on the Texas
+coast studying first-hand the situation that obtains there in order
+that I might be in a position to compare it with what I have learned
+from observations elsewhere in the region of the Gulf of Mexico,
+Louisiana, Florida, Yucatán, and eastern Mexico.
+
+Although the concentrations of birds on some days near the mouth of
+the Mississippi River are almost incalculable, the fact remains that
+in Texas the densities of transient species on the ground are more
+consistently high from day to day. The reason for this may be simple.
+As birds move up daily from Mexico overland, a certain percentage
+would be destined to come down at all points along the route but so
+dispersed in the inland forest that they might pass unnoticed.
+However, that part of the same flight settling down in coastal areas,
+where trees are scarce, would produce visible concentrations of
+woodland species. With the advent of a cold-front storm, two
+diametrically opposite effects of the same meteorological phenomenon
+would tend to pile up great concentrations of migrants of two
+classes--the overland and the trans-Gulf flights. During the
+prepolar-front weather the strong southerly (from the south) and
+southeasterly winds would tend to displace much of the trans-Gulf
+segment to the western part of the Gulf. With the shift of the winds
+to the north and northwest, which always occurs as the front passes,
+the overland flight still in the air would tend to be banked up
+against the coast, and the incoming trans-Gulf flight would be
+confronted with a barrier, resulting in the precipitation of birds on
+the first available land.
+
+These postulated conditions are duplicated in part in autumn along the
+Atlantic coast of the eastern United States. There, as a result of the
+excellent work of Allen and Peterson (1936) and Stone (1937), a
+similar effect has been demonstrated when northwest winds shove the
+south-bound flights up against the coast of New Jersey and concentrate
+large aggregations of migrants there.
+
+_Interior of the United States_
+
+Attention has been drawn already to the nature of the nightly flights
+at stations immediately inland from the Gulf coast, where densities
+decline abruptly well before midnight. I have suggested that this
+early drop-off is mainly a result of the small amount of terrain south
+of these stations from which birds may be contributed to a night's
+flight. At Oak Grove, Louisiana, the flight exhibited a strong
+directional trend with no significant aberrant components. Therefore,
+one may infer that a considerable part of the flight was derived from
+regions to the south of the station.
+
+At Mansfield, Louisiana, thirty-eight hours of observation in April
+and May resulted in flight densities that are surprisingly low--much
+lower, in fact, than at Oak Grove. In eleven of the hours of
+observation no birds at all were seen. A possible explanation for
+these low densities lies in the fact that eastern Texas and western
+Louisiana, where, probably, the Mansfield flights originated, is not
+an especially attractive region to migrants because of the great
+amount of deforested and second growth pine land. Oak Grove, in
+contrast, is in the great Tensas-Mississippi River flood plain,
+characterized by an almost solid stand of deciduous forest extending
+over thousands of square miles in the lower Mississippi valley.
+
+    [Illustration: FIG. 37. Sector density representation on
+       two nights at Rosedale, Mississippi, in 1948. The white lines
+       are the vector resultants.]
+
+In further contrast to the considerable flight densities and
+pronounced directional trend at Oak Grove, we have the results from
+Rosedale, Mississippi, only seventy miles to the north and slightly to
+the east. At Rosedale the densities were mediocre and the flight
+directions were extremely divergent. Many of the nights of observation
+at this locality were seriously interrupted by clouds, but such counts
+as were made on those dates indicated little migration taking place.
+On two nights, however, April 21-22 and May 20-21, visibility was
+almost continuous and densities were moderately high. In Figure 37 I
+have shown the flight directions for these two nights. The lengths of
+the individual sector vectors are plotted as a percentage of the total
+station density for each of the two nights (5,800 and 6,800 birds,
+respectively). Although the vector resultants show a net movement of
+birds to the northeast, there are important divergent components of
+the flights. This "round-the-compass" pattern is characteristic of
+stations on the edge of meteorological disturbances, as was Rosedale
+on April 21-22, but not on the night of May 20-21. If bats are
+presumed to have played a rôle in these latter observations, their
+random flights would tend to cancel out and the vector resultant
+would emerge as a graphic representation of the actual net trend
+density of the birds and its prevailing direction of flow. Although I
+do not believe that bats are the real reason for the diverse
+directional patterns at Rosedale, I can offer no alternative
+explanation consistent with data from other stations.
+
+Moving northward in the valley of the Mississippi and its tributaries,
+we find a number of stations that yielded significantly high densities
+on most nights when weather conditions were favorable for migration.
+Louisville and Murray, Kentucky, and Knoxville, Tennessee, each show
+several nights with many birds flying, but only Lawrence, Kansas, and
+Ottumwa, Iowa, had migrations that approach in magnitude the record
+station densities at Tampico. Indeed, these were the only two stations
+in the United States that produced flights exceeding the densities at
+Progreso, Yucatán. The densities at Lawrence are unique in one
+respect, in that they were extremely high in the month of March. Since
+there were very few stations in operation then, these high densities
+would be of little significance were it not for the fact that at no
+time in the course of this study from 1945 to the present have
+comparable densities been obtained this early in the migration period.
+Examination of the "Remarks" section of the original data sheets from
+Lawrence show frequent mention of "duck-like" birds passing before the
+moon. We may infer from these notations that a considerable part of
+the overhead flight was composed of ducks and other aquatic birds that
+normally leave the southern United States before the main body of
+transient species reach there. The heavy flight densities at Lawrence
+may likewise have contained certain Fringillidae, Motacillidae,
+Sylviidae, and other passerine birds that winter mainly in the
+southern United States and which are known to begin their return
+northward in March or even earlier. Observations in 1948 at Lawrence
+in April were hindered by clouds, and in May no studies were
+attempted. However, we do have at hand two excellent sets of data
+recorded at Lawrence on the nights of May 3-4 and May 5-6, 1947, when
+the density was also extremely high.
+
+At Ottumwa, Iowa, where a splendid cooperative effort on the part of
+the local ornithologists resulted in forty-four hours of observation
+in April and May, densities were near the maximum for all stations.
+Considering this fact along with results at Lawrence and other
+mid-western stations where cloud cover did not interfere at the
+critical periods of observation, we have here evidence supporting the
+generally held thesis that eastern Kansas, Missouri, and Iowa lie on a
+principal migratory flyway. Stations in Minnesota, Illinois,
+Michigan, Massachusetts, and Ontario were either operated for only
+parts of one or two nights, or else clouds seriously interfered with
+observations, resulting in discontinuous counts. It may be hoped that
+future studies will include an adequate representation of stations in
+these states and that observations will be extensive enough to permit
+conclusions regarding the density and direction of migration.
+
+Charleston, South Carolina, which does not conveniently fall in any of
+the geographic regions so far discussed, had, to me, a surprisingly
+low flight density; twenty-two hours of observation there in March,
+April, and May yielded a total flight density of only 3,000 birds.
+This is less, for example, than the number of birds computed to have
+passed Lawrence, Kansas, in one hour, or to have passed Progreso,
+Yucatán, in one twenty-minute interval! Possibly observations at
+Charleston merely chanced to fall on nights of inexplicably low
+densities; further observations will be required to clear up this
+uncertainty.
+
+
+E. MIGRATION AND METEOROLOGICAL CONDITIONS
+
+The belief that winds affect the migration of birds is an old one. The
+extent to which winds do so, and the precise manner in which they
+operate, have not until rather recently been the subject of real
+investigation. With modern advances in aerodynamics and the
+development of the pressure-pattern system of flying in aviation,
+attention of ornithologists has been directed anew to the part that
+air currents may play in the normal migrations of birds. In America, a
+brief article by Bagg (1948), correlating the observed abundance of
+migrants in New England with the pressure pattern obtaining at the
+time, has been supplemented by the unpublished work of Winnifred
+Smith. Also Landsberg (1948) has pointed out the close correspondence
+between the routes of certain long-distance migrants and prevailing
+wind trajectories. All of this is basis for the hypothesis that most
+birds travel along definite air currents, riding with the wind. Since
+the flow of the air moves clockwise around a high pressure area and
+counterclockwise around a low pressure area, the birds are directed
+away from the "high" and toward the center of the "low." The arrival
+of birds in a particular area can be predicted from a study of the
+surrounding meteorological conditions, and the evidence in support of
+the hypothesis rests mainly upon the success of these predictions in
+terms of observations in the field.
+
+From some points of view, this hypothesis is an attractive one. It
+explains how long distances involved in many migrations may be
+accomplished with a minimum of effort. But the ways in which winds
+affect migration need analysis on a broader scale than can be made
+from purely local vantage points. Studies of the problem must be
+implemented by data accumulated from a study of the process in action,
+not merely from evidence inferred from the visible results that follow
+it. Although several hundred stations operating simultaneously would
+surely yield more definite results, the telescopic observations in
+1948 offer a splendid opportunity to test the theory on a continental
+scale.
+
+The approach employed has been to plot on maps sector vectors and
+vector resultants that express the directional trends of migration in
+the eastern United States and the Gulf region, and to compare the data
+on these maps with data supplied by the U. S. Weather Bureau regarding
+the directions and velocities of the winds, the location of high and
+low pressure areas, the movement of cold and warm fronts, and the
+disposition of isobars or lines of equal pressure. It should be borne
+in mind when interpreting these vectors that they are intended to
+represent the directions of flight only at the proximal ends, or
+junction points, of the arrows. The tendency of the eye to follow a
+vector to its distal extremity should not be allowed to create the
+misapprehension that the actual flight is supposed to have continued
+on in a straight line to the map location occupied by the arrowhead.
+
+A fundamental difficulty in the pressure-pattern theory of migration
+has no doubt already suggested itself to the reader. The difficulty to
+which I refer is made clear by asking two questions. How can the birds
+ever get where they are going if they are dependent upon the whim of
+the winds? How can pressure-pattern flying be reconciled with the
+precision birds are supposed to show in returning year after year to
+the same nesting area? The answer is, in part, that, if the wind is a
+major controlling influence on the routes birds follow, there must be
+a rather stable pattern of air currents prevailing from year to year.
+Such a situation does in fact exist. There are maps showing wind roses
+at 750 and 1,500 meters above mean sea level during April and May
+(Stevens, 1933, figs. 13-14, 17-18). Similarly, the "Airway
+Meteorological Atlas for the United States" (Anonymous, 1941) gives
+surface wind roses for April (Chart 6) and upper wind roses at 500 and
+1,000 meters above mean sea level for the combined months of March,
+April, and May (Charts 81 and 82). The same publication shows wind
+resultants at 500 and 1,000 meters above mean sea level (Charts 108
+and 109). Further information permitting a description in general
+terms of conditions prevailing in April and May is found in the
+"Monthly Weather Review" covering these months (_cf._ Anonymous,
+1948 _a_, Charts 6 and 8; 1948 _b_, Charts 6 and 8).
+
+    [Illustration: FIG. 38. Over-all sector vectors at major
+       stations in the spring 1948. See text for explanation of
+       system used in determining the length of vectors. For
+       identification of stations, see Figure 34.]
+
+    [Illustration: FIG. 39. Over-all net trend of flight
+       directions at stations shown in Figure 38. The arrows
+       indicate direction only and their slants were obtained by
+       vector analysis of the over-all sector densities.]
+
+First, however, it is helpful as a starting point to consider the
+over-all picture created by the flight trends computed from this
+study. In Figure 38, the individual sector vectors are mapped for the
+season for all stations with sufficient data. The length of each
+sector vector is determined as follows: the over-all seasonal density
+for the station is regarded as 100 percent, and the total for the
+season of the densities in each individual sector is then expressed as
+a percentage. The results show the directional spread at each station.
+In Figure 39, the direction of the over-all vector resultant, obtained
+from the sector vectors on the preceding map, is plotted to show the
+net trend at each station.
+
+As is evident from the latter figure, the direction of the net trend
+at Progreso, Yucatán, is decidedly west of north (N 26° W). At Tampico
+this trend is west of north (N 11° W), but not nearly so much so as at
+Progreso. In Texas, Louisiana, Georgia, Tennessee, and Kentucky, it is
+decidedly east of north. In the upper Mississippi Valley and in the
+eastern part of the Great Plains, the flow appears to be northward or
+slightly west of north. At Winter Park, Florida, migration follows in
+general the slant of the Florida Peninsula, but, the meager data from
+Thomasville, Georgia, do not indicate a continuation of this trend.
+
+It might appear, on the basis of the foregoing data, that birds
+migrate along or parallel to the southeast-northwest extension of the
+land masses of Central America and southern Mexico. This would carry
+many of them west of the meridian of their ultimate goal, obliging
+them to turn back eastward along the lines of net trend in the Gulf
+states and beyond. This curved trajectory is undoubtedly one of the
+factors--but certainly not the only factor--contributing to the effect
+known as the "coastal hiatus." The question arises as to whether this
+northwestward trend in the southern part of the hemisphere is a
+consequence of birds following the land masses or whether instead it
+is the result of some other natural cause such as a response to
+prevailing winds. I am inclined to the opinion that both factors are
+important. Facts pertinent to this opinion are given below.
+
+In April and May a high pressure area prevails over the region of the
+Gulf of Mexico. As the season progresses, fewer and fewer cold-front
+storms reach the Gulf area, and as a result the high pressure area
+over the Gulf is more stable. Since the winds move clockwise around a
+"high," this gives a general northwesterly trajectory to the air
+currents in the vicinity of the Yucatán Peninsula. In the western area
+of the Gulf, the movement of the air mass is in general only slightly
+west of north, but in the central Gulf states and lower Mississippi
+Valley the trend is on the average northeasterly. In the eastern part
+of the Great Plains, however, the average circulation veers again
+slightly west of north. The over-all vector resultants of bird
+migration at stations in 1948, as mapped in Figure 39, correspond
+closely to this general pattern.
+
+Meteorological data are available for drawing a visual comparison
+between the weather pattern and the fight pattern on individual
+nights. I have plotted the directional results of four nights of
+observation on the Daily Weather Maps for those dates, showing surface
+conditions (Figures 40, 42, 44 and 46). Each sector vector is drawn in
+proportion to its percentage of the corresponding nightly station
+density; hence the vectors at each station are on an independent
+scale. The vector resultants, distinguished by the large arrowheads,
+are all assigned the same length, but the nightly and average hourly
+station densities are tabulated in the legends under each figure. For
+each map showing the directions of flight, there is on the facing page
+another map showing the directions of winds aloft at 2,000 and 4,000
+feet above mean sea level on the same date (see Figures 41-47). The
+maps of the wind direction show also the velocities.
+
+Unfortunately, since there is no way of analyzing the sector trends in
+terms of the elevations of the birds involved, we have no certain way
+of deciding whether to compare a given trend with the winds at 2,000,
+1,000, or 0 feet. Nor do we know exactly what wind corresponds to the
+average or median flight level, which would otherwise be a good
+altitude at which to study the net trend or vector resultant.
+Furthermore, the Daily Weather Map illustrates conditions that
+obtained at 12:30 A. M. (CST); the winds aloft are based on
+observations made at 10:00 P. M. (CST); and the data on birds covers
+in most cases the better part of the whole night. Add to all this the
+fact that the flight vectors, their resultants, and the wind
+representations themselves are all approximations, and it becomes
+apparent that only the roughest sort of correlations are to be
+expected.
+
+However, as will be seen from a study of the accompanying maps
+(Figures 40-47), the shifts in wind direction from the surface up to
+4,000 feet above sea level are not pronounced in most of the
+instances at issue, and such variations as do occur are usually in a
+clockwise direction. All in all, except for regions where frontal
+activity is occurring, the weather maps give a workable approximation
+to the average meteorological conditions on a given night.
+
+The maps (Figures 40-47) permit, first, study of the number of
+instances in which the main trend of flight, as shown by the vector
+resultant, parallels the direction of wind at a reasonable potential
+mean flight elevation, and, second, comparison of the larger
+individual sector vectors and the wind currents at any elevation below
+the tenable flight ceiling--one mile.
+
+On the whole, inspection of the trend of bird-flight and wind
+direction on specific nights supports the principle that the flow of
+migration is in general coincident with the flow of air. It might be
+argued that when the flow of air is toward the north, and when birds
+in spring are proceeding normally in that direction, no significance
+can be attached to the agreement of the two trends. However, the same
+coincidence of wind directions and bird flights seems to be maintained
+when the wind currents deviate markedly from a northward trajectory.
+Figures 46 and 47, particularly in regard to the unusual slants of the
+flight vectors at Ottumwa, Knoxville, and Memphis, illustrate that
+this coincidence holds even when the wind is proceeding obliquely
+eastward or westward. On the night of May 22-23, when a high pressure
+area prevailed from southern Iowa to the Atlantic coast, and the
+trajectory of the winds was northward, migration activity at Knoxville
+and Ottumwa was greatly increased and the flow of birds was again
+northward in the normal seasonal direction of migration.
+
+Further study of the data shows fairly conclusively that maximum
+migration activity occurs in the regions of high barometric pressure
+and that the volume of migration is either low or negligible in
+regions of low pressure. The passage of a cold-front storm may almost
+halt migration in spring. This was demonstrated first to me by the
+telescopic method at Baton Rouge, on April 12, 1946, following a
+strong cold front that pushed southeastward across the Gulf coastal
+plain and over the eastern Gulf of Mexico. The winds, as usual,
+shifted and became strong northerly. On this night, following the
+shift of the wind, only three birds were seen in seven hours of
+continuous observation. Three nights later, however, on April 15, when
+the warm air of the Gulf was again flowing from the south, I saw 104
+birds through the telescope in two hours. Apropos of this
+consideration in the 1948 data are the nights of May 21-22 and 22-23.
+
+    [Illustration: FIG. 40. Comparison of flight trends and
+       surface weather conditions on April 22-23, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on April 23. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          5. Louisville: 9,100 (1,100)
+          6. Murray: 16,300 (2,700)
+          8. Stillwater: 1,900 (500)
+          9. Knoxville: 15,200 (1,700)
+         13. Oak Grove: 13,600 (1,700)
+         16. College Station: 13,300 (1,900)
+         17. Baton Rouge: 6,200 (1,000)
+         19. Lafayette: 2,800 (600)
+         21. Winter Park: 6,200 (700)
+         23. Tampico: 11,100 (3,700)]
+
+    [Illustration: FIG. 41. Winds aloft at 10:00 P. M. on
+       April 22 (CST). Winds at 2,000 feet above mean sea level are
+       shown in black; those at 4,000 feet, in white. Velocities are
+       indicated by standard Beaufort Scale of Wind Force. The
+       numbers in circles refer to the stations shown in Figure 40.]
+
+    Correction: Figures 41 and 45 were inadvertently transposed.
+
+    [Illustration: FIG. 42. Comparison of flight trends and
+       surface weather conditions on April 23-24, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on April 24. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          1. Albion: 1,100 (300)
+          2. Ottumwa: 5,500 (900)
+          4. Lawrence: 5,400 (1,400)
+          5. Louisville: 13,300 (2,700)
+          6. Murray: 9,800 (1,400)
+          8. Stillwater: 800 (100)
+          9. Knoxville: 8,000 (900)
+         10. Memphis: 7,900 (1,000)
+         14. Mansfield: 4,900 (1,200)
+         16. College Station: 700 (100)
+         17. Baton Rouge: 1,700 (400)
+         18. Pensacola: migration negligible
+         20. New Orleans: 1,600 (800)
+         21. Winter Park: 2,700 (300)
+         23. Tampico: 63,600 (6,300)
+         24. Progreso: 31,300 (3,900)]
+
+    [Illustration: FIG. 43. Winds aloft at 10:00 P. M. on
+       April 23 (CST). Winds at 2,000 feet above mean sea level are
+       shown in black; those at 4,000 feet, in white. Velocities are
+       indicated by standard Beaufort Scale of Wind Force. The
+       numbers in circles refer to the stations shown in Figure 42.]
+
+    [Illustration: FIG. 44. Comparison of flight trends and
+       surface weather conditions on April 24-25, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on April 25. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          1. Albion: migration negligible
+          2. Ottumwa: 4,600 (1,500)
+          3. Columbia: 1,400 (400)
+          5. Louisville: 1,700 (200)
+         10. Memphis: 6,600 (900)
+         12. Rosedale: 1,100 (100)
+         14. Mansfield: 1,700 (400)
+         18. Pensacola: migration negligible
+         21. Winter Park: 600 (100)
+         24. Progreso: 27,300 (3,000)]
+
+    [Illustration: FIG. 45. Winds aloft at 10:00 P. M. on
+       April 24 (CST). Winds at 2,000 feet above mean sea level are
+       shown in black; those at 4,000 feet, in white. Velocities are
+       indicated by standard Beaufort Scale of Wind Force. The
+       numbers in circles refer to the stations shown in Figure 44.]
+
+    Correction: Figures 41 and 45 were inadvertently transposed.
+
+    [Illustration: FIG. 46. Comparison of flight trends and
+       surface weather conditions on May 21-22, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on May 22. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          2. Ottumwa: 6,900 (1,400)
+          5. Louisville: 1,500 (200)
+          9. Knoxville: 3,200 (500)
+         10. Memphis: 7,000 (1,200)
+         13. Oak Grove: 5,800 (800)
+         14. Mansfield: 2,500 (800)
+         18. Pensacola: migration negligible
+         21. Winter Park: 1,200 (200)]
+
+    [Illustration: FIG. 47. Winds aloft at 10:00 P. M. on May
+       21 (CST). Winds at 2,000 feet above mean sea level are shown.
+       Velocities are indicated by standard Beaufort Scale of Wind
+       Force. The numbers in circles refer to the stations shown in
+       Figure 46.]
+
+On the first night, following the passage of a cold front, migration
+at Ottumwa was comparatively low (6,900 birds in five hours). On the
+following night, when the trajectory of the winds was toward the
+north, the volume of migration was roughly twice as high (22,300 birds
+in eight hours). At Louisville, on May 21-22, the nightly station
+density was only 1,500 birds in seven hours, whereas on the following
+night, it was 8,400 birds in the same length of time, or about six
+times greater.
+
+The evidence adduced from the present study gives support to the
+hypothesis that the continental pattern of spring migration in eastern
+North America is regulated by the movement of air masses. The
+clockwise circulation of warm air around an area of high pressure
+provides, on its western edge, tail winds which are apparently
+favorable to northward migration. High pressure areas exhibit a
+centrifugal force outward from the center, which may tend to disperse
+the migratory flight originating at any given point. In contrast, the
+circulation of air in the vicinity of a low pressure area is
+counterclockwise with the force tending to be directed inward toward
+the center. Since the general movement of the air is from the high
+pressure area toward a low pressure area, birds starting their
+migrations with favorable tail winds, are often ultimately carried to
+a region where conditions are decidedly less favorable. In the
+vicinity of an area of low pressure the greater turbulence and high
+wind velocities, combined with the possibly slightly less buoyant
+property of the air, cause birds to descend. Since low pressure areas
+in spring generally precede cold fronts, with an attending shift of
+the wind to the north, an additional barrier to the northward
+migration of birds is imposed. The extreme manifestation of low
+pressure conditions and the manner in which they operate against bird
+flight, are associated with tropical hurricanes. There, the
+centripetal force of the wind is so great that it appears to draw
+birds into the "eye" of the hurricane. A classic example of this
+effect is seen in the case of the birds that came aboard the "West
+Quechee" when this vessel passed through the "eye" of a hurricane in
+the Gulf of Mexico in August, 1927. I have already discussed the
+details of this incident in a previous paper (1946:192). There is also
+the interesting observation of Mayhew (1949), in which a similar
+observation was made of large numbers of birds aboard a ship passing
+through one of these intense low-pressure areas.
+
+Although the forces associated with an ordinary low-pressure area are
+by no means as intense as those associated with a tropical hurricane,
+the forces operating are much the same. Consequently birds conceivably
+might tend to be drawn toward a focal point near the center of the
+low, where the other factors already mentioned would tend to
+precipitate the entire overhead flight. Visible evidence of migration
+would then manifest itself to the field ornithologists.
+
+
+
+
+CONCLUSIONS
+
+
+  1. Telescopic counts of birds passing before the moon may be used
+     to determine reliable statistical expressions of the volume of
+     migration in terms of direction and of definite units of time
+     and space.
+
+  2. Night migrants fly singly more often than in flocks, creating a
+     remarkably uniform dispersion on a local scale throughout the
+     sky, quite unlike the scattered distributions observable in the
+     daytime.
+
+  3. The nocturnal migration of birds is apparently preceded by a
+     resting or feeding pause during which there are few migrants in
+     the air. It is not to an important degree a non-stop continuation
+     of flights begun in the daylight.
+
+  4. Nightly migrational activity in North America varies from hour to
+     hour according to a definite temporal pattern, corresponding to
+     the _Zugunruhe_ of caged European birds, and expressed by
+     increasingly heavy flights up until the hour before midnight,
+     followed by a pronounced decline.
+
+  5. The visible effects of the time pattern are subject to
+     modification at a particular station by its location with respect
+     to the resting areas from which the night's flight originates.
+
+  6. Quantitative and directional studies have so far failed to prove
+     that nocturnal migrants favor narrow, topographically-determined
+     flight lanes to an important degree.
+
+  7. Flight densities on the east coast of Mexico, though of first
+     magnitude, have not yet been demonstrated in the volume demanded
+     by the premise that almost all migrants returning to the
+     United States from regions to the south do so by coastal routes.
+
+  8. Heavy flights have been recorded from the northern coast of
+     Yucatán under circumstances leading inevitably to the conclusion
+     that birds migrate across the Gulf of Mexico in considerable
+     numbers.
+
+  9. There is reason to believe that the importance of the Florida
+     Peninsula as an April and May flyway has been over-estimated,
+     as regards the numbers of birds using it in comparison with the
+     numbers of birds using the Mexican and Gulf routes.
+
+ 10. The amount of migration is apparently seldom sufficient to produce
+     heavy densities of transient species on the ground without
+     the operation of concentrative factors such as ecological patterns
+     and meteorological forces.
+
+ 11. The absence or scarcity of transients in some areas in fine
+     weather may be explained by this consideration.
+
+ 12. A striking correlation exists between air currents and the
+     directional flight trends of birds, suggesting that most night
+     migrants travel by a system of pressure-pattern flying.
+
+
+
+LITERATURE CITED
+
+
+  ALLEN, R. P., AND R. T. PETERSON
+
+    1936.    The hawk migrations at Cape May Point, New Jersey. Auk,
+             53:393-404.
+
+
+  ANONYMOUS
+    1936-1941. Tables of computed altitude and azimuth. U. S. Navy
+             Department Hydrographic Office. U. S. Govt. Printing
+             Office, Washington, D. C., vols. 3-5.
+
+    1941.    Airway meteorological atlas for the United States.
+             Weather Bureau Publ. 1314. U. S. Dept. Commerce,
+             Washington, D. C.
+
+    1945-1948. The American air almanac. U. S. Naval Observatory.
+             U. S. Govt. Printing Office, Washington, D. C., 3 vols.,
+             issued annually.
+
+    1948_a_. Meteorological and climatological data for April 1948.
+             Monthly Weather Review, April 1948, 76:65-84, 10 charts.
+
+    1948_b_. Meteorological and climatological data for May 1948.
+             Monthly Weather Review, May 1948, 76:85-103, 11 charts.
+
+
+  BAGG, A. M.
+
+    1948.    Barometric pressure-patterns and spring migration.
+             Auk, 65:147.
+
+
+  BERGMAN, G.
+
+    1941.    Der Fruhlingszug von _Clangula hyemalis_ (L.) und
+             _Oidemia nigra_ (L.) bei Helsingfors. Eine Studie über
+             Zugverlauf und Witterung sowie Tagesrhythmus und Flughöhe.
+             Ornis Fennica, 18:1-26.
+
+
+  BRAY, R. A.
+
+    1895.    A remarkable flight of birds. Nature (London), 52:415.
+
+
+  CARPENTER, F. W.
+
+    1906.    An astronomical determination of the height of birds
+             during nocturnal migration. Auk, 23:210-217.
+
+
+  CHAPMAN, F. M.
+
+    1888.    Observations on the nocturnal migration of birds.
+             Auk, 5:37-39.
+
+
+  DAVIS, L. I.
+
+    1936-1940. The season: lower Rio Grande Valley region. Bird-Lore
+             (now  Audubon Mag.), 38-42.
+
+
+  F. [ARNER], D. [ONALD] S.
+
+    1947.    Studies on daily rhythm of caged migrant birds (review of
+             Palmgren article). Bird-Banding, 18:83-84.
+
+
+  GATES, W. H.
+
+    1933.    Hailstone damage to birds. Science, 78:263-264.
+
+
+  HOWELL, A. H.
+
+    1932.    Florida bird life. Florida Department Game and Fresh Water
+             Fish, Tallahassee, 1-579 + 14 pp., 58 pls., 72 text figs.
+
+
+  LANSBERG, H.
+
+    1948.    Bird migration and pressure patterns. Science, 108:708-709.
+
+
+  LIBBY, O. G.
+
+    1899.    The nocturnal flight of migratory birds. Auk, 16:140-146.
+
+
+  LOWERY, G. H., JR.
+
+    1945.    Trans-Gulf spring migration of birds and the coastal
+             hiatus. Wilson Bull., 57:92-121.
+
+    1946.    Evidence of trans-Gulf migration. Auk, 63:175-211.
+
+
+  MAYHEW, D. F.
+
+    1949.    Atmospheric pressure and bird flight. Science, 109:403.
+
+
+  OVERING, R.
+
+    1938.    High mortality at the Washington Monument. Auk, 55:679.
+
+
+  PALMGREN, P.
+
+    1944.    Studien über die Tagesrhythmik gekäfigter Zugvögel.
+             Zeitschrift für Tierpsychologie, 6:44-86.
+
+
+  POUGH, R. H.
+
+    1948.    Out of the night sky. Audubon Mag., 50:354-355.
+
+
+  PUTKONEN, T. A.
+
+    1942.    Kevätmuutosta Viipurinlakdella. Ornis Fennica, 19:33-44.
+
+
+  RENSE, W. A.
+
+    1946.    Astronomy and ornithology. Popular Astronomy, 54:55-73.
+
+
+  SCOTT, W. E. D.
+
+    1881_a._ Some observations on the migration of birds. Bull. Nuttall
+             Orni. Club, 6:97-100.
+
+    1881_b._ Migration of birds at night. Bull. Nuttall Orni. Club,
+             6:188.
+
+
+  SIIVONEN, L.
+
+    1936.    Die Stärkevariation des Nächtlichen Zuges bei _Turdus ph.
+             philomelos_ Brehn und _T. musicus_ L. auf Grund der
+             Zuglaute geschätz und mit der Zugunruhe einer gekäfigten
+             Singdrossel Verglichen. Ornis Fennica, 13:59-63.
+
+
+  SPOFFORD, W. R.
+
+    1949.    Mortality of birds at the ceilometer of the Nashville
+             airport. Wilson Bull., 61:86-90.
+
+
+  STEBBINS, J.
+
+    1906.    A method of determining height of migrating birds.
+             Popular Astronomy, 14:65-70.
+
+
+  STEVENS, LLOYD A.
+
+    1933.    Upper-air wind roses and resultant winds for the eastern
+             United States. Monthly Weather Review, Supplement No. 35,
+             November 13, pp. 1-3, 65 figs.
+
+
+  STONE, W.
+
+    1906.    Some light on night migration. Auk, 23:249-252.
+
+    1937.    Bird studies at Old Cape May. Delaware Valley Orni. Club,
+             Philadelphia, Vol. 1, 1-520 + 15 pp., pls. 1-46 and frontis.
+
+
+  THOMSON, A. L.
+
+    1926.    Problems of bird migration. Houghton Mifflin Company,
+             Boston.
+
+
+  VAN OORDT, G.
+
+    1943.    Vogeltrek. E. J. Brill, Leiden, xii + 145 pp.
+
+
+  VERY, F. W.
+
+    1897.    Observations of the passage of migrating birds across the
+             lunar disc on the nights of September 23 and 24, 1896.
+             Science, 6:409-411.
+
+
+  WALTERS, W.
+
+    1927.    Migration and the telescope. Emu, 26:220-222.
+
+
+  WEST, R. H.
+
+    1896.    Flight of birds across the moon's disc. Nature (London),
+             53:131.
+
+
+  WILLIAMS, G. G.
+
+    1941-1948. The season: Texas coastal region. Audubon Mag., 43-50.
+
+    1945.    Do birds cross the Gulf of Mexico in spring? Auk,
+             62:98-111.
+
+    1947.    Lowery on trans-Gulf migration. Auk, 64:217-238.
+
+
+  WINKENWERDER, H. A.
+
+    1902_a_. The migration of birds with special reference to nocturnal
+             flight. Bull. Wisconsin Nat. Hist. Soc., 2:177-263.
+
+    1902_b_. Some recent observations on the migration of birds. Bull.
+             Wisconsin Nat. Hist. Soc., 2:97-107.
+
+
+  Transmitted June 1, 1949.
+
+
+
+    []
+  23-1020
+
+
+
+
+UNIVERSITY OF KANSAS PUBLICATIONS
+
+
+The University of Kansas Publications, Museum of Natural History, are
+offered in exchange for the publications of learned societies and
+institutions, universities and libraries. For exchanges and
+information, address the Exchange Desk, University of Kansas Library,
+Lawrence, Kansas, U. S. A.
+
+MUSEUM OF NATURAL HISTORY.--E. Raymond Hall, Chairman, Editorial
+Committee.
+
+This series contains contributions from the Museum of Natural History.
+Cited as Univ. Kans. Publ., Mus. Nat. Hist.
+
+  Vol. 1.  (Complete) Nos. 1-26.  Pp. 1-638. August 15, 1946-January 20,
+           1951.
+
+  Vol. 2.  (Complete)  Mammals of Washington. By Walter W. Dalquest.
+           Pp. 1-444, 140 figures in text. April 9, 1948.
+
+  Vol. 3.  1.  The avifauna of Micronesia, its origin, evolution, and
+               distribution. By Rollin H. Baker. Pp. 1-359, 16 figures
+               in text. June 12, 1951.
+
+           2.  A quantitative study of the nocturnal migration of birds.
+               By George H. Lowery, Jr. Pp. 361-472, 47 figures in text.
+               June 29, 1951.
+
+
+
+
+
+  Transcriber's Notes
+
+  With the exception of the typographical corrections detailed below
+  and some minor corrections for missing periods or extra punctuation
+  (item 28 in List of Figures), the text presented here is that
+  contained in the original printed version. A transcription of the
+  Data presented in Figure 12 was added to illustrate the information
+  contained on that sheet. Some text was moved to rejoin paragraphs.
+  The list of UK publications was moved to the end of the document.
+
+  In writing variables for formulae, superscripted characters are
+  shown using a caret (^). So, X squared would be X^2. Subscripts are
+  shown using an underscore. Carbon dioxide is CO_2. Where several
+  superscript or subscript character(s) are required or to aid in
+  clarity, they are placed in braces (ex., H_{2}O for water and
+  [theta]_{Npt.} for theta degrees from the North point).
+
+  Emphasis Notation
+
+      _Text_  = Italics
+
+  Typographical Corrections
+
+  Page   Correction
+
+  385   flght => flight
+  394   diargrams => diagrams
+  404   Determinaton => Determination
+  411   obsever => observer
+  419   Morover => Moreover
+  425   Mississippii => Mississippi
+  425   a => as
+  430   at => and
+  431   inserted "a"
+         ("...traveling along a certain topographic feature...")
+  442   concensus => consensus
+  472   Stephens, Loyd A. => Stevens, Lloyd A.
+
+
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of A Quantitative Study of the Nocturnal
+Migration of Birds., by George H. Lowery.
+
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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
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+    <title>
+      The Project Gutenberg eBook of A Quantitative Study of the Nocturnal Migration of Birds, by George H. Lowery, Jr..
+    </title>
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+
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+  p                {text-align: justify; text-indent: 1.5em;}
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+  .bt              {border-top: solid #000 1px;}
+  .pagenum         {position: absolute; left: 92%; text-indent:0; font-size: 0.75em; text-align: right; color: #b0b0b0;}
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+  .caption1        {font-weight: bold; font-size:2.00em; text-align: center;}
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+  ins              {text-decoration:none;  border-bottom: thin dotted #D0D0F0; background-color: #e0ffe0;}
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+<pre>
+
+The Project Gutenberg EBook of A Quantitative Study of the Nocturnal
+Migration of Birds., by George H. Lowery.
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Quantitative Study of the Nocturnal Migration of Birds.
+       Vol.3 No.2
+
+Author: George H. Lowery.
+
+Editor: E. Raymond Hall
+
+Release Date: October 31, 2011 [EBook #37894]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A QUANTITATIVE STUDY OF THE ***
+
+
+
+
+Produced by Chris Curnow, Tom Cosmas, Joseph Cooper, The
+Internet Archive for some images and the Online Distributed
+Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<div class="book"><!-- Begin Book -->
+<p><span class="pagenum"><a name="Cover" id="Cover">[Cover]</a></span></p>
+<div class="center">
+<img src="images/cover.jpg" width="272" height="452" alt="" title="" />
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_361" id="Page_361">[Pg_361]</a></span></p>
+
+<div class="center">
+<div class="caption1">A Quantitative Study of the Nocturnal<br />
+Migration of Birds</div>
+<br />
+<div class="caption3">BY</div>
+<br />
+<div class="caption2">GEORGE H. LOWERY, JR.</div>
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<div class="caption2">
+University of Kansas Publications<br />
+Museum of Natural History<br />
+</div>
+<br />Volume 3, No. 2, pp. 361-472, 47 figures in text<br />
+June 29, 1951<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+University of Kansas<br />
+LAWRENCE<br />
+1951<br />
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_362" id="Page_362">[Pg_362]</a></span></p>
+
+<div class="center">
+UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY<br />
+<br />
+Editors: E. Raymond Hall, Chairman; A. Byron Leonard,<br />
+Edward H. Taylor, Robert W. Wilson<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+UNIVERSITY OF KANSAS<br />
+Lawrence, Kansas<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+<br />
+
+PRINTED BY<br />
+FERD VOILAND, JR., STATE PRINTER<br />
+TOPEKA, KANSAS<br />
+1951<br />
+<br />
+<img src="images/union_label.png" width="71" height="26" alt="Look for the Union Label" title="Look for the Union Label" /><br />
+<br />
+23-1020<br />
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_363" id="Page_363">[Pg_363]</a></span></p>
+<br />
+<br />
+<br />
+<br />
+
+<div class="center">
+<div class="caption1">A Quantitative Study of the Nocturnal Migration of Birds</div>
+<br />
+<div class="caption3">By</div>
+<br />
+<div class="caption2">GEORGE H. LOWERY, JR.<br />
+</div>
+</div>
+<br />
+<br />
+<div class="caption2"><a name="CONTENTS" id="CONTENTS"></a>CONTENTS</div>
+<br />
+<div class="center">
+<table width="100%" cellpadding="4" cellspacing="0" summary="ToC">
+<tr>
+  <td align="left">&nbsp;</td>
+  <td class="text_rt">Page</td>
+</tr>
+<tr>
+  <td class="text_lf smcap"><a href="#Introduction">Introduction</a></td>
+  <td class="text_rt">365</td>
+</tr>
+<tr>
+  <td class="text_lf smcap"><a href="#Acknowledgments">Acknowledgments</a></td>
+  <td class="text_rt">367</td>
+</tr>
+<tr>
+  <td class="text_lf smcap"><a href="#Flight_Densities_and_Their_Determination">Part i. Flight Densities and Their Determination</a></td>
+  <td class="text_rt">370</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Lunar_Observations_of_Birds">Lunar Observations of Birds and the Flight Density Concept</a></td>
+  <td class="text_rt">370</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Observational_Procedure">Observational Procedure and the Processing of Data</a></td>
+  <td class="text_rt">390</td>
+</tr>
+<tr>
+  <td class="text_lf smcap"><a href="#Nature_of_Nocturnal_Migration">Part ii. The Nature of Nocturnal Migration</a></td>
+  <td class="text_rt">408</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Horizontal_Distribution_of_Birds">Horizontal Distribution of Birds on Narrow Fronts</a></td>
+  <td class="text_rt">409</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Density_as_a_Function">Density as a Function of the Hour of the Night</a></td>
+  <td class="text_rt">413</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Migration_in_Relation_to_Topography">Migration in Relation to Topography</a></td>
+  <td class="text_rt">424</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Geographical_Factors">Geographical Factors and the Continental Density Pattern</a></td>
+  <td class="text_rt">432</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;<a href="#Migration_and_Meteorological_Conditions">Migration and Meteorological Conditions</a></td>
+  <td class="text_rt">453</td>
+</tr>
+<tr>
+  <td class="text_lf smcap"><a href="#Conclusions">Conclusions</a></td>
+  <td class="text_rt">469</td>
+</tr>
+<tr>
+  <td class="text_lf smcap"><a href="#Literature_Cited">Literature Cited</a></td>
+  <td class="text_rt">470</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_364" id="Page_364">[Pg_364]</a></span></p>
+
+<div class="caption2">LIST OF FIGURES</div>
+<br />
+<div class="center">
+<table width="100%" cellpadding="4" cellspacing="0" summary="List of Figures">
+<tr>
+  <td class="text_rt">&nbsp;</td>
+  <td class="text_lf smcap">Figure</td>
+  <td class="text_rt smcap">page</td>
+</tr>
+<tr>
+  <td class="text_rt"> 1</td>
+  <td class="text_lf"><a href="#Fig_1">The field of observation as it appears to the observer</a></td>
+  <td class="text_rt">374</td>
+</tr>
+<tr>
+  <td class="text_rt"> 2</td>
+  <td class="text_lf"><a href="#Fig_2">Determination of diameter of cone at any point</a></td>
+  <td class="text_rt">375</td>
+</tr>
+<tr>
+  <td class="text_rt"> 3</td>
+  <td class="text_lf"><a href="#Fig_3">Temporal change in size of the field of observation</a></td>
+  <td class="text_rt">376</td>
+</tr>
+<tr>
+  <td class="text_rt"> 4</td>
+  <td class="text_lf"><a href="#Fig_4">Migration at Ottumwa, Iowa</a></td>
+  <td class="text_rt">377</td>
+</tr>
+<tr>
+  <td class="text_rt"> 5</td>
+  <td class="text_lf"><a href="#Fig_5">Geographic variation in size of cone of observation</a></td>
+  <td class="text_rt">378</td>
+</tr>
+<tr>
+  <td class="text_rt"> 6</td>
+  <td class="text_lf"><a href="#Fig_6">The problem of sampling migrating birds</a></td>
+  <td class="text_rt">380</td>
+</tr>
+<tr>
+  <td class="text_rt"> 7</td>
+  <td class="text_lf"><a href="#Fig_7">The sampling effect of a square</a></td>
+  <td class="text_rt">381</td>
+</tr>
+<tr>
+  <td class="text_rt"> 8</td>
+  <td class="text_lf"><a href="#Fig_8">Rectangular samples of square areas</a></td>
+  <td class="text_rt">382</td>
+</tr>
+<tr>
+  <td class="text_rt"> 9</td>
+  <td class="text_lf"><a href="#Fig_9">The effect of vertical components in bird flight</a></td>
+  <td class="text_rt">383</td>
+</tr>
+<tr>
+  <td class="text_rt">10</td>
+  <td class="text_lf"><a href="#Fig_10">The interceptory potential of slanting lines</a></td>
+  <td class="text_rt">384</td>
+</tr>
+<tr>
+  <td class="text_rt">11</td>
+  <td class="text_lf"><a href="#Fig_11">Theoretical possibilities of vertical distribution</a></td>
+  <td class="text_rt">388</td>
+</tr>
+<tr>
+  <td class="text_rt">12</td>
+  <td class="text_lf"><a href="#Fig_12">Facsimile of form used to record data in the field</a></td>
+  <td class="text_rt">391</td>
+</tr>
+<tr>
+  <td class="text_rt">13</td>
+  <td class="text_lf"><a href="#Fig_13">The identification of co-ordinates</a></td>
+  <td class="text_rt">392</td>
+</tr>
+<tr>
+  <td class="text_rt">14</td>
+  <td class="text_lf"><a href="#Fig_14">The apparent pathways of birds seen in one hour</a></td>
+  <td class="text_rt">393</td>
+</tr>
+<tr>
+  <td class="text_rt">15</td>
+  <td class="text_lf"><a href="#Fig_15">Standard form for plotting the apparent paths of flight</a></td>
+  <td class="text_rt">395</td>
+</tr>
+<tr>
+  <td class="text_rt">16</td>
+  <td class="text_lf"><a href="#Fig_16">Standard sectors for designating flight trends</a></td>
+  <td class="text_rt">398</td>
+</tr>
+<tr>
+  <td class="text_rt">17</td>
+  <td class="text_lf"><a href="#Fig_17">The meaning of symbols used in the direction formula</a></td>
+  <td class="text_rt">399</td>
+</tr>
+<tr>
+  <td class="text_rt">18</td>
+  <td class="text_lf"><a href="#Fig_18">Form used to compute zenith distance and azimuth of the moon</a></td>
+  <td class="text_rt">400</td>
+</tr>
+<tr>
+  <td class="text_rt">19</td>
+  <td class="text_lf"><a href="#Fig_19">Plotting sector boundaries on diagrammatic plots</a></td>
+  <td class="text_rt">402</td>
+</tr>
+<tr>
+  <td class="text_rt">20</td>
+  <td class="text_lf"><a href="#Fig_20">Form to compute sector densities</a></td>
+  <td class="text_rt">403</td>
+</tr>
+<tr>
+  <td class="text_rt">21</td>
+  <td class="text_lf"><a href="#Fig_21">Determination of the angle &#945;</a><!-- Greek: alpha --></td>
+  <td class="text_rt">404</td>
+</tr>
+<tr>
+  <td class="text_rt">22</td>
+  <td class="text_lf"><a href="#Fig_22">Facsimile of form summarizing sector densities</a></td>
+  <td class="text_rt">405</td>
+</tr>
+<tr>
+  <td class="text_rt">23</td>
+  <td class="text_lf"><a href="#Fig_23">Determination of net trend density</a></td>
+  <td class="text_rt">406</td>
+</tr>
+<tr>
+  <td class="text_rt">24</td>
+  <td class="text_lf"><a href="#Fig_24">Nightly station density curve at Progreso, Yucatán</a></td>
+  <td class="text_rt">407</td>
+</tr>
+<tr>
+  <td class="text_rt">25</td>
+  <td class="text_lf"><a href="#Fig_25">Positions of the cone of observation at Tampico, Tamps</a></td>
+  <td class="text_rt">411</td>
+</tr>
+<tr>
+  <td class="text_rt">26</td>
+  <td class="text_lf"><a href="#Fig_26">Average hourly station densities in spring of 1948</a></td>
+  <td class="text_rt">414</td>
+</tr>
+<tr>
+  <td class="text_rt">27</td>
+  <td class="text_lf"><a href="#Fig_27">Hourly station densities plotted as a percentage of peak</a></td>
+  <td class="text_rt">415</td>
+</tr>
+<tr>
+  <td class="text_rt">28</td>
+  <td class="text_lf"><a href="#Fig_28">Incidence of maximum peak at the various hours of the night in 1948</a></td>
+  <td class="text_rt">416</td>
+</tr>
+<tr>
+  <td class="text_rt">29</td>
+  <td class="text_lf"><a href="#Fig_29">Various types of density-time curves</a></td>
+  <td class="text_rt">418</td>
+</tr>
+<tr>
+  <td class="text_rt">30</td>
+  <td class="text_lf"><a href="#Fig_30">Density-time curves on various nights at Baton Rouge</a></td>
+  <td class="text_rt">422</td>
+</tr>
+<tr>
+  <td class="text_rt">31</td>
+  <td class="text_lf"><a href="#Fig_31">Directional components in the flight at Tampico, Tamps</a></td>
+  <td class="text_rt">428</td>
+</tr>
+<tr>
+  <td class="text_rt">32</td>
+  <td class="text_lf"><a href="#Fig_32">Hourly station density curve at Tampico, Tamps</a></td>
+  <td class="text_rt">429</td>
+</tr>
+<tr>
+  <td class="text_rt">33</td>
+  <td class="text_lf"><a href="#Fig_33">The nightly net trend of migrations at three stations in 1948</a></td>
+  <td class="text_rt">431</td>
+</tr>
+<tr>
+  <td class="text_rt">34</td>
+  <td class="text_lf"><a href="#Fig_34">Stations at which telescopic observations were made in 1948</a></td>
+  <td class="text_rt">437</td>
+</tr>
+<tr>
+  <td class="text_rt">35</td>
+  <td class="text_lf"><a href="#Fig_35">Positions of the cone of observation at Progreso, Yucatán</a></td>
+  <td class="text_rt">443</td>
+</tr>
+<tr>
+  <td class="text_rt">36</td>
+  <td class="text_lf"><a href="#Fig_36">Hourly station density curve at Progreso, Yucatán</a></td>
+  <td class="text_rt">444</td>
+</tr>
+<tr>
+  <td class="text_rt">37</td>
+  <td class="text_lf"><a href="#Fig_37">Sector density representation on two nights at Rosedale, Miss.</a></td>
+  <td class="text_rt">451</td>
+</tr>
+<tr>
+  <td class="text_rt">38</td>
+  <td class="text_lf"><a href="#Fig_38">Over-all sector vectors at major stations in spring of 1948</a></td>
+  <td class="text_rt">455</td>
+</tr>
+<tr>
+  <td class="text_rt">39</td>
+  <td class="text_lf"><a href="#Fig_39">Over-all net trend of flight directions shown in Figure 38</a></td>
+  <td class="text_rt">456</td>
+</tr>
+<tr>
+  <td class="text_rt">40</td>
+  <td class="text_lf"><a href="#Fig_40">Comparison of flight trends and surface weather conditions on April 22-23, 1948</a></td>
+  <td class="text_rt">460</td>
+</tr>
+<tr>
+  <td class="text_rt">41</td>
+  <td class="text_lf"><a href="#Fig_41">Winds aloft at 10:00 <span class="smcap">P.&nbsp;M.</span> on April 22 (CST)</a></td>
+  <td class="text_rt">461</td>
+</tr>
+<tr>
+  <td class="text_rt">42</td>
+  <td class="text_lf"><a href="#Fig_42">Comparison of flight trends and surface weather conditions on April 23-24, 1948</a></td>
+  <td class="text_rt">462</td>
+</tr>
+<tr>
+  <td class="text_rt">43</td>
+  <td class="text_lf"><a href="#Fig_43">Winds aloft at 10:00 <span class="smcap">P.&nbsp;M.</span> on April 23 (CST)</a></td>
+  <td class="text_rt">463</td>
+</tr>
+<tr>
+  <td class="text_rt">44</td>
+  <td class="text_lf"><a href="#Fig_44">Comparison of flight trends and surface weather conditions on April 24-25, 1948</a></td>
+  <td class="text_rt">464</td>
+</tr>
+<tr>
+  <td class="text_rt">45</td>
+  <td class="text_lf"><a href="#Fig_45">Winds aloft at 10:00 <span class="smcap">P.&nbsp;M.</span> on April 24 (CST)</a></td>
+  <td class="text_rt">465</td>
+</tr>
+<tr>
+  <td class="text_rt">46</td>
+  <td class="text_lf"><a href="#Fig_46">Comparison of flight trends and surface weather conditions on May 21-22, 1948</a></td>
+  <td class="text_rt">466</td>
+</tr>
+<tr>
+  <td class="text_rt">47</td>
+  <td class="text_lf"><a href="#Fig_47">Winds aloft at 10:00 <span class="smcap">P.&nbsp;M.</span> on May 21 (CST)</a></td>
+  <td class="text_rt">467</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Introduction"></a>
+<p><span class="pagenum"><a name="Page_365" id="Page_365">[Pg_365]</a></span></p>
+<div class="caption2">INTRODUCTION</div>
+
+
+<p>The nocturnal migration of birds is a phenomenon that long has
+intrigued zoologists the world over. Yet, despite this universal interest,
+most of the fundamental aspects of the problem remain
+shrouded in uncertainty and conjecture.</p>
+
+<p>Bird migration for the most part, whether it be by day or by night,
+is an unseen movement. That night migrations occur at all is a conclusion
+derived from evidence that is more often circumstantial than
+it is direct. During one day in the field we may discover hundreds
+of transients, whereas, on the succeeding day, in the same situation,
+we may find few or none of the same species present. On cloudy
+nights we hear the call notes of birds, presumably passing overhead
+in the seasonal direction of migration. And on stormy nights birds
+strike lighthouses, towers, and other tall obstructions. Facts such
+as these are indisputable evidences that migration is taking place,
+but they provide little basis for evaluating the flights in terms of
+magnitude or direction.</p>
+
+<p>Many of the resulting uncertainties surrounding the nocturnal
+migration of birds have a quantitative aspect; their resolution
+hinges on how many birds do one thing and how many do another.
+If we knew, for instance, how many birds are usually flying between
+2 and 3 A.&nbsp;M. and how this number compares with other one-hour
+intervals in the night, we would be in a position to judge to what
+extent night flight is sustained from dusk to dawn. If we could
+measure the number of birds passing selected points of observation,
+we could find out whether such migration in general proceeds more
+or less uniformly on a broad front or whether it follows certain
+favored channels or flyways. This in turn might give us a clearer
+insight into the nature of the orienting mechanism and the extent
+to which it depends on visual clues. And, if we had some valid way
+of estimating the number of birds on the wing under varying weather
+conditions, we might be able to understand better the nature and
+development of migration waves so familiar to field ornithologists.
+These are just random examples suggesting some of the results that
+may be achieved in a broad field of inquiry that is still virtually
+untouched&mdash;the quantitative study of migratory flights.</p>
+
+<p>This paper is a venture into that field. It seeks to evaluate on a
+more factual basis the traditional ideas regarding these and similar
+problems, that have been developed largely from circumstantial
+<span class="pagenum"><a name="Page_366" id="Page_366">[Pg_366]</a></span>
+criteria. It is primarily, therefore, a study of comparative quantities
+or volumes of migration&mdash;or what may be conveniently called flight
+densities, if this term be understood to mean simply the number of
+birds passing through a given space in a given interval of time.</p>
+
+<p>In the present study, the basic data permitting the numerical expression
+of such migration rates from many localities under many
+different sets of circumstances were obtained by a simple method.
+When a small telescope, mounted on a tripod, is focused on the moon,
+the birds that pass before the moon's disc may be seen and counted,
+and their apparent pathways recorded in terms of coördinates. In
+bare outline, this approach to the problem is by no means new.
+Ornithologists and astronomers alike have recorded the numbers of
+birds seen against the moon in stated periods of time (Scott, 1881a
+and 1881b; Chapman, 1888; Libby, 1889; West, 1896; Very, 1897;
+Winkenwerder, 1902a and 1902b; Stebbins, 1906; Carpenter, 1906).
+Unfortunately, as interesting as these observations are, they furnish
+almost no basis for important generalizations. Most of them lack
+entirely the standardization of method and the continuity that would
+make meaningful comparisons possible. Of all these men, Winkenwerder
+appears to have been the only one to follow up an initial one
+or two nights of observation with anything approaching an organized
+program, capable of leading to broad conclusions. And even he was
+content merely to reproduce most of his original data without correlation
+or comment and without making clear whether he fully grasped
+the technical difficulties that must be overcome in order to estimate
+the important flight direction factor accurately.</p>
+
+<p>The present study was begun in 1945, and early results obtained
+were used briefly in a paper dealing with the trans-Gulf migration
+of birds (Lowery, 1946). Since that time the volume of field data,
+as well as the methods by which they can be analyzed, has been
+greatly expanded. In the spring of 1948, through the cooperation
+and collaboration of a large number of ornithologists and astronomers,
+the work was placed on a continent-wide basis. At more
+than thirty stations (<a href="#Fig_34">Figure 34</a>, page 437) on the North American
+continent, from Yucatán to Ontario, and from California to South
+Carolina, observers trained telescopes simultaneously on the moon
+and counted the birds they saw passing before its disc.</p>
+
+<p>Most of the stations were in operation for several nights in the full
+moon periods of March, April, and May, keeping the moon under
+constant watch from twilight to dawn when conditions permitted.
+They have provided counts representing more than one thousand
+<span class="pagenum"><a name="Page_367" id="Page_367">[Pg_367]</a></span>
+hours of observation, at many places in an area of more than a
+million square miles. But, as impressive as the figures on the record
+sheets are, they, like the published observations referred to above,
+have dubious meaning as they stand. Were we to compare them
+directly, station for station, or hour for hour, we would be almost
+certain to fall into serious errors. The reasons for this are not
+simple, and the measures that must be taken to obtain true comparisons
+are even less so. When I first presented this problem to my
+colleague, Professor William A. Rense, of the Department of Physics
+and Astronomy at Louisiana State University, I was told that mathematical
+means exist for reducing the data and for ascertaining the
+desired facts. Rense's scholarly insight into the mathematics of the
+problem resulted in his derivation of formulae that have enabled me
+to analyze on a comparable basis data obtained from different
+stations on the same night, and from the same station at different
+hours and on different nights. Astronomical and technical aspects
+of the problem are covered by Rense in his paper (1946), but the
+underlying principles are discussed at somewhat greater length in
+this paper.</p>
+
+<p>Part I of the present paper, dealing with the means by which the
+data were obtained and processed, will explore the general nature of
+the problem and show by specific example how a set of observations
+is prepared for analysis. Part II will deal with the results obtained
+and their interpretation.</p>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Acknowledgments" id="Acknowledgments"></a>
+<div class="caption2">ACKNOWLEDGMENTS</div>
+
+<p>In the pursuit of this research I have received a tremendous amount of
+help from my colleagues, students, and other friends. In the first place, in
+order to obtain much of the data on which the study was based, it was necessary
+to enlist the aid of many persons in various parts of the country and to draw
+heavily on their time and patience to get all-night telescopic counts of migrating
+birds. Secondly, the processing of the primary data and its subsequent
+analysis demanded that I delve into the fields of astronomy and mathematics.
+Here, from the outset, I have enjoyed the constant and untiring help of Professor
+W. A. Rense of the Department of Physics and Astronomy at Louisiana
+State University. Without his collaboration, I would not have been able to do
+this work, for he not only supplied formulae whereby I was able to make desired
+computations, but time and again he maneuvered me through my difficulties
+in the mathematical procedures. Moreover, Professor Rense has manifested
+a great interest in the ornithological aspect of the problem, and his
+trenchant advice has been of inestimable value to me. No less am I indebted
+to my associate, Robert J. Newman, with whom I have spent untold hours
+discussing the various aspects of the problem. Indeed, most of the concepts
+that have evolved in the course of this study have grown out of discussions
+<span class="pagenum"><a name="Page_368" id="Page_368">[Pg_368]</a></span>
+over a four-year period with both Rense and Newman. Whatever merit this
+work may have may be attributable in no small part to the help these two men
+have given me. In the preparation of many of the illustrations, I am further
+obligated to Newman for his excellent creative ideas as well as draftsmanship,
+and to Miss Helen Behrnes and A. Lowell Wood for their assistance.</p>
+
+<p>The mathematical computations required in this study have been laborious
+and time-consuming. It is estimated that more than two thousand man-hours
+have gone into this phase of the work alone. Whereas I have necessarily done
+most of this work, I have received a tremendous amount of help from A.
+Lowell Wood. Further assistance in this regard came from Herman Fox,
+Donald Norwood, and Lewis Kelly.</p>
+
+<p>The recording of the original field data in the spring of 1948 from the thirty-odd
+stations in North America involved the participation of more than 200
+ornithologists and astronomers. This collaboration attests to the splendid cooperative
+spirit that exists among scientists. Many of these persons stayed
+at the telescope, either as observer or as recorder, hours on end in order to
+get sets of data extending through a whole night.</p>
+
+<p>The following were responsible for much of the field data herein used:
+J. R. Andrews, S. A. Arny, M. Dale Arvey, H. V. Autrey, Charles C. Ayres,
+Mr. and Mrs. Roy Bailey, Irwin L. Baird, Maurice F. Baker, Rollin H. Baker,
+Bedortha and Edna Baldwin, Mrs. A. Marguerite Baumgartner, T. A. Becket,
+Paul Bellington, Donald Bird, Carl Black, Jr., Lea Black, Lytle Blankenship,
+Mr. and Mrs. J. Stewart Boswell, Bruce Boudreaux, Frank Bray, Mr. and Mrs.
+Leonard Brecher, Homer Brewer, Mrs. Harvey Broome, Heyward Brown,
+Floyd Browning, Cyril Broussard, Paul Buress, Ralph M. Burress, Robert
+Cain, Don Carlos, Mrs. Reba Campbell, Mr. and Mrs. E. Burnham Chamberlain,
+Laura Chaney, Van B. Chaney, Jr., Edward Clebsch, Mr. and Mrs. Ben
+B. Coffey, William Cook, Dr. Jack Craven, Hugh C. and William Davis,
+Katherine Davis, Richard Davis, Richard DeArment, Robert E. Delphia, J. C.
+Dickinson, Mr. and Mrs. Otto Dietrich, John Dietrich, Clara Dixon, Nina
+Driven, John J. Duffy, Mr. and Mrs. R. J. Dunbar, Betty Dupre, Bernard E.
+Eble, Jr., Robert G. Eble, Dr. and Mrs. William H. Elder, C. C. Emory,
+Davis Emory, Alice H. Farnsworth, James Fielding, William R. Fish, Mr.
+and Mrs. Myron Ford, W. G. Fuller, Louis Gainey, Dr. Mary E. Gaulden,
+Mr. and Mrs. John J. Giudice, Lt. L. E. Goodnight, Earl R. Greene, Max
+Grilkey, W. W. H. Gunn, Noel Maxwell Hall, Jr., A. J. Hanna, Paul Hansen,
+Harold W. Harry, Joseph Healy, Dorothy Helmer, Mr. and Mrs. John H.
+Helmer, Philip E. Hoberecht, William D. Hogan, Dr. and Mrs. Joseph C.
+Howell, E. J. Huggins, Mrs. Walter Huxford, Hugh Iltis, W. S. Jennings,
+William M. Johnson, William Kasler, Luther F. Keeton, Lawrence C. Kent,
+W. H. Kiel, L. P. Kindler, Mr. and Mrs. Joseph E. King, Harriet Kirby, E. J.
+Koestner, Roy Komarek, Ann Knight, Mr. and Mrs. N. B. Langworthy, Mr.
+and Mrs. C. F. Lard, Prentiss D. Lewis, Ernest Liner, Dr. and Mrs. R. W.
+Lockwood, Dr. Harvey B. Lovell, William J. Lueck, Don Luethy, James
+Major, Mr. and Mrs. Russell L. Mannette, Mrs. John B. Mannix, Donald
+Mary, Dale E. McCollum, Stewart McConnell, Mr. and Mrs. M. L. McCroe,
+Robert L. McDaniel, Mr. and Mrs. Frank McGill, Thomas Merimer, Mr. and
+Mrs. I. S. H. Metcalf, Ann Michener, John Michener, T. H. Milby, D. S.
+Miller, <ins title="TN: Last comma added">Burt Monroe, Jr.,</ins> Burt Monroe, Sr., Mrs. R. A. Monroe, Gordon
+Montague, Duryea Morton, James Mosimonn, Don L. Moyle, Grant Murphy,
+<span class="pagenum"><a name="Page_369" id="Page_369">[Pg_369]</a></span>
+John T. Murphy, Mrs. H. F. Murphy, Mrs. Hill Myers, Mr. and Mrs. Robert
+J. Newman, William Nichols, R. A. Norris, Floyd Oaks, Eugene P. Odum, Mrs.
+E. E. Overton, Lennie E. Pate, Kenneth Patterson, Ralph Paxton, Louis
+Peiper, Marie Peiper, Mr. and Mrs. Harold S. Peters, Mary Peters, Mr. and
+Mrs. D. W. Pfitzer, Betty Plice, Max Plice, Lestar Porter, D. R. Power,
+Kenneth Price, George Rabb, Marge Reese, Wayne L. Reeve, C. L. Riecke,
+R. D. Ritchie, V. E. Robinson, Beverly J. Rose, Mary Jane Runyon, Roger
+Rusk, Bernd Safinsley, Mr. and Mrs. Glen C. Sanderson, Lewis L. Sandidge,
+John Sather, J. Benton Schaub, Evelyn Schneider, Henry W. Setzer, Mr. and
+Mrs. Walter Shackleton, Mr. and Mrs. Francis P. Shannon, Mr. and Mrs.
+Charles Shaw, Paul H. Shepard, Jr., Alan C. Sheppard, Mabel Slack, Alice
+Smith, R. Demett Smith, Jr., Nat Smith, Major and Mrs. Charles H. Snyder,
+Albert Springs, Dr. and Mrs. Fred W. Stamm, J. S. Steiner, Mrs. Paul Stephenson,
+Herbert Stern, Jr., Herbert Stoddard, Mr. and Mrs. F. W. Stomm, Charles
+Strull, Harold P. Strull, Mrs. Fan B. Tabler, Dr. and Mrs. James T. Tanner,
+S. M. H. Tate, David Taylor, Hall Tennin, Scott Terry, Mr. and Mrs. S.
+Charles Thacher, Olive Thomas, G. A. Thompson, Jr., Dr. and Mrs. S. R.
+Tipton, Robert Tucker, Tom Uzzel, Mr. and Mrs. M. G. Vaiden, Richard
+Vaught, Edward Violante, Brother I. Vincent, Marilyn L. Walker, Mr. and
+Mrs. Willis Weaver, Mr. and Mrs. W. L. Webb, Margaret M. L. Wehking,
+W. A. Welshans, Jr., Mrs. J. F. Wernicke, Francis M. Weston, Miss G. W.
+Weston, Dr. James W. White, John A. White, A. F. Wicke, Jr., Oren Williams,
+J. L. Wilson III, W. B. Wilson, Dr. and Mrs. Leonard Wing, Sherry Woo,
+Rodney Wuthnow, Grace Wyatt, Mr. and Mrs. Malcom Young, Mr. and Mrs.
+A. J. Zimmerman. To the scores of other people who assisted in making these
+observations I extend my hearty thanks.</p>
+
+<p>Drs. E. R. Hall, Edward H. Taylor, and H. B. Hungerford of the University
+of Kansas have read the manuscript and have made valuable suggestions, as have
+also Dr. W. H. Gates of Louisiana State University and Dr. Donald S. Farner
+of the State College of Washington. Dr. Farner has also been of great help,
+together with Drs. Ernst Mayr, J. Van Tyne, and Ernst Schüz, in suggesting
+source material bearing on the subject in foreign literature. Dr. N. Wyaman
+Storer, of the University of Kansas, pointed out a short-cut in the method for
+determining the altitude and azimuth of the moon, which resulted in much
+time being saved. For supplying climatological data and for guidance in the
+interpretation thereof, I am grateful to Dr. Richard Joel Russell, Louisiana
+State University; Commander F. W. Reichelderfer, Chief of the U. S. Weather
+Bureau, Washington, D. C.; Mr. Merrill Bernard, Chief of the Climatological
+and Hydrologic Services; and Mr. Ralph Sanders, U. S. Weather Bureau at
+New Orleans, Louisiana.</p>
+
+<p>Acknowledgment is made to Bausch and Lomb Optical Company for the
+loan of six telescopes for use in this project. Messrs. G. V. Cutler and George
+Duff of Smith and Johnson Steamship Company, operators of the Yucatan
+Line, are to be thanked for granting me free passage on the "S. S. Bertha
+Brřvig" to Progreso, Yucatán, where I made observations in 1945 and 1948. I
+am also indebted to the Louisiana State University Committee on Faulty Research
+for a grant-in-aid.</p>
+<br />
+<br />
+
+<a name="Flight_Densities_and_Their_Determination"></a>
+<a name="Lunar_Observations_of_Birds"></a>
+<span class="pagenum"><a name="Page_370" id="Page_370">[Pg_370]</a></span>
+<div class="caption2">PART I. FLIGHT DENSITIES AND THEIR DETERMINATION</div>
+<br />
+<div class="caption3 smcap">A. Lunar Observations of Birds and the Flight Density Concept</div>
+
+<p>The subject matter of this paper is wholly ornithological. It is
+written for the zoologist interested in the activities of birds. But its
+bases, the principles that make it possible, lie in other fields, including
+such rather advanced branches of mathematics as analytical
+geometry, spherical geometry, and differential calculus. No exhaustive
+exposition of the problem is practicable, that does not take
+for granted some previous knowledge of these disciplines on the part
+of all readers.</p>
+
+<p>There are, however, several levels of understanding. It is possible
+to appreciate <i>what</i> is being done without knowing <i>how</i> to do it; and
+it is possible to learn how to carry out the successive steps of a
+procedure without entirely comprehending <i>why</i>. Some familiarity
+with the concepts underlying the method is essential to a full understanding
+of the results achieved, and details of procedure must be
+made generally available if the full possibilities of the telescopic
+approach are to be realized. Without going into proof of underlying
+propositions or actual derivation of formulae, I shall accordingly
+present a discussion of the general nature of the problem, conveyed
+as much as possible in terms of physical visualization. The development
+begins with the impressions of the student when he first attempts
+to investigate the movements of birds by means of the moon.</p>
+
+
+<div class="caption3nci">What the Observer Sees</div>
+
+<p>Watched through a 20-power telescope on a cloudless night, the
+full moon shines like a giant plaster hemisphere caught in the full
+glare of a floodlight. Inequalities of surface, the rims of its craters,
+the tips of its peaks, gleam with an almost incandescent whiteness;
+and even the darker areas, the so-called lunar seas, pale to a clear,
+glowing gray.</p>
+
+<p>Against this brilliant background, most birds passing in focus
+appear as coal-black miniatures, only 1/10 to 1/30 the apparent
+diameter of the moon. Small as these silhouettes are, details of form
+are often beautifully defined&mdash;the proportions of the body, the shape
+of the tail, the beat of the wings. Even when the images are so far
+away that they are pin-pointed as mere flecks of black against the
+illuminated area, the normal eye can follow their progress easily.
+<span class="pagenum"><a name="Page_371" id="Page_371">[Pg_371]</a></span>
+In most cases the birds are invisible until the moment they "enter,"
+or pass opposite, the rim of the moon and vanish the instant they
+reach the other side. The interval between is likely to be inestimably
+brief. Some birds seem fairly to flash by; others, to drift; yet
+seldom can their passing be counted in seconds, or even in measureable
+fractions of seconds. During these short glimpses, the flight
+paths tend to lie along straight lines, though occasionally a bird may
+be seen to undulate or even to veer off course.</p>
+
+<p>Now and again, in contrast to this typical picture, more eerie effects
+may be noted. Some of them are quite startling&mdash;a minute, inanimate-looking
+object drifting passively by like a corpuscle seen in
+the field of a microscope; a gigantic wing brushing across half the
+moon; a ghost-like suggestion of a bird so transparent it seems
+scarcely more than a product of the imagination; a bird that pauses
+in mid-flight to hang suspended in the sky; another that beats its
+way ineffectually forward while it moves steadily to the side; and
+flight paths that sweep across the vision in astonishingly geometric
+curves. All of these things have an explanation. The "corpuscle"
+is possibly a physical entity of some sort floating in the fluid of the
+observer's eye and projected into visibility against the whiteness
+of the moon. The winged transparency may be an insect unconsciously
+picked up by the unemployed eye and transferred by the
+<i>camera lucida</i> principle to the field of the telescope. It may be a
+bird flying very close, so drastically out of focus that the observer
+sees right through it, as he would through a pencil held against his
+nose. The same cause, operating less effectively, gives a characteristic
+gray appearance with hazy edges to silhouettes passing just
+beneath the limits of sharp focus. Focal distortions doubtless also
+account for the precise curvature of some flight paths, for this
+peculiarity is seldom associated with distinct images. Suspended
+flight and contradictory directions of drift may sometimes be attributable
+to head winds or cross winds but more often are simply
+illusions growing out of a two-dimensional impression of a three-dimensional
+reality.</p>
+
+<p>Somewhat more commonplace are the changes that accompany
+clouds. The moon can be seen through a light haze and at times
+remains so clearly visible that the overcast appears to be behind, instead
+of in front of, it. Under these circumstances, birds can still be
+readily discerned. Light reflected from the clouds may cause the silhouettes
+to fade somewhat, but they retain sufficient definition to distinguish
+them from out-of-focus images. On occasion, when white
+<span class="pagenum"><a name="Page_372" id="Page_372">[Pg_372]</a></span>
+cloud banks lie at a favorable level, they themselves provide a backdrop
+against which birds can be followed all the way across the field
+of the telescope, whether or not they directly traverse the main area
+of illumination.</p>
+
+
+<div class="caption3nci">Types of Data Obtained</div>
+
+<p>The nature of the observations just described imposes certain
+limitations on the studies that can be made by means of the moon.
+The speed of the birds, for instance, is utterly beyond computation
+in any manner yet devised. Not only is the interval of visibility
+extremely short, but the rapidity with which the birds go by depends
+less on their real rate of motion than on their proximity to the
+observer. The identification of species taking part in the migration
+might appear to offer more promise, especially since some of the
+early students of the problem frequently attempted it, but there
+are so many deceptive elements to contend with that the results
+cannot be relied upon in any significant number of cases. Shorn
+of their bills by the diminution of image, foreshortened into unfamiliar
+shape by varying angles of perspective, and glimpsed for an
+instant only, large species at distant heights may closely resemble
+small species a few hundred feet away. A sandpiper may appear
+as large as a duck; or a hawk, as small as a sparrow. A goatsucker
+may be confused with a swallow, and a swallow may pass as a tern.
+Bats, however, can be consistently recognized, if clearly seen, by
+their tailless appearance and the forward tilt of their wings, as well
+as by their erratic flight. And separations of nocturnal migrants
+into broad categories, such as seabirds and passerine birds, are often
+both useful and feasible.</p>
+
+<p>It would be a wonderful convenience to be able to clock the speed
+of night-flying birds accurately and to classify them specifically,
+but neither of these things is indispensable to the general study of
+nocturnal migration, nor as important as the three kinds of basic
+data that <i>are</i> provided by telescopes directed at the moon. These
+concern:&mdash;(1) the direction in which the birds are traveling; (2)
+their altitude above the earth; (3) the number per unit of space
+passing the observation station.</p>
+
+<p>Unfortunately none of these things can be perceived directly,
+except in a very haphazard manner. Direction is seen by the
+observer in terms of the slant of a bird's pathway across the face of
+the moon, and may be so recorded. But the meaning of every such
+slant in terms of its corresponding compass direction on the plane of
+<span class="pagenum"><a name="Page_373" id="Page_373">[Pg_373]</a></span>
+the earth constantly changes with the position of the moon. Altitude
+is only vaguely revealed through a single telescope by the size and
+definition of images whose identity and consequent real dimensions
+are subject to serious misinterpretation, for reasons already explained.
+The number of birds per unit of space, seemingly the
+easiest of all the features of migration to ascertain, is actually the
+most difficult, requiring a prior knowledge of both direction and
+altitude. To understand why this is so, it will be necessary to consider
+carefully the true nature of the field of observation.</p>
+
+
+<div class="caption3nci">The Changing Field of Observation</div>
+
+<p>Most of the observations used in this study were made in the week
+centering on the time of the full moon. During this period the lunar
+disc progresses from nearly round to round and back again with little
+change in essential aspect or apparent size. To the man behind the
+telescope, the passage of birds looks like a performance in two dimensions
+taking place in this area of seemingly constant diameter&mdash;not
+unlike the movement of insects scooting over a circle of paper on
+the ground. Actually, as an instant's reflection serves to show, the
+two situations are not at all the same. The insects are all moving
+in one plane. The birds only appear to do so. They may be flying
+at elevations of 500, 1000, or 2000 feet; and, though they give the
+illusion of crossing the same illuminated area, the actual breadth of
+the visible space is much greater at the higher, than at the lower,
+level. For this reason, other things being equal, birds nearby cross
+the moon much more swiftly than distant ones. The field of observation
+is not an area in the sky but a volume in space, bounded by the
+diverging field lines of the observer's vision. Specifically, it is an
+inverted cone with its base at the moon and its vertex at the telescope.</p>
+
+<p>Since the distance from the moon to the earth does not vary a
+great deal, the full dimensions of the Great Cone determined by the
+diameter of the moon and a point on the earth remain at all times
+fairly constant. Just what they are does not concern us here, except
+as regards the angle of the apex (roughly &frac12;&deg;), because obviously
+the effective field of observation is limited to that portion
+of the Great Cone below the maximum ceiling at which birds fly, a
+much smaller cone, which I shall refer to as the Cone of Observation
+(<a href="#Fig_1">Figure 1</a>).</p>
+
+<a name="Fig_1"></a>
+<span class="pagenum"><a name="Page_374" id="Page_374">[Pg_374]</a></span>
+<div class="center">
+<img src="images/fig_1.png" width="437" height="606" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 1.</span> The field of observation, showing its two-dimensional aspect as it
+appears to the observer and its three-dimensional actuality. The breadth of
+the cone is greatly exaggerated.</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_2"></a>
+<span class="pagenum"><a name="Page_375" id="Page_375">[Pg_375]</a></span>
+<div class="center">
+<img src="images/fig_2.png" width="483" height="283" alt="Method for determining the diameter of the cone at any
+point." title="Method for determining the diameter of the cone at any
+point." /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 2.</span> Method for determining the diameter of the cone at any
+point. The angular diameter of the moon may be expressed in radians,
+or, in other words, in terms of lengths of arc equivalent to the
+radius of a circle. In the diagram, the arc between C and E, being
+equivalent to the radius CO, represents a radian. If we allow the arc
+between A and B to be the diameter of the moon, it is by astronomical
+calculation about .009 radian, or .009 CO. This ratio will hold for
+any smaller circle inscribed about the center O; that is, the arc between
+A´B´ equals .009 C´O. Thus the width of the cone of observation
+at any point, expressed in degrees of arc, is .009 of the axis of
+the cone up to that point. The cone is so slender that the arc between
+A and B is essentially equal to the chord AB. Exactly the
+same consideration holds true for the smaller circle where the chord
+A´B´ represents part of the flight ceiling.</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_3"></a>
+<div class="center">
+<img src="images/fig_3.png" width="471" height="549" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 3.</span> Temporal change in the effective size of the field of observation.
+The sample sections, A and B, represent the theoretical densities of flight at
+8:20 and 12:00 P.&nbsp;M., respectively. Though twice as many birds are assumed
+to be in the air at midnight when the moon is on its zenith (Z) as there were
+at the earlier hour, only half as many are visible because of the decrease in size
+of the cone of observation.</div>
+</div>
+<br />
+<br />
+
+<p>The problem of expressing the number of passing birds in terms
+of a definite quantity of space is fundamentally one of finding out
+the critical dimensions of this smaller cone. The diameter at any
+distance from the observer may be determined with enough accuracy
+for our purposes simply by multiplying the distance by .009, a convenient
+approximation of the diameter of the moon, expressed in
+radians (see <a href="#Fig_2">Figure 2</a>). One hundred feet away, it is approximately
+11 inches; 1000 feet away, nine feet; at one mile, 48 feet; at two
+miles, 95 feet. Estimating the effective length of the field of observation
+presents more formidable difficulties, aggravated by the
+fact that the lunar base of the Great Cone does not remain stationary.
+The moon rises in the general direction of east and sets somewhere
+in the west, the exact points where it appears and disappears
+on the horizon varying somewhat throughout the year. As it drifts
+across the sky it carries the cone of observation with it like the slim
+beam of an immense searchlight slowly probing space. This situation
+is ideal for the purpose of obtaining a random sample of the
+number of birds flying out in the darkness, yet it involves great
+complications; for the size of the sample is never at two consecutive
+instants the same. The nearer the ever-moving great cone of the
+moon moves toward a vertical position, the nearer its intersection
+with the flight ceiling approaches the observer, shortening, therefore,
+the cone of observation (<a href="#Fig_3">Figure 3</a>). The effect on the number of
+<span class="pagenum"><a name="Page_376" id="Page_376">[Pg_376]</a></span>
+birds seen is profound. In extreme instances it may completely reverse
+the meaning of counts. Under the conditions visualized in
+<a href="#Fig_3">Figure 3</a>, the field of observation at midnight is only one-fourth as
+large as the field of observation earlier in the evening. Thus the
+twenty-four birds seen from 7 to 8 P.&nbsp;M., represent not twice as many
+birds actually flying per unit of space as the twelve observed from
+11:30 to 12:30 A.&nbsp;M., but only half the amount. <a href="#Fig_4">Figure 4</a>, based on observations
+<span class="pagenum"><a name="Page_377" id="Page_377">[Pg_377]</a></span>
+at Ottumwa, Iowa, on the night of May 22-23, shows a
+similar effect graphically. Curve A represents the actual numbers of
+birds per hour seen; Curve B shows the same figures expressed as
+flight densities, that is, corrected to take into account the changing
+size of the field of observation. It will be noted that the trends are
+almost exactly opposite. While A descends, B rises, and <i>vice-versa</i>.
+In this case, inferences drawn from the unprocessed data lead to a
+complete misinterpretation of the real situation.</p>
+
+<a name="Fig_4"></a>
+<div class="center">
+<img src="images/fig_4.png" width="482" height="345" alt="" title="" />
+<div class="fig_text"><span class="bold smcap">Fig. 4.</span> Migration at Ottumwa, Iowa, on the night of May 22-23, 1948.
+Curve A is a graphic representation of the actual numbers of birds seen
+hourly through the telescope. Curve B represents the same figures corrected
+for the variation in the size of the cone of observation. The dissimilarity
+in the two curves illustrates the deceptive nature of untreated
+telescopic counts.</div>
+</div>
+<br />
+<br />
+
+<p>Nor does the moon suit our convenience by behaving night after
+night in the same way. On one date we may find it high in the sky
+between 9 and 10 P.&nbsp;M.; on another date, during the same interval of
+time, it may be near the horizon. Consequently, the size of the cone
+is different in each case, and the direct comparison of flights in the
+same hour on different dates is no more dependable than the misleading
+comparisons discussed in the preceding paragraph.</p>
+
+<p>The changes in the size of the cone have been illustrated in <a href="#Fig_3">Figure 3</a> as though the moon were traveling in a plane vertical to the earth's
+surface, as though it reached a point directly over the observer's
+head. In practice this least complicated condition seldom obtains
+in the regions concerned in this study. In most of the northern
+<span class="pagenum"><a name="Page_378" id="Page_378">[Pg_378]</a></span>
+hemisphere, the path of the moon lies south of the observer so that
+the cone is tilted away from the vertical plane erected on the
+parallel of latitude where the observer is standing. In other words
+it never reaches the zenith, a point directly overhead. The farther
+north we go, the lower the moon drops toward the horizon and the
+more, therefore, the cone of observation leans away from us. Hence,
+at the same moment, stationed on the same meridian, two observers,
+one in the north and one in the south, will be looking into different
+effective volumes of space (<a href="#Fig_5">Figure 5</a>).</p>
+
+<a name="Fig_5"></a>
+<div class="center">
+<img src="images/fig_5.png" width="471" height="334" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 5.</span> Geographical variation in the size of the cone of observation. The
+cones A and B represent the effective fields of observation at two stations
+situated over 1,200 miles apart. The portions of the great cones included here
+appear nearly parallel, but if extended far enough would be found to have a
+common base on the moon. Because of the continental scale of the drawing,
+the flight ceiling appears as a curved surface, equidistant above each station.
+The lines to the zenith appear to diverge, but they are both perpendicular to
+the earth. Although the cones are shown at the same instant in time, and
+have their origin on the same meridian, the dimensions of B are less than one-half
+as great as those of A, thus materially decreasing the opportunity to see
+birds at the former station. This effect results from the different slants at which
+the zenith distances cause the cones to intersect the flight ceiling. The diagram
+illustrates the principle that northern stations, on the average, have a better
+chance to see birds passing in their vicinity than do southern stations</div>
+</div>
+<br />
+<br />
+
+<p>As a further result of its inclination, the cone of observation,
+seldom affords an equal opportunity of recording birds that are flying
+in two different directions. This may be most easily understood by
+<span class="pagenum"><a name="Page_379" id="Page_379">[Pg_379]</a></span>
+considering what happens on a single flight level. The plane parallel
+to the earth representing any such flight level intersects the slanting
+cone, not in a circle, but in an ellipse. The proportions of this ellipse
+are very variable. When the moon is high, the intersection on the
+plane is nearly circular; when the moon is low, the ellipse becomes
+greatly elongated. Often the long axis may be more than twice the
+length of the short axis. It follows that, if the long axis happens
+to lie athwart the northward direction of flight and the short axis
+across the eastward direction, we will get on the average over twice
+as large a sample of birds flying toward the north as of birds flying
+toward the east.</p>
+
+<p>In summary, whether we wish to compare different stations,
+different hours of the night, or different directions during the same
+hour of the night, no conclusions regarding even the relative numbers
+of birds migrating are warranted, unless they take into account the
+ever-varying dimensions of the field of observation. Otherwise we
+are attempting to measure migration with a unit that is constantly
+expanding or contracting. Otherwise we may expect the same kind
+of meaningless results that we might obtain by combining measurements
+in millimeters with measurements in inches. Some method
+must be found by which we can reduce all data to a standard basis
+for comparison.</p>
+
+
+<div class="caption3nci">The Directional Element in Sampling</div>
+
+<p>In seeking this end, we must immediately reject the simple logic of
+sampling that may be applied to density studies of animals on land.
+We must not assume that, since the field of observation is a volume
+in space, the number of birds therein can be directly expressed in
+terms of some standard volume&mdash;a cubic mile, let us say. Four
+birds counted in a cone of observation computed as 1/500 of a cubic
+mile are not the equivalent of 500 × 4, or 2000, birds per cubic mile.
+Nor do four birds flying over a sample 1/100 of a square mile mathematically
+represent 400 birds passing over the square mile. The
+reason is that we are not dealing with static bodies fixed in space but
+with moving objects, and the objects that pass through a cubic mile
+are not the sum of the objects moving through each of its 500 parts.
+If this fact is not immediately apparent, consider the circumstances
+in Figures 6 and 7, illustrating the principle as it applies to areas.
+The relative capacity of the sample and the whole to intercept
+bodies in motion is more closely expressed by the ratio of their perimeters
+in the case of areas and the ratio of their surface areas in the
+case of volumes. But even these ratios lead to inaccurate results
+<span class="pagenum"><a name="Page_380" id="Page_380">[Pg_380]</a></span>
+unless the objects are moving in all directions equally (see <a href="#Fig_8">Figure 8</a>).
+Since bird migration exhibits strong directional tendencies, I have
+come to the conclusion that no sampling procedure that can be applied
+to it is sufficiently reliable short of handling each directional
+trend separately.</p>
+
+<a name="Fig_6"></a>
+<div class="center">
+<img src="images/fig_6.png" width="412" height="383" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 6.</span> The problem of sampling migrating birds. The
+large square in the diagram may be thought of as a square
+mile on the earth's surface, divided into four equal smaller
+squares. Birds are crossing over the area in three directions,
+equally spaced, so that each of the subdivisions is traversed by
+three of them. We might be tempted to conclude that 4 × 3,
+or 12, would pass over the large square. Actually there are
+only seven birds involved all told. Obviously, the interceptive
+potential of a small square and a larger square do not
+stand in the same ratio as their areas.</div>
+</div>
+<br />
+<br />
+
+<p>For this reason, the success of the whole quantitative study of
+migration depends upon our ability to make directional analyses of
+primary data. As I have already pointed out, the flight directions of
+birds may be recorded with convenience and a fair degree of objectivity
+by noting the slant of their apparent pathways across the disc
+of the moon. But these apparent pathways are seldom the real
+pathways. Usually they involve the transfer of the flight line from
+a horizontal plane of flight to a tilted plane represented by the face
+of the moon, and so take on the nature of a projection. They are
+<span class="pagenum"><a name="Page_381" id="Page_381">[Pg_381]</a></span>
+clues to directions, but they are not the directions themselves. For
+each compass direction of birds flying horizontally above the earth,
+there is one, and only one, slant of the pathway across the moon at
+a given time. It is possible, therefore, knowing the path of a bird
+in relation to the lunar disc and the time of the observation, to compute
+the direction of its path in relation to the earth. The formula
+employed is not a complicated one, but, since the meaning of the
+lunar coördinates in terms of their corresponding flight paths parallel
+to the earth is constantly changing with the position of the moon,
+the calculation of each bird's flight separately would require a tremendous
+amount of time and effort.</p>
+
+<a name="Fig_7"></a>
+<div class="center">
+<img src="images/fig_7.png" width="454" height="274" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 7.</span> The sampling effect of a square. In Diagram A eight evenly
+distributed birds are flying from south to north, and another four are proceeding
+from east to west. Three appear in each of the smaller squares.
+Thus, if we were to treat any of these smaller sections as a directly proportionate
+sample of the whole, we would be assuming that 3 × 16, or 48,
+birds had traversed the square mile&mdash;four times the real total of 12. If
+we consider the paths separately as in Diagram B, we see quite clearly what
+is wrong. Every bird crosses four plots the size of the sample and is being
+computed into the total over and over a corresponding number of times.
+Patently, just as many south-north birds cross the bottom tier of squares
+as cross the four tiers comprising the whole area. Just as many west-east
+birds traverse one side of the large square as cross the whole square. In
+other words, the inclusion of additional sections <i>athwart</i> the direction of
+flight involves the inclusion of additional birds proceeding in that direction,
+while the inclusion of additional sections <i>along</i> the direction does not.
+The correct ratio of the sample to the whole would seem to be the ratio of
+their perimeters, in this case the ratio of one to four. When this factor
+of four is applied to the problem it proves correct: 4 × 3 (the number of
+birds that have been seen in the sample square) equals 12 (the exact
+number of birds that could be seen in the square mile).</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_8"></a>
+<p><span class="pagenum"><a name="Page_382" id="Page_382">[Pg_382]</a></span></p>
+<div class="center">
+<img src="images/fig_8.png" width="496" height="296" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 8.</span> Rectangular samples of square areas. In Diagram A, where as many
+birds are flying from west to east as are flying from south to north, the perimeter
+ratio (three to eight) correctly expresses the number of birds that have
+traversed the whole area relative to the number that have passed through the
+sample. But in Diagram B, where all thirty-two birds are flying from south
+to north, the correct ratio is the ratio of the base of the sample to the base of
+the total area (one to four), and use of the perimeter ratio would lead to an
+inaccurate result (forty-three instead of thirty-two birds). Perimeter ratios
+do not correctly express relative interceptory potential, unless the shape of the
+sample is the same as the shape of the whole, or unless the birds are flying in
+all directions equally.</div>
+</div>
+<br />
+<br />
+
+<p>Whatever we do, computed individual flight directions must be
+frankly recognized as approximations. Their anticipated inaccuracies
+are not the result of defects in the mathematical procedure employed.
+This is rigorous. The difficulty lies in the impossibility of
+reading the slants of the pathways on the moon precisely and in the
+three-dimensional nature of movement through space. The observed
+coördinates of birds' pathways across the moon are the projected
+product of two component angles&mdash;the compass direction of the
+flight and its slope off the horizontal, or gradient. These two factors
+cannot be dissociated by any technique yet developed. All we can
+do is to compute what a bird's course would be, if it were flying horizontal
+to the earth during the interval it passes before the moon.
+We cannot reasonably assume, of course, that all nocturnal migration
+takes place on level planes, even though the local distractions
+so often associated with sloping flight during the day are minimized
+in the case of migrating birds proceeding toward a distant destination
+in darkness. We may more safely suppose, however, that deviations
+from the horizontal are random in nature, that it is mainly
+a matter of chance whether the observer happens to see an ascending
+segment of flight or a descending one. Over a series of observations,
+we may expect a fairly even distribution of ups and downs. It follows
+that, although departures from the horizontal may distort individual
+directions, they tend to average out in the computed trend
+of the mean. The working of this principle applied to the undulating
+flight of the Goldfinch (<i>Spinus</i>) is illustrated in <a href="#Fig_9">Figure 9</a>.</p>
+
+<p><span class="pagenum"><a name="Page_383" id="Page_383">[Pg_383]</a></span></p>
+
+<a name="Fig_9"></a>
+<div class="center">
+<img src="images/fig_9.png" width="396" height="440" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 9.</span> The effect of vertical components in bird flight. The four diagrams
+illustrate various effects that might result if a bird with an undulating flight,
+such as a Goldfinch, flew before a moon 45&deg; above the horizon. In each case the
+original profile of the pathways, illustrated against the dark background, is
+flattened considerably as a result of projection. In the situation shown in
+Diagram A, where the high point of the flight line, GHJ, occurs within the field
+of the telescope, it is not only obvious that a deviation is involved, but the
+line GJ drawn between the entry and departure points coincides with the normal
+coördinates of a bird proceeding on a horizontal plane. In Diagrams B
+and C, one which catches an upward segment of flight, and the other, a downward
+segment, the nature of the deviation would not be detectable, and an
+incorrect direction would be computed from the coördinates. Over a series of
+observations, including many Goldfinches, one would expect a fairly even distribution
+of ups and downs. Since the average between the coördinate angles
+in Diagrams B and C, +19&deg; and -19&deg;, is the angle of the true coördinate, we
+have here a situation where the errors tend to compensate. In Diagram D,
+where the bird is so far away that several undulations are encompassed within
+the diameter of the field of view, the coördinate readings do not differ materially
+from those of a straight line.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_384" id="Page_384">[Pg_384]</a></span>
+Since <i>individually</i> computed directions are not very reliable in any
+event, little is to be lost by treating the observed pathways in groups.
+Consequently, the courses of all the birds seen in a one-hour period
+may be computed according to the position of the moon at the middle
+of the interval and expressed in terms of their general positions on
+the compass, rather than their exact headings. For this latter purpose,
+the compass has been divided into twelve fixed sectors, 22&frac12;
+degrees wide. The trends of the flight paths are identified by the
+mid-direction of the sector into which they fall. The sectoring
+method is described in detail in the section on procedures.</p>
+
+<a name="Fig_10"></a>
+<div class="center">
+<img src="images/fig_10.png" width="469" height="269" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 10.</span> The interceptory potential of slanting lines. The diagram
+deals with one direction of flight and its incidence across lines of six
+different slants, lines of identical length oriented in six different ways.
+Obviously, the number of birds that cross a line depends not only on
+the length of the line, but also on its slant with respect to the flight
+paths.</div>
+</div>
+<br />
+<br />
+
+<p>The problem remains of converting the number of birds involved
+in each directional trend to a fixed standard of measurement. <a href="#Fig_7">Figure 7</a>A contains the partial elements of a solution. All of the west-east
+flight paths that cross the large square also cross one of its mile-long
+sides and suggest the practicability of expressing the amount of migration
+<span class="pagenum"><a name="Page_385" id="Page_385">[Pg_385]</a></span>
+in any certain direction in terms of the assumed quantity
+passing over a one-mile line in a given interval of time. However,
+many lines of that length can be included within the same set of
+flight paths (<a href="#Fig_10">Figure 10</a>); and the number of birds intercepted depends
+in part upon the orientation of the line. The 90&deg; line is the only one
+that fully measures the amount of <ins title="TN: flght => flight">flight</ins> per linear unit of front; and
+so I have chosen as a standard an imaginary mile on the earth's
+surface lying at right angles to the direction in which the birds are
+traveling.</p>
+
+
+<div class="caption3nci">Definitions of Flight Density</div>
+
+<p>When the count of birds in the cone of observation is used as a
+sample to determine the theoretical number in a sector passing over
+such a mile line, the resulting quantity represents what I shall call
+a Sector Density. It is one of several expressions of the more general
+concept of Flight Density, which may be defined as the passage of
+migration past an observation station stated in terms of the theoretical
+number of birds flying over a one-mile line on the earth's
+surface in a given interval of time. Note that a flight density is
+primarily a theoretical number, a statistical expression, a <i>rate</i> of
+passage. It states merely that birds were moving through the effective
+field of observation at the <i>rate</i> of so many per mile per unit of
+time. It may or may not closely express the amount of migration
+occurring over an actual mile or series of miles. The extent to which
+it does so is to be decided by other general criteria and by the circumstances
+surrounding a given instance. Its basic function is to take
+counts of birds made at different times and at different places, in
+fields of observation of different sizes, and to put them on the statistically
+equal footing that is the first requisite of any sound comparison.</p>
+
+<p>The idea of a one-mile line as a standard spacial measurement
+is an integral part of the basic concept, as herein propounded. But,
+within these limitations, flight density may be expressed in many
+different ways, distinguished chiefly by the directions included and
+the orientation of the one-mile line with respect to them. Three
+such kinds of density have been found extremely useful in subsequent
+analyses and are extensively employed in this paper: Sector, Net
+Trend, and Station Density, or Station Magnitude.</p>
+
+<p>Sector Density has already been referred to. It may be defined
+as the flight density within a 22&frac12;&deg; directional spread, or sector,
+measured across a one-mile line lying at right angles to the mid-direction
+of the sector. It is the basic type of density from the point
+<span class="pagenum"><a name="Page_386" id="Page_386">[Pg_386]</a></span>
+of view of the computer, the others being derived from it. In
+analysis it provides a means of comparing directional trends at the
+same station and of studying variation in directional fanning.</p>
+
+<p>Net Trend Density represents the maximum net flow of migration
+over a one-mile line. It is found by plotting the sector densities
+directionally as lines of thrust, proportioned according to the density
+in each sector, and using vector analysis to obtain a vector resultant,
+representing the density and direction of the net trend. The mile
+line defining the spacial limits lies at right angles to this vector resultant,
+but the density figure includes all of the birds crossing the
+line, not just those that do so at a specified angle. Much of the
+directional spread exhibited by sector densities undoubtedly has no
+basis in reality but results from inaccuracies in coördinate readings
+and from practical difficulties inherent in the method of computation.
+By reducing all directions to one major trend, net trend density
+has the advantage of balancing errors one against the other and
+may often give the truer index to the way in which the birds are
+actually going. On the other hand, if the basic directions are too
+widely spread or if the major sector vectors are widely separated
+with little or no representation between, the net trend density may
+become an abstraction, expressing the idea of a mean direction but
+pointing down an avenue along which no migrants are traveling. In
+such instances, little of importance can be learned from it. In others,
+it gives an idea of general trends indispensable in comparing station
+with station to test the existence of flyways and in mapping the continental
+distribution of flight on a given night to study the influence
+of weather factors.</p>
+
+<p>Station Density, or Station Magnitude, represents all of the migration
+activity in an hour in the vicinity of the observation point,
+regardless of direction. It expresses the sum of all sector densities.
+It includes, therefore, the birds flying at right angles over several
+one-mile lines. One way of picturing its physical meaning is to
+imagine a circle one-mile in diameter lying on the earth with the
+observation point in the center. Then all of the birds that fly over
+this circle in an hour's time constitute the hourly station density.
+While its visualization thus suggests the idea of an area, it is derived
+from linear expressions of density; and, while it involves no limitation
+with respect to direction, it could not be computed without taking
+every component direction into consideration. Station density
+is adapted to studies involving the total migration activity at various
+stations. So far it has been the most profitable of all the density
+<span class="pagenum"><a name="Page_387" id="Page_387">[Pg_387]</a></span>
+concepts, throwing important light on nocturnal rhythm, seasonal
+increases in migration, and the vexing problem of the distribution
+of migrating birds in the region of the Gulf of Mexico.</p>
+
+<p>Details of procedure in arriving at these three types of flight density
+will be explained in Section B of this discussion. For the moment,
+it will suffice to review and amplify somewhat the general idea
+involved.</p>
+
+
+<div class="caption3nci">Altitude as a Factor in Flight Density</div>
+
+<p>A flight density, as we have seen, may be defined as the number of
+birds passing over a line one mile long; and it may be calculated
+from the number of birds crossing the segment of that line included
+in an elliptical cross-section of the cone of observation. It may be
+thought of with equal correctness, without in any way contradicting
+the accuracy of the original definition, as the number of birds passing
+through a vertical plane one mile long whose upper limits are
+its intersection with the flight ceiling and whose base coincides with
+the one mile line of the previous visualization. From the second
+point of view, the sample becomes an area bounded by the triangular
+projection of the cone of observation on the density plane. The
+dimensions of two triangles thus determined from any two cones of
+observation stand in the same ratio as the dimensions of their elliptical
+sections on any one plane; so both approaches lead ultimately
+to the same result. The advantage of this alternative way of looking
+at things is that it enables us to consider the vertical aspects of migration&mdash;to
+comprehend the relation of altitude to bird density.</p>
+
+<p>If the field of observation were cylindrical in shape, if it had
+parallel sides, if its projection were a rectangle or a parallelogram,
+the height at which birds are flying would not be a factor in finding
+out their number. Then the sample would be of equal breadth
+throughout, with an equally wide representation of the flight at all
+levels. Since the field of observation is actually an inverted cone,
+triangular in section, with diverging sides, the opportunity to detect
+birds increases with their distance from the observer. The chances
+of seeing the birds passing below an elevation midway to the flight
+ceiling are only one-third as great as of seeing those passing above
+that elevation, simply because the area of that part of the triangle
+below the mid-elevation is only one-third as great as the area of that
+part above the mid-elevation. If we assume that the ratio of the
+visible number of birds to the number passing through the density
+plane is the same as the ratio of the triangular section of the cone
+<span class="pagenum"><a name="Page_388" id="Page_388">[Pg_388]</a></span>
+to the total area of the plane, we are in effect assuming that the
+density plane is made up of a series of triangles the size of the
+sample, each intercepting approximately the same number of birds.
+We are assuming that the same number of birds pass through the
+inverted triangular sample as through the erect and uninvestigable
+triangle beside it (as in <a href="#Fig_11">Figure 11</a>, Diagram II). In reality, the assumption
+is sound only if the altitudinal distribution of migrants is
+uniform.</p>
+
+<a name="Fig_11"></a>
+<div class="center">
+<img src="images/fig_11.png" width="490" height="373" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 11.</span> Theoretical possibilities of vertical distribution. Diagram
+I shows the effect of a uniform vertical distribution of birds.
+The figures indicate the number of birds in the respective areas.
+Here the sample triangle, ABD, contains the same number of birds
+as the upright triangle, ACD, adjacent to it; the density plane
+may be conceived of as a series of such alternating triangles, equal
+in their content of birds. Diagram II portrays, on an exaggerated
+scale, the situation when many more birds are flying below the median
+altitude than above it. In contrast to the 152 birds occurring
+in the triangle A´C´D´, only seventy-two are seen in the triangle
+A´B´D´. Obviously, the latter triangle does not provide a representative
+sample of the total number of birds intersecting the density
+plane. Diagram III illustrates one method by which this difficulty
+may be overcome. By lowering the line F´G´ to the median altitude
+of bird density, F´´G´´ (the elevation above which there are
+just as many birds as below), we are able to determine a rectangular
+panel, HIJK, whose content of birds provides a representative
+sample of the vertical distribution.</div>
+</div>
+<br />
+<br />
+
+<p>The definite data on this subject are meagre. Nearly half a century
+ago, Stebbins worked out a way of measuring the altitude of
+<span class="pagenum"><a name="Page_389" id="Page_389">[Pg_369]</a></span>
+migrating birds by the principle of parallax. In this method, the
+distance of a bird from the observers is calculated from its apparent
+displacement on the moon as seen through two telescopes. Stebbins
+and his colleague, Carpenter, published the results of two nights
+of observation at Urbana, Illinois (Stebbins, 1906; Carpenter,
+1906); and then the idea was dropped until 1945, when Rense and I
+briefly applied an adaptation of it to migration studies at Baton
+Rouge. Results have been inconclusive. This is partly because
+sufficient work has not been done, partly because of limitations in
+the method itself. If the two telescopes are widely spaced, few
+birds are seen by both observers, and hence few parallaxes are
+obtained. If the instruments are brought close together, the displacement
+of the images is so reduced that extremely fine readings
+of their positions are required, and the margin of error is greatly
+increased. Neither alternative can provide an accurate representative
+sample of the altitudinal distribution of migrants at a station
+on a single night. New approaches currently under consideration
+have not yet been perfected.</p>
+
+<p>Meanwhile the idea of uniform vertical distribution of migrants
+must be dismissed from serious consideration on logical grounds.
+We know that bird flight cannot extend endlessly upward into the
+sky, and the notion that there might be a point to which bird density
+extends in considerable magnitude and then abruptly drops off to
+nothing is absurd. It is far more likely that the migrants gradually
+dwindle in number through the upper limits at which they fly,
+and the parallax observations we have seem to support this view.</p>
+
+<p>Under these conditions, there would be a lighter incidence of
+birds in the sample triangle than in the upright triangle beside it
+(<a href="#Fig_11">Figure 11</a>, Diagram III). Compensation can be made by deliberately
+scaling down the computed size of the sample area below
+its actual size. A procedure for doing this is explained in <a href="#Fig_11">Figure 11</a>.
+If it were applied to present altitudinal data, it would place the
+computational flight ceiling somewhere below 4000 feet. In arriving
+at the flight densities used in this paper, however, I have used an
+assumed ceiling of one mile. When the altitude factor is thus assigned
+a value of 1, it disappears from the formula, simplifying
+computations. Until the true situation with respect to the vertical
+distribution of flight is better understood, it seems hardly worthwhile
+to sacrifice the convenience of this approximation to a
+rigorous interpretation of scanty data. This particular uncertainty,
+however, does not necessarily impair the analytical value of the
+<span class="pagenum"><a name="Page_390" id="Page_390">[Pg_390]</a></span>
+computations. Provided that the vertical pattern of migration is
+more or less constant, flight densities still afford a sound basis for
+comparisons, wherever we assume the upper flight limits to be.
+Raising or lowering the flight ceiling merely increases or reduces
+all sample cones or triangles proportionately.</p>
+
+<p>A more serious possibility is that the altitudinal pattern may
+vary according to time or place. This might upset comparisons. If
+the divergencies were severe enough and frequent enough, they
+could throw the study of flight densities into utter confusion.</p>
+
+<p>This consideration of possible variation in the altitudinal pattern
+combines with accidents of sampling and the concessions to perfect
+accuracy, explained on pages 379-385, to give to small quantities
+of data an equivocal quality. As large-scale as the present survey is
+from one point of view, it is only a beginning. Years of intensive
+work and development leading to a vast accumulation of data must
+elapse before the preliminary indications yet discernible assume
+the status of proved principles. As a result, much of the discussion
+in Part II of this paper is speculative in intent, and most of the
+conclusions suggested are of a provisional nature. Yet, compared
+with similar procedures in its field, flight density study is a highly
+objective method, and a relatively reliable one. In no other type
+of bird census has there ever been so near a certainty of recording
+<i>all</i> of the individuals in a specified space, so nearly independently
+of the subjective interpretations of the observer. The best assurance
+of the essential soundness of the flight density computations lies in
+the coherent results and the orderly patterns that already emerge
+from the analyses presented in Part II.</p>
+<br />
+<br />
+
+<a name="Observational_Procedure"></a>
+<div class="caption3 smcap">B. Observational Procedure And The Processing Of Data</div>
+
+<p>At least two people are required to operate an observation station&mdash;one
+to observe, the other to record the results. They should exchange
+duties every hour to avoid undue eye fatigue. Additional
+personnel are desirable so that the night can be divided into shifts.</p>
+
+<p>Essential materials and equipment include: (1) a small telescope;
+(2) a tripod with pan-tilt or turret head and a mounting cradle; (3)
+data sheets similar to the one illustrated in <a href="#Fig_12">Figure 12</a>. Bausch and
+Lomb or Argus spotting scopes (19.5 ×) and astronomical telescopes
+up to 30- or 40-power are ideal. Instruments of higher
+magnification are subject to vibration, unless very firmly mounted,
+and lead to difficulties in following the progress of the moon, unless
+powered by clockwork. Cradles usually have to be devised. An
+<span class="pagenum"><a name="Page_391" id="Page_391">[Pg_391]</a></span>
+adjustable lawn chair is an important factor in comfort in latitudes
+where the moon reaches a point high overhead.</p>
+<br />
+<br />
+
+<a name="Fig_12"></a>
+<div class="center">
+<a href="#Fig_12_Trans"><img src="images/fig_12.png" width="445" height="589" alt="" title="" /><br />
+<span class="smaller">Click here to see a transcription.</span></a><br />
+<div class="fig_text"><span class="bold smcap">Fig. 12.</span> Facsimile of form used to record data in the field. One sheet of
+the actual observations obtained at Progreso, Yucatán, on April 24-25, 1948, is
+reproduced here. The remainder of this set of data, which is to be used
+throughout the demonstration of procedures, is shown in <a href="#Tbl_1">Table 1</a>.</div>
+</div>
+<br />
+<br />
+
+<p>As much detail as possible should be entered in the space provided
+at the top of the data sheet. Information on the weather should
+include temperature, description of cloud cover, if any, and the
+<span class="pagenum"><a name="Page_392" id="Page_392">[Pg_392]</a></span>
+direction and apparent speed of surface winds. Care should be
+taken to specify whether the telescope used has an erect or inverted
+image. The entry under "Remarks" in the heading should describe
+the location of the observation station with respect to
+watercourses, habitations, and prominent terrain features.</p>
+
+<p>The starting time is noted at the top of the "Time" column, and
+the observer begins the watch for birds. He must keep the disc
+of the moon under unrelenting scrutiny all the while he is at the
+telescope. When interruptions do occur as a result of changing
+positions with the recorder, re-adjustments of the telescope, or
+the disappearance of the moon behind clouds, the exact duration
+of the "time out" must be set down.</p>
+<br />
+<br />
+
+<a name="Fig_13"></a>
+<div class="center">
+<img src="images/fig_13.png" width="465" height="332" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 13.</span> The identification of coördinates. These diagrams illustrate how
+the moon may be envisioned as a clockface, constantly oriented with six
+o'clock nearest the horizon and completely independent of the rotation of the
+moon's topographic features.</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_14"></a>
+<div class="center">
+<img src="images/fig_14.png" width="455" height="595" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 14.</span> The apparent pathways of the birds seen in one hour. The observations
+are those recorded in the 11:00-12:00 P.&nbsp;M. interval on April 24-25, 1948,
+at Progreso, Yucatán (see <a href="#Tbl_1">Table 1</a>).</div>
+</div>
+<br />
+<br />
+
+<p>Whenever a bird is seen, the exact time must be noted, together
+with its apparent pathway on the moon. These apparent pathways
+can be designated in a simple manner. The observer envisions the
+disc of the moon as the face of a clock, with twelve equally spaced
+points on the circumference marking the hours (<a href="#Fig_13">Figure 13</a>). He calls
+the bottommost point 6 o'clock and the topmost, 12. The intervals
+in between are numbered accordingly. As this lunar clockface moves
+across the sky, it remains oriented in such a way that 6 o'clock continues
+<span class="pagenum"><a name="Page_393" id="Page_393">[Pg_393]</a></span>
+to be the point nearest the horizon, unless the moon reaches a
+position directly overhead. Then, all points along the circumference
+are equidistant from the horizon, and the previous definition of clock
+values ceases to have meaning. This situation is rarely encountered
+in the northern hemisphere during the seasons of migration, except
+<span class="pagenum"><a name="Page_394" id="Page_394">[Pg_394]</a></span>
+in extreme southern latitudes. It is one that has never actually been
+dealt with in the course of this study. But, should the problem arise,
+it would probably be feasible to orient the clock during this interval
+with respect to the points of the compass, calling the south point
+6 o'clock.</p>
+
+<p>When a bird appears in front of the moon, the observer identifies
+its entry and departure points along the rim of the moon with respect
+to the nearest half hour on the imaginary clock and informs the recorder.
+In the case of the bird shown in <a href="#Fig_13">Figure 13</a>, he would simply
+call out, "5 to 10:30." The recorder would enter "5" in the "In" column
+on the data sheet (see <a href="#Fig_12">Figure 12</a>) and 10:30 in the "Out" column.
+Other comment, offered by the observer and added in the remarks
+column, may concern the size of the image, its speed, distinctness,
+and possible identity. Any deviation of the pathway from a
+straight line should be described. This information has no bearing
+on subsequent mathematical procedure, except as it helps to eliminate
+objects other than birds from computation.</p>
+
+<p>The first step in processing a set of data so obtained is to blue-pencil
+all entries that, judged by the accompanying remarks, relate
+to extraneous objects such as insects or bats. Next, horizontal lines
+are drawn across the data sheets marking the beginning and the end
+of each even hour of observation, as 8 <span class="smcap">P.&nbsp;M.</span>-9 <span class="smcap">P.&nbsp;M.</span>, 9 <span class="smcap">P.&nbsp;M.</span>-10 <span class="smcap">P.&nbsp;M.</span>,
+etc. The coördinates of the birds in each one-hour interval
+may now be plotted on separate diagrammatic clockfaces, just as
+they appeared on the moon. Tick marks are added to each line to
+indicate the number of birds occurring along the same coördinate.
+The slant of the tick marks distinguishes the points of departure
+from the points of entry. <a href="#Fig_14">Figure 14</a> shows the plot for the 11 P.&nbsp;M.-12
+P.&nbsp;M. observations reproduced in <a href="#Tbl_1">Table 1</a>. The standard form,
+illustrated in <a href="#Fig_15">Figure 15</a>, includes four such <ins title="TN: diargrams => diagrams">diagrams</ins>.</p>
+
+<p>Applying the self-evident principle that all pathways with the
+same slant represent the same direction, we may further consolidate
+the plots by shifting all coördinates to the corresponding lines passing
+through the center of the circle, as in <a href="#Fig_15">Figure 15</a>. To illustrate,
+the 6 to 8, 5 to 9, 3 to 11, and 2 to 12 pathways all combine on the
+4 to 10 line. Experienced computers eliminate a step by directly
+plotting the pathways through center, using a transparent plastic
+straightedge ruled off in parallel lines.</p>
+
+<p><span class="pagenum"><a name="Page_395" id="Page_395">[Pg_395]</a></span></p>
+<a name="Fig_15"></a>
+<div class="center">
+<img src="images/fig_15.png" width="432" height="573" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 15.</span> Standard form for plotting the apparent paths of flight. On these
+diagrams the original coördinates, exemplified by <a href="#Fig_14">Figure 14</a>, have been moved
+to center. In practice the sector boundaries are drawn over the circles in
+red pencil, as shown by the white lines in <a href="#Fig_19">Figure 19</a>, making it possible to count
+the number of birds falling within each zone. These numbers are then tallied
+in the columns at the lower right of each hourly diagram.</div>
+</div>
+<br />
+<br />
+
+<a name="Tbl_1"></a>
+<p><span class="pagenum"><a name="Page_396" id="Page_396">[Pg_396]</a></span></p>
+<div class="caption3nb"><span class="smcap">Table 1.</span>&mdash;Continuation of Data in <a href="#Fig_12">Figure 12</a>, Showing Time and Readings of
+Observations on 24-25 April 1948, Progreso, Yucatán</div>
+<br />
+<div style='margin-left:15%; font-family: "Courier New", monospace;'>
+<pre>
+=============================     =============================
+Time          In       Out        Time          In       Out
+-----------------------------     -----------------------------
+10:37-10:41  Time out             11:15        8          9:30
+10:45        5:30      10         11:16        4         11
+             6          9                      5          9
+             5:30      10         11:17        5         11:30
+10:46        6          8         11:18        5         12
+             3:30      11                      6         11:30
+             5         12         11:19        5:30      11:30
+10:47        3:15       1         11:20        6         10
+             6          8:30                   3         12
+             5:45      11:45                   5         12
+             5         10         11:21        5:45      11
+10:48        6          9:45                   5         11
+10:50        5:30      11         11:23        5         12
+10:51        4         11         11:25        5         10:30
+10:52        4          2                      6         11
+             5:30      11                      6         12
+10:53        5:30      11:30      11:27        6         10
+             5         11         11:28        6         11:30
+10:55        5         12                      5:30      12:30
+             5         11         11:29        6         11:30
+10:56        6         10                      4         12
+10:58        4:30      11:30                   6:30      10:30
+             5:45      11:45                   6         11
+10:59        6:30      10:30      11:30        3         10
+11:00        3:30      12               (2 birds at once)
+             6:30      11         11:31        5         10:30
+      (2 birds at once)                        5:30      10:30
+11:03        6         11         11:32        6         11:30
+11:04        3         12         11:33        7:30       9:30
+             5         12                      4         10:30
+11:05        6         10                      6         11:30
+             5         11                      8          9:30
+11:06        6         10:30      11:35        7         10
+11:07        3         10                      4:30       1
+11:08        6         11         11:38        6:30      11
+11:10        7          9:30      11:40        5:30      12
+11:11        5          9:15      11:42        4          2
+11:13        5         12                      5         12
+11:14        6:30      10                      6         10
+             5:30       1                      4          2
+             4         12                      5         12
+</pre>
+</div>
+<br />
+
+<p><span class="pagenum"><a name="Page_397" id="Page_397">[Pg_397]</a></span></p>
+<div class="caption3nb"><span class="smcap">Table 1.</span>&mdash;<i>Concluded</i></div>
+<br />
+<div style='margin-left:15%; font-family: "Courier New", monospace;'>
+<pre>
+=============================     =============================
+Time          In       Out        Time          In       Out
+-----------------------------     -----------------------------
+11:44        8          9:30                   8         10:15
+             7         11         12:16        3:30       1:30
+             6         10                      8         11
+11:45        5         12         12:23        7          1:30
+             6         10:30                   6         12:30
+             5:45      11         12:36        8         11
+             4         12         12:37        7:30       1
+11:46        7         11         12:38        7         12:30
+             6         12         12:40        8          1
+11:47        8         10         12:45        7:30       1
+11:48        6         10         12:47        5:30       1
+11:49        6:30      10:30      12:48        7          1
+11:51        8         10         12:52        5:30       1:30
+             8         10         12:54-12:55  Time out
+             8         10         12:56        8         10:45
+             8         10         12:58        5:30       1:30
+             6         10                      7          1:30
+             8         10                      7          2
+             6         11         12:59        5          3
+             7         12          1:00-1:30   Time out
+11:52        5          1          1:37        8         12
+11:54        7         11          1:38        8         12
+             6         12:30       1:48        7          1
+11:55        5         12                      7          1
+11:56        7         10          1:51        5:30      11
+             5         12          1:57        8          1
+11:58        8         11          2:07        7          2
+11:59        5:30      12          2:09        9         12
+12:00-12:03  Time out              2:10        8          1
+12:03        5:30      11:30       2:17        9         12
+12:04        8         11          2:21        6          2
+12:07        6         12:30       2:30        5:30       3:15
+             7:30       1          2:32        8          2
+12:08        5         10:30       2:46        7          1
+12:09        5:30       1          3:36        9          2
+             7:30       2          3:39        8:30       2
+12:10        6:30      12:45       3:45        6          4
+12:13        8         11          3:55        9          2
+12:14        7          1          4:00        8          3
+12:15        7         12:30       4:03        9          2
+             7:15       1:30       4:30        Closed station
+-----------------------------     -----------------------------
+</pre>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_398" id="Page_398">[Pg_398]</a></span></p>
+
+<p>We now have a concise picture of the apparent pathways of all
+the birds recorded in each hour of observation. But the coördinates
+do not have the same meaning as readings of a horizontal clock on
+the earth's surface, placed in relation to the points of the compass.
+They are merely projections of the birds' courses. An equation is
+available for reversing the effect of projection and discovering the
+true directions of flight. This formula, requiring thirty-five separate
+computations for the pathways reproduced in <a href="#Fig_12">Figure 12</a> alone,
+is far too-consuming for the handling of large quantities of data. A
+simpler procedure is to divide the compass into sectors and, with the
+aid of a reverse equation, to draw in the projected boundaries of
+these divisions on the circular diagrams of the moon. A standardized
+set of sectors, each 22&frac12;&deg; wide and bounded by points of the
+compass, has been evolved for this purpose. They are identified as
+shown in <a href="#Fig_16">Figure 16</a>. The zones north of the east-west line are known
+as the North, or N, Sectors, as N<sub>1</sub>, N<sub>2</sub>, N<sub>3</sub>, etc. Each zone south
+of the east-west line bears the same number as the sector opposite,
+but is distinguished by the designation S.</p>
+
+<a name="Fig_16"></a>
+<div class="center">
+<img src="images/fig_16.png" width="437" height="404" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 16.</span> Standard sectors for designating flight trends. Each
+zone covers a span of 22&frac12;&deg;. The N<sub>6</sub> and N<sub>8</sub>, the N<sub>5</sub> and N<sub>7</sub>, and
+their south complements, where usually few birds are represented,
+can be combined and identified as N<sub>6-8</sub> and N<sub>5-7</sub>, etc.</div>
+</div>
+<br />
+<br />
+
+<p>Several methods may be used to find the projection of the sector
+boundaries on the plot diagrams of <a href="#Fig_15">Figure 15</a>. Time may be saved
+by reference to graphic tables, too lengthy for reproduction here,
+showing the projected reading in degrees for every boundary, at
+every position of the moon; and a mechanical device, designed by
+<span class="pagenum"><a name="Page_399" id="Page_399">[Pg_399]</a></span>
+C. M. Arney, duplicating the conditions of the original projection,
+speeds up the work even further. Both methods are based on the
+principle of the following formula:</p>
+
+<table width="100%" summary="formula 1">
+<tr>
+  <td class="center">tan <!--Greek: theta-->&#952; = tan (<!--Greek: eta-->&#951; - <!--Greek: psi-->&#968;) / cos Z<sub>0</sub></td>
+  <td class="text_rt">(1)</td>
+</tr>
+</table>
+<br />
+<br />
+
+<a name="Fig_17"></a>
+<div class="center">
+<img src="images/fig_17.png" width="486" height="495" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 17.</span> The meaning of symbols used in the direction formula.</div>
+</div>
+<br />
+<br />
+
+<p>The symbols have these meanings:</p>
+
+<p><!--Greek: theta-->&#952; is the position angle of the sector boundary on the lunar clock,
+with positive values measured counterclockwise from 12 o'clock,
+negative angles clockwise (<a href="#Fig_17">Figure 17A</a>).</p>
+
+<p><!--Greek: eta-->&#951; is the compass direction of the sector boundary expressed in degrees
+reckoned west from the south point (<a href="#Fig_17">Figure 17B</a>).</p>
+
+<p><span class="pagenum"><a name="Page_400" id="Page_400">[Pg_400]</a></span>
+Z<sub>0</sub> is the zenith distance of the moon's center midway through the
+hour of observation, that is, at the half hour. It represents the
+number of degrees of arc between the center of the moon and a
+point directly over the observer's head (<a href="#Fig_17">Figure 17C</a>).</p>
+
+<p><!--Greek: psi-->&#968; is the azimuth of the moon midway through the hour of observation,
+measured from the south point, positive values to the west,
+negative values to the east (<a href="#Fig_17">Figure 17D</a>).</p>
+<br />
+<br />
+
+<a name="Fig_18"></a>
+<div class="center">
+<img src="images/fig_18.png" width="463" height="617" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 18.</span> Form used in the computation of the zenith distance and azimuth of the moon.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_401" id="Page_401">[Pg_401]</a></span>
+The angle <!--Greek: eta-->&#951; for any sector boundary can be found immediately by
+measuring its position in the diagram (<a href="#Fig_16">Figure 16</a>). The form (<a href="#Fig_18">Figure 18</a>) for the "Computation of Zenith Distance and Azimuth of the
+Moon" illustrates the steps in calculating the values of Z<sub>0</sub> and <!--Greek: psi-->&#968;<sub>0</sub>.
+From the American Air Almanac (Anonymous, 1945-1948), issued
+annually by the U. S. Naval Observatory in three volumes, each
+covering four months of the year, the Greenwich Hour Angle (GHA)
+and the declination of the moon may be obtained for any ten-minute
+interval of the date in question. The Local Hour Angle (LHA) of
+the observation station is determined by subtracting the longitude
+of the station from the GHA. Reference is then made to the
+"Tables of Computed Altitude and Azimuth," published by the
+U. S. Navy Department, Hydrographic Office (Anonymous, 1936-1941),
+and better known as the "H.O. 214," to locate the altitude
+and azimuth of the moon at the particular station for the middle
+of the hour during which the observations were made. The tables
+employ three variables&mdash;the latitude of the locality measured to
+the nearest degree, the LHA as determined above, and the declination
+of the moon measured to the nearest 30 minutes of arc.
+Interpolations can be made, but this exactness is not required. When
+the latitude of the observation station is in the northern hemisphere,
+the H.O. 214 tables entitled "Declinations Contrary Name
+to Latitude" are used with south declinations of the moon, and the
+tables "Declinations Same Name as Latitude," with north declinations.
+In the sample shown in <a href="#Fig_15">Figure 15</a>, the declination of the
+moon at 11:30 P.&nbsp;M., midway through the 11 to 12 o'clock interval,
+was S 20&deg; 22´. Since the latitude of Progreso, Yucatán is N 21&deg; 17´,
+the "Contrary Name" tables apply to this hour.</p>
+
+<p>Because the H.O. 214 expresses the vertical position of the moon
+in terms of its altitude, instead of its zenith distance, a conversion
+is required. The former is the number of arc degrees from the horizon
+to the moon's center; therefore Z<sub>0</sub> is readily obtained by subtracting
+the altitude from 90&deg;. Moreover, the azimuth given in the
+H.O. 214 is measured on a 360&deg; scale from the north point, whereas
+the azimuth used here (<!--Greek: psi-->&#968;<sub>0</sub>) is measured 180&deg; in either direction from
+the south point, negative values to the east, positive values to the
+west. I have designated the azimuth of the tables as Az<sub>n</sub> and obtained
+the desired azimuth (<!--Greek: psi-->&#968;<sub>0</sub>) by subtracting 180&deg; from Az<sub>n</sub>. The
+sign of <!--Greek: psi-->&#968;<sub>0</sub> may be either positive or negative, depending on whether
+or not the moon has reached its zenith and hence the meridian of
+the observer. When the GHA is greater than the local longitude
+<span class="pagenum"><a name="Page_402" id="Page_402">[Pg_402]</a></span>
+(that is, the longitude of the observation station), the azimuth is
+positive. When the GHA is less than the local longitude, the
+azimuth is negative.</p>
+
+<p>Locating the position of a particular sector boundary now becomes
+a mere matter of substituting the values in the equation (1)
+and reducing. The computation of the north point for 11 to 12 P.&nbsp;M.
+in the sample set of data will serve as an example. Since the north
+point reckoned west from the south point is 180&deg;, its <!--Greek: eta-->&#951; has a value
+of 180&deg;.</p>
+<br />
+<br />
+
+<a name="Fig_19"></a>
+<div class="center">
+<img src="images/fig_19.png" width="477" height="554" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 19.</span> Method of plotting sector boundaries on the diagrammatic plots.
+The example employed is the 11:00 to 12:00 P.&nbsp;M. diagram of <a href="#Fig_15">Figure 15</a>.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_403" id="Page_403">[Pg_403]</a></span></p>
+
+<div class="center">
+<table summary="formula">
+<tr>
+  <td rowspan="2">tan <!--Greek: theta-->&#952;<sub>Npt.</sub> =&nbsp;&nbsp;</td>
+  <td class="bb">tan (180&deg; - <!--Greek: psi-->&#968;<sub>0</sub>)</td>
+</tr>
+<tr>
+  <td class="center">cos Z<sub>0</sub></td>
+</tr>
+</table>
+</div>
+
+<p>Substituting values of <!--Greek: psi-->&#968;<sub>0</sub> found on the form (<a href="#Fig_18">Figure 18</a>):</p>
+
+<div class="center">
+<table summary="formula">
+<tr>
+  <td rowspan="2">tan <!--Greek: theta-->&#952;<sub>Npt.</sub> =&nbsp;&nbsp;</td>
+  <td class="bb">tan [180&deg; - (-35&deg;)]</td>
+  <td rowspan="2">&nbsp;&nbsp;=&nbsp;&nbsp;</td>
+  <td class="bb">tan 215&deg;</td>
+  <td rowspan="2">&nbsp;&nbsp;=&nbsp;&nbsp;</td>
+  <td class="bb">.700</td>
+  <td rowspan="2">&nbsp;&nbsp;= 1.09</td>
+</tr>
+<tr>
+  <td class="center">cos 50&deg;</td>
+  <td class="center">cos 50&deg;</td>
+  <td>.643</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<div class="center"><!--Greek: theta-->&#952;<sub>Npt.</sub> = 47&deg;28´</div>
+<br />
+<br />
+
+<a name="Fig_20"></a>
+<div class="center">
+<img src="images/fig_20.png" width="450" height="590" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 20.</span> Form for computing sector densities.</div>
+</div>
+<br />
+<br />
+
+
+<p>Four angles, one in each quadrant, have the same tangent value.
+<span class="pagenum"><a name="Page_404" id="Page_404">[Pg_404]</a></span>
+Since, in processing spring data, we are dealing mainly with north
+sectors, it is convenient to choose the acute angle, in this instance
+47&deg;28´. In doubtful cases, the value of the numerator of the equation
+(here 215&deg;) applied as an angular measure from 6 o'clock will
+tell in which quadrant the projected boundary must fall. The fact
+that projection always draws the boundary closer to the 3-9 line
+serves as a further check on the computation.</p>
+<br />
+<br />
+
+<a name="Fig_21"></a>
+<div class="center">
+<img src="images/fig_21.png" width="476" height="455" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 21.</span> <ins title='Correction: was "Determinaton"'>Determination</ins>
+of the angle <!--Greek: alpha-->&#945;</div>
+</div>
+<br />
+<br />
+
+<p>In the same manner, the projected position angles of all the pertinent
+sector boundaries for a given hour may be calculated and plotted
+in red pencil with a protractor on the circular diagrams of <a href="#Fig_15">Figure 15</a>. To avoid confusion in lines, the zones are not portrayed in the
+black and white reproduction of the sample plot form. They are
+shown, however, in the shaded enlargement (<a href="#Fig_19">Figure 19</a>) of the 11 to 12
+P.&nbsp;M. diagram. The number of birds recorded for each sector may
+be ascertained by counting the number of tally marks between each
+pair of boundary lines and the information may be entered in the
+columns provided in the plot form (<a href="#Fig_15">Figure 15</a>).
+
+<span class="pagenum"><a name="Page_405" id="Page_405">[Pg_405]</a></span></p>
+
+<p>We are now prepared to turn to the form for "Computations of
+Sector Densities" (<a href="#Fig_20">Figure 20</a>), which systematizes the solution of
+the following equation:</p>
+
+<div class="center">
+<table width="100%" summary="formula 2">
+<tr>
+  <td class="center"><img src="images/formula_2.png" width="298" height="81" title="D = (220*(60/T)(No. of Birds)(cos^2 Z_{0}))/(1 - sin^2 Z_{0} cos^2 [alpha])^0.5" alt="Complex Formula" /></td>
+  <td  class="text_rt">(2)</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+
+<a name="Fig_22"></a>
+<div class="center">
+<img src="images/fig_22.png" width="444" height="595" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 22.</span> Facsimile of form summarizing sector densities. The totals at the bottom of each column give the station densities.</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_23"></a>
+<p><span class="pagenum"><a name="Page_406" id="Page_406">[Pg_406]</a></span></p>
+<div class="center">
+<img src="images/fig_23.png" width="479" height="508" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 23.</span> Determination of Net Trend Density.</div>
+</div>
+<br />
+<br />
+
+<p>Some of the symbols and factors, appearing here for the first
+time, require brief explanation. D stands for Sector Density. The
+constant, 220, is the reciprocal of the quotient of the angular diameter
+of the moon divided by 2. T is Time In, arrived at by subtracting
+the total number of minutes of time out, as noted for each
+hour on the original data sheets, from 60. "No. of Birds" is the
+number for the sector and hour in question as just determined on
+the plot form. The symbol <!--Greek: a--> represents the angle between the mid-line
+of the sector and the azimuth line of the moon. The quantity
+is found by the equation:</p>
+
+<p><span class="pagenum"><a name="Page_407" id="Page_407">[Pg_407]</a></span></p>
+
+<table width="100%" summary="formula 3">
+<tr>
+  <td class="center"><!--Greek: alpha-->&#945; = 180&deg; - <!--Greek: eta-->&#951; + <!--Greek: psi-->&#968;<sub>0</sub></td>
+  <td class="text_rt">(3)</td>
+</tr>
+</table>
+
+<p>The symbol <!--Greek: eta-->&#951; here represents the position of the mid-line of the
+sector expressed in terms of its 360&deg; compass reading. This equation
+is illustrated in <a href="#Fig_21">Figure 21</a>. The values of <!--Greek: eta-->&#951; for various zones are
+given in the upper right-hand corner of the form (<a href="#Fig_20">Figure 20</a>). The
+subsequent reductions of the equations, as they appear in the figure
+for four zones, are self-explanatory. The end result, representing
+the sector density, is entered in the rectangular box provided.</p>
+
+<p>After all the sector densities have been computed, they are tabulated
+on a form for the "Summary of Sector Densities" (<a href="#Fig_22">Figure 22</a>).
+By totaling each vertical column, sums are obtained, expressing the
+Station Density or Station Magnitude for each hour.</p>
+<br />
+<br />
+
+<a name="Fig_24"></a>
+<div class="center">
+<img src="images/fig_24.png" width="481" height="370" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 24.</span> Nightly station density curve at Progreso, Yucatán, on April 24-25, 1948.</div>
+</div>
+<br />
+<br />
+
+<p>An informative way of depicting the densities in each zone is to
+plot them as lines of thrust, as in <a href="#Fig_23">Figure 23</a>. Each sector is represented
+by the directional slant of its mid-line drawn to a length expressing
+the flight density per zone on some chosen scale, such as
+100 birds per millimeter. Standard methods of vector analysis are
+then applied to find the vector resultant. This is done by considering
+the first two thrust lines as two sides of an imaginary parallelogram
+and using a drawing compass to draw intersecting arcs locating
+the position of the missing corner. In the same way, the third vector
+<span class="pagenum"><a name="Page_408" id="Page_408">[Pg_408]</a></span>
+is combined with the invisible resultant whose distal end is represented
+by the intersection of the first two arcs. The process is repeated
+successively with each vector until all have been taken into
+consideration. The final intersection of arcs defines the length and
+slant of the Vector Resultant, whose magnitude expresses the Net
+Trend Density in terms of the original scale.</p>
+
+<p>The final step in the processing of a set of observations is to plot
+on graph paper the nightly station density curve as illustrated by
+<a href="#Fig_24">Figure 24</a>.</p>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Nature_of_Nocturnal_Migration" id="Nature_of_Nocturnal_Migration"></a>
+<div class="caption2">PART II. THE NATURE OF NOCTURNAL MIGRATION</div>
+
+<p>Present day concepts of the whole broad problem of bird migration
+are made up of a few facts and many guesses. The evolutionary
+origin of migration, the modern necessities that preserve its biologic
+utility, the physiological processes associated with it, the sensory
+mechanisms that make it possible, the speed at which it is achieved,
+and the routes followed, all have been the subject of some investigation
+and much conjecture. All, to a greater or less extent, remain
+matters of current controversy. All must be considered unknowns
+in every logical equation into which they enter. Since all aspects of
+the subject are intimately interrelated, since all have a bearing on
+the probabilities relating to any one, and since new conjectures must
+be judged largely in the light of old conjectures rather than against
+a background of ample facts, the whole field is one in which many
+alternative explanations of the established phenomena remain equally
+tenable. Projected into this uncertain atmosphere, any statistical
+approach such as determinations of flight density will require the
+accumulation of great masses of data before it is capable of yielding
+truly definitive answers to those questions that it is suited to solve.
+Yet, even in their initial applications, density analyses can do much
+to bring old hypotheses regarding nocturnal migration into sharper
+definition and to suggest new ones.</p>
+
+<p>The number of birds recorded through the telescope at a particular
+station at a particular time is the product of many potential
+variables. Some of these&mdash;like the changing size of the field of
+observation and the elevation of flight&mdash;pertain solely to the capacity
+of the observer to see what is taking place. It is the function
+of the density and direction formulae to eliminate the influences of
+these two variables insofar as is possible, so that the realities of the
+situation take shape in a nearly statistically true form. There remain
+to be considered those influences potentially responsible for
+<span class="pagenum"><a name="Page_409" id="Page_409">[Pg_409]</a></span>
+variations in the real volume of migration at different times and
+places&mdash;things like the advance of season, geographic location,
+disposition of terrain features, hourly activity rhythm, wind currents,
+and other climatological causes. The situation represented
+by any set of observations probably is the end result of the interaction
+of several such factors. It is the task of the discussions that
+follow to analyze flight densities in the light of the circumstances
+surrounding them and by statistical insight to isolate the effects of
+single factors. When this has been done, we shall be brought
+closer to an understanding of these influences themselves as they
+apply to the seasonal movements of birds. Out of data that is
+essentially quantitative, conclusions of a qualitative nature will
+begin to take form. It should be constantly borne in mind, however,
+that such conclusions relate to the movement of birds <i>en masse</i>
+and that caution must be used in applying these conclusions to
+any one species.</p>
+
+<p>Since the dispersal of migrants in the night sky has a fundamental
+bearing on the sampling procedure itself, and therefore on the reliability
+of figures on flight density, consideration can well be given
+first to the horizontal distribution of birds on narrow fronts.</p>
+<br />
+<br />
+
+<a name="Horizontal_Distribution_of_Birds"></a>
+<div class="caption3 smcap">A. Horizontal Distribution Of Birds On Narrow Fronts</div>
+
+<p>Bird migration, as we know it in daytime, is characterized by
+spurts and uneven spatial patterns. Widely separated V's of geese
+go honking by. Blackbirds pass in dense recurrent clouds, now on
+one side of the observer, now on the other. Hawks ride along in
+narrow file down the thermal currents of the ridges. Herons, in
+companies of five to fifty, beat their way slowly along the line of the
+surf. And an unending stream of swallows courses low along the
+levees. Everywhere the impression is one of birds in bunches, with
+vast spaces of empty sky between.</p>
+
+<p>Such a situation is ill-suited to the sort of sampling procedure on
+which flight density computations are based. If birds always
+traveled in widely separated flocks, many such flocks might pass
+near the cone of observation and still, by simple chance, fail to
+enter the sliver of space where they could be seen. Chance would
+be the dominating factor in the number of birds recorded, obscuring
+the effects of other influences. Birds would seldom be seen, but,
+when they did appear, a great many would be observed simultaneously
+or in rapid succession.
+
+<span class="pagenum"><a name="Page_410" id="Page_410">[Pg_410]</a></span>
+When these telescopic studies were first undertaken at Baton
+Rouge in 1945, some assurance already existed, however, that night
+migrants might be so generally dispersed horizontally in the darkness
+above that the number passing through the small segment of sky
+where they could be counted would furnish a nearly proportionate
+sample of the total number passing in the neighborhood of the
+observation station. This assurance was provided by the very
+interesting account of Stone (1906: 249-252), who enjoyed the
+unique experience of viewing a nocturnal flight as a whole. On the
+night of March 27, 1906, a great conflagration occurred in Philadelphia,
+illuminating the sky for a great distance and causing the
+birds overhead to stand out clearly as their bodies reflected the light.
+Early in the night few birds were seen in the sky, but thereafter they
+began to come in numbers, passing steadily from the southwest to
+the northeast. At ten o'clock the flight was at its height. The
+observer stated that two hundred birds were in sight at any given
+moment as he faced the direction from which they came. This unparalleled
+observation is of such great importance that I quote it in
+part, as follows: "They [the birds] flew in a great scattered, wide-spread
+host, never in clusters, each bird advancing in a somewhat
+zigzag manner&hellip;. Far off in front of me I could see them
+coming as mere specks&hellip;gradually growing larger as they
+approached&hellip;. Over the illuminated area, and doubtless for
+great distances beyond, they seemed about evenly distributed&hellip;.
+I am inclined to think that the migrants were not influenced
+by the fire, so far as their flight was concerned, as those far
+to the right were not coming toward the blaze but keeping steadily
+on their way&hellip;. Up to eleven o'clock, when my observations
+ceased, it [the flight] continued apparently without abatement, and
+I am informed that it was still in progress at midnight."</p>
+
+<p>Similarly, in rather rare instances in the course of the present
+study, the combination of special cloud formations and certain atmospheric
+conditions has made it possible to see birds across the
+entire field of the telescope, whether they actually passed before the
+moon or not. In such cases the area of the sky under observation is
+greatly increased, and a large segment of the migratory movement
+can be studied. In my own experience of this sort, I have been forcibly
+impressed by the apparent uniformity and evenness of the procession
+of migrants passing in review and the infrequence with which
+birds appeared in close proximity.</p>
+
+<p>As striking as these broader optical views of nocturnal migration
+are, they have been too few to provide an incontestable basis for
+<span class="pagenum"><a name="Page_411" id="Page_411">[Pg_411]</a></span>
+generalizations. A better test of the prevailing horizontal distribution
+of night migrants lies in the analysis of the telescopic data
+themselves.</p>
+<br />
+<br />
+
+<a name="Fig_25"></a>
+<div class="center">
+<img src="images/fig_25.png" width="483" height="487" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 25.</span> Positions of the cone of observation at Tampico, Tamps., on April
+21-22, 1948. Essential features of this diagrammatic map are drawn to scale, the
+triangular white lines representing the projections of the cone of observation
+on the actual terrain at the mid-point of each hour of observation. If the
+distal ends of the position lines were connected, the portion of the map encompassed
+would represent the area over which all the birds seen between
+8:30 P.&nbsp;M. and 3:30 A.&nbsp;M. must have flown.</div>
+</div>
+<br />
+<br />
+
+<p>The distribution in time of birds seen by a single <ins title="TN: obsever => observer">observer</ins> may be
+studied profitably in this connection. Since the cone of observation
+is in constant motion, swinging across the front of birds migrating
+from south to north, each interval of time actually represents a different
+position in space. This is evident from the map of the progress
+of the field of observation across the terrain at Tampico, Tamaulipas,
+on April 21-22, 1948 (<a href="#Fig_25">Figure 25</a>). At this station on this
+night, a total of 259 birds were counted between 7:45 P.&nbsp;M. and 3:45
+<span class="pagenum"><a name="Page_412" id="Page_412">[Pg_412]</a></span>
+A.&nbsp;M. The number seen in a single hour ranged from three to
+seventy-three, as the density overhead mounted to a peak and then
+declined. The number of birds seen per minute was not kept with
+stop watch accuracy; consequently, analysis of the number of birds
+that passed before the moon in short intervals of time is not justified.
+It appears significant, however, that in the ninety minutes of heaviest
+flight, birds were counted at a remarkably uniform rate per fifteen
+minute interval, notwithstanding the fact that early in the period
+the flight rate overhead had reached a peak and had begun to
+decline. The number of birds seen in successive fifteen-minute periods
+was twenty-six, twenty-five, nineteen, eighteen, fifteen, and
+fifteen.</p>
+
+<p>Also, despite the heavy volume of migration at this station on this
+particular night, the flight was sufficiently dispersed horizontally so
+that only twice in the course of eight hours of continuous observation
+did more than one bird simultaneously appear before the moon.
+These were "a flock of six birds in formation" seen at 12:09 A.&nbsp;M.
+and "a flock of seven, medium-sized and distant," seen at 2:07 A.&nbsp;M.
+In the latter instance, as generally is the case when more than one
+bird is seen at a time, the moon had reached a rather low altitude,
+and consequently the cone of observation was approaching its maximum
+dimensions.</p>
+
+<p>The comparative frequency with which two or more birds simultaneously
+cross before the moon would appear to indicate whether
+or not there is a tendency for migrants to fly in flocks. It is significant,
+therefore, that in the spring of 1948, when no less than 7,432
+observations were made of birds passing before the moon, in only
+seventy-nine instances, or 1.1 percent of the cases, was more than
+one seen at a time. In sixty percent of these instances, only two
+birds were involved. In one instance, however, again when the
+moon was low and the cone of observation near its maximum size,
+a flock estimated at twenty-five was recorded.</p>
+
+<p>The soundest approach of all to the study of horizontal distribution
+at night, and one which may be employed any month, anywhere,
+permitting the accumulation of statistically significant quantities
+of data, is to set up two telescopes in close proximity. Provided
+the flight overhead is evenly dispersed, each observer should
+count approximately the same number of birds in a given interval
+of time. Some data of this type are already available. On May 19-20,
+at Urbana, Illinois, while stationed twenty feet apart making
+parallax studies with two telescopes to determine the height above
+<span class="pagenum"><a name="Page_413" id="Page_413">[Pg_413]</a></span>
+the earth of the migratory birds, Carpenter and Stebbins (<i>loci cit.</i>)
+saw seventy-eight birds in two and one-half hours. Eleven were
+seen by both observers, thirty-three by Stebbins only, and thirty-four
+by Carpenter only. On October 10, 1905, at the same place, in
+two hours, fifty-seven birds were counted, eleven being visible
+through both telescopes. Of the remainder, Stebbins saw seventeen
+and Carpenter, twenty-nine. On September 12, 1945, at Baton
+Rouge, Louisiana, in an interval of one hour and forty minutes, two
+independent observers each counted six birds. Again, on October 17,
+1945, two observers each saw eleven birds in twenty-two minutes.
+On April 10, 1946, in one hour and five minutes, twenty-four birds
+were seen through one scope and twenty-six through the other. Likewise
+on May 12, 1946, in a single hour, seventy-three birds were
+counted by each of two observers. The Baton Rouge observations
+were made with telescopes six to twelve feet apart. These results
+show a remarkable conformity, though the exceptional October observation
+of Carpenter and Stebbins indicates the desirability of continuing
+these studies, particularly in the fall.</p>
+
+<p>On the whole, the available evidence points to the conclusion that
+night migration differs materially from the kind of daytime migration
+with which we are generally familiar. Birds are apparently
+evenly spread throughout the sky, with little tendency to fly in
+flocks. It must be remembered, however, that only in the case of
+night migration have objective and truly quantitative studies been
+made of horizontal distribution. There is a possibility that our impressions
+of diurnal migration are unduly influenced by the fact
+that the species accustomed to flying in flocks are the ones that attract
+the most attention.</p>
+
+<p>These conclusions relate to the uniformity of migration in terms
+of short distances only, in the immediate vicinity of an observation
+station. The extent to which they may be applied to broader fronts
+is a question that may be more appropriately considered later, in
+connection with continental aspects of the problem.</p>
+<br />
+<br />
+
+<a name="Density_as_a_Function"></a>
+<div class="caption3 smcap">B. Density As Function Of The Hour Of The Night</div>
+
+<p>There are few aspects of nocturnal migration about which there is
+less understanding than the matter of when the night flight begins,
+at what rate it progresses, and for what duration it continues. One
+would think, however, that this aspect of the problem, above most
+others, would have been thoroughly explored by some means of
+objective study. Yet, this is not the case. Indeed, I find not a
+<span class="pagenum"><a name="Page_414" id="Page_414">[Pg_414]</a></span>
+single paper in the American literature wherein the subject is discussed,
+although some attention has been given the matter by
+European ornithologists. Siivonen (1936) recorded in Finland the
+frequency of call notes of night migrating species of <i>Turdus</i> and
+from these data plotted a time curve showing a peak near midnight.
+Bergman (1941) and Putkonen (1942), also in Finland, studied the
+night flights of certain ducks (<i>Clangula hyemalis</i> and <i>Oidemia fusca</i>
+and <i>O. nigra</i>) and a goose (<i>Branta bernicla</i>) and likewise demonstrated
+a peak near midnight. However, these studies were made
+at northern latitudes and in seasons characterized by evenings of
+long twilight, with complete darkness limited to a period of short
+duration around midnight. Van Oordt (1943: 34) states that in
+many cases migration lasts all night; yet, according to him, most
+European investigators are of the opinion that, in general, only a
+part of the night is used, that is, the evening and early morning
+hours. The consensus of American ornithologists seems to be that
+migratory birds begin their flights in twilight or soon thereafter
+and that they remain on the wing until dawn. Where this idea
+has been challenged at all, the implication seems to have been that
+the flights are sustained even longer, often being a continuation far
+into the night of movements begun in the daytime. The telescopic
+method fails to support either of these latter concepts.</p>
+<br />
+<br />
+
+<a name="Fig_26"></a>
+<div class="center">
+<img src="images/fig_26.png" width="458" height="222" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 26.</span> Average hourly station densities in spring of 1948. This curve
+represents the arithmetic mean obtained by adding all the station densities
+for each hour, regardless of date, and dividing the sum by the number of sets
+of observations at that hour (CST).</div>
+</div>
+<br />
+<br />
+
+<div class="caption3nci">The Time Pattern</div>
+
+<p>When the nightly curves of density at the various stations are
+plotted as a function of time, a salient fact emerges&mdash;that the flow
+<span class="pagenum"><a name="Page_415" id="Page_415">[Pg_415]</a></span>
+of birds is in no instance sustained throughout the night. The
+majority of the curves rise smoothly from near zero at the time of
+twilight to a single peak and then decline more or less symmetrically
+to near the base line before dawn. The high point is reached in or
+around the eleven to twelve o'clock interval more often than at any
+other time.</p>
+<br />
+<br />
+
+<a name="Fig_27"></a>
+<div class="center">
+<img src="images/fig_27.png" width="489" height="235" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 27.</span> Hourly station densities plotted as a percentage of peak. The
+curve is based only on those sets of data where observations were continued
+long enough to include the nightly peak. In each set of data the station
+density for each hour has been expressed as a percentage of the peak for the
+night at the station in question. All percentages for the same hour on all
+dates have been averaged to obtain the percentile value of the combined
+station density at each hour (CST).</div>
+</div>
+<br />
+<br />
+
+<p><a href="#Fig_26">Figure 26</a>, representing the average hourly densities for all stations
+on all nights of observation, demonstrates the over-all effect of
+these tendencies. Here the highest density is reached in the hour
+before midnight with indications of flights of great magnitude also
+in the hour preceding and the hour following the peak interval.
+The curve ascends somewhat more rapidly than it declines, which
+fact may or may not be significant. Since there is a great disproportion
+in the total volume of migration at different localities, the
+thought might be entertained that a few high magnitude stations,
+such as Tampico and Progreso, have imposed their own characteristics
+on the final graph. Fortunately, this idea may be tested by
+subjecting the data to a second treatment. If hourly densities are
+expressed as a percentage of the nightly peak, each set of observations,
+regardless of the number of birds involved, carries an equal
+weight in determining the character of the over-all curve. <a href="#Fig_27">Figure 27</a>
+shows that percentage analysis produces a curve almost identical
+with the preceding one. To be sure, all of the individual curves do
+not conform with the composite, either in shape or incidence of
+<span class="pagenum"><a name="Page_416" id="Page_416">[Pg_416]</a></span>
+peak. The extent of this departure in the latter respect is evident
+from <a href="#Fig_28">Figure 28</a>, showing the number of individual nightly station
+curves reaching a maximum peak in each hour interval. Even this
+graph demonstrates that maximum densities near midnight represent
+the typical condition.</p>
+<br />
+<br />
+
+<a name="Fig_28"></a>
+<div class="center">
+<img src="images/fig_28.png" width="472" height="227" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 28.</span> Incidence of maximum peak at the various hours of the night in
+1948. "Number of stations" represents the total for all nights of the numbers
+of station peaks falling within a given hour.</div>
+</div>
+<br />
+<br />
+
+<p>The remarkable smoothness and consistency of the curves shown
+in Figures <a href="#Fig_26">26</a> and <a href="#Fig_27">27</a> seem to lead directly to the conclusion that
+the volume of night migration varies as a function of time. Admittedly
+other factors are potentially capable of influencing the
+number of birds passing a given station in a given hour. Among
+these are weather conditions, ecological patterns, and specific topographical
+features that might conceivably serve as preferred avenues
+of flight. However, if any of these considerations were alone
+responsible for changes in the numbers of birds seen in successive
+intervals, the distribution of the peak in time could be expected to
+be haphazard. For example, there is no reason to suppose that the
+cone of observation would come to lie over favored terrain at precisely
+the hour between eleven and twelve o'clock at so many widely
+separated stations. Neither could the topographical hypothesis explain
+the consistently ascending and descending pattern of the ordinates
+in <a href="#Fig_28">Figure 28</a>. This is not to say that other factors are without
+effect; they no doubt explain the divergencies in the time pattern
+exhibited by <a href="#Fig_28">Figure 28</a>. Nevertheless, the underlying circumstances
+are such that when many sets of data are merged these other influences
+are subordinated to the rise and fall of an evident time pattern.
+<span class="pagenum"><a name="Page_417" id="Page_417">[Pg_417]</a></span>
+Stated in concrete terms, the time frequencies shown in the graphs
+suggest the following conclusions: first, nocturnal migrations are
+not a continuation of daytime flights; second, nearly all night migrants
+come to earth well before dawn; and, third, in each hour of
+the night up until eleven or twelve o'clock there is typically a progressive
+increase in the number of birds that have taken wing and
+in each of the hours thereafter there is a gradual decrease. Taken at
+its face value, the evidence seems to indicate that birds do not begin
+their night migrations <i>en masse</i> and remain on the wing until dawn
+and that in all probability most of them utilize less than half of the
+night.</p>
+
+<p>Interestingly enough, the fact that the plot points in <a href="#Fig_26">Figure 26</a>
+lie nearly in line tempts one to a further conclusion. The curve behaves
+as an arithmetic progression, indicating that approximately
+the same number of birds are leaving the ground in each hour interval
+up to a point and that afterwards approximately the same number
+are descending within each hour. However, some of the components
+making up this curve, as later shown, are so aberrant in
+this regard that serious doubt is cast on the validity of this generalization.</p>
+
+<p>Because the results of these time studies are unexpected and startling,
+I have sought to explore other alternative explanations and
+none appears to be tenable. For example, the notion that the varying
+flight speeds of birds might operate in some way to produce a
+cumulative effect as the night progresses must be rejected on close
+analysis. If birds of varying flight speeds are continuously and
+evenly distributed in space, a continuous and even flow would result
+all along their line of flight. If they are haphazardly distributed in
+space, a correspondingly haphazard density pattern would be expected.</p>
+
+<p>Another explanation might be sought in the purely mathematical
+effects of the method itself. The computational procedure assumes
+that the effective area of the sample is extremely large when the
+moon is low, a condition that usually obtains in the early hours of
+the evening in the days surrounding the full moon. Actually no
+tests have yet been conducted to ascertain how far away a silhouette
+of a small bird can be seen as it passes before the moon. Consequently,
+it is possible that some birds are missed under these conditions
+and that the effective field of visibility is considerably smaller
+than the computed field of visibility. The tendency, therefore, may
+be to minimize the densities in such situations more than is justified.
+<span class="pagenum"><a name="Page_418" id="Page_418">[Pg_418]</a></span>
+However, in many, if not most, cases, the plotting of the actual
+number of birds seen, devoid of any mathematical procedures, results
+in an ascending and descending curve.</p>
+<br />
+<br />
+
+<a name="Fig_29"></a>
+<div class="center">
+<img src="images/fig_29.png" width="484" height="541" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 29.</span> Various types of density-time curves. (A) Near typical, Ottumwa,
+April 22-23; (B) random fluctuation, Stillwater, April 23-24; (C) bimodal,
+Knoxville, April 22-23; (D) sustained peak, Ottumwa, April 21-22; (E) early
+peak, Oak Grove, May 21-22; (F) late peak, Memphis, April 23-24.</div>
+</div>
+<br />
+<br />
+
+<p>A third hypothesis proposes that all birds take wing at nearly the
+same time, gradually increase altitude until they reach the mid-point
+of their night's journey, and then begin a similarly slow descent.
+Since the field of observation of the telescope is conical, it is assumed
+that the higher the birds arise into the sky the more they increase
+their chances of being seen. According to this view, the changes
+<span class="pagenum"><a name="Page_419" id="Page_419">[Pg_419]</a></span>
+in the density curve represent changes in the opportunity to see
+birds rather than an increase or decrease in the actual number of
+migrants in the air. Although measurements of flight altitude at
+various hours of the night have not been made in sufficient number
+to subject this idea to direct test, it is hardly worthy of serious
+consideration. The fallacy in the hypothesis is that the cone of
+observation itself would be rising with the rising birds so that
+actually the greatest proportion of birds flying would still be seen
+when the field of observation is in the supine position of early
+evening.</p>
+
+<p>It cannot be too strongly emphasized that the over-all time
+curves just discussed have been derived from a series of individual
+curves, some of which differ radically from the composite pattern.
+In <a href="#Fig_29">Figure 29</a>, six dissimilar types are shown. This variation is not
+surprising in view of the fact that many other causative factors
+aside from time operate on the flow of birds from hour to hour.
+<a href="#Fig_29">Figure 29A</a> illustrates how closely some individual patterns conform
+with the average. <a href="#Fig_29">Figure 29B</a> is an example of a random type
+of fluctuation with no pronounced time character. It is an effect
+rarely observed, occurring only in the cases where the number of
+birds observed is so small that pure chance has a pronounced effect
+on the computed densities; its vacillations are explicable on that
+account alone. Errors of sampling may similarly account for some,
+though not all, of the curves of the bimodal type shown in <a href="#Fig_29">Figure 29C</a>.
+Some variation in the curves might be ascribed to the variations
+in kinds of species comprising the individual flights at different
+times at different places, provided that it could be demonstrated
+that different species of birds show dissimilar temporal patterns.
+The other atypical patterns are not so easily dismissed and will be
+the subject of inquiry in the discussions that follow. It is significant
+that in spite of the variety of the curves depicted, which represent
+every condition encountered, in not a single instance is the density
+sustained at a high level throughout the night. <ins title="TN: Morover => Moreover">Moreover</ins>, these
+dissident patterns merge into a remarkably harmonious, almost
+normal, average curve.</p>
+
+<p>When, at some future date, suitable data are available, it would
+be highly desirable to study the average monthly time patterns to
+ascertain to what extent they may deviate from the over-all average.
+At present this is not justifiable because there are not yet enough
+sets of data in any two months representing the same selection of
+stations.</p>
+
+<div class="caption3nc"><i>Correlations with Other Data</i><span class="pagenum"><a name="Page_420" id="Page_420">[Pg_420]</a></span></div>
+
+<p>It is especially interesting to note that the data pertaining to this
+problem derived from other methods of inquiry fit the conclusions
+adduced by the telescopic method. Overing (1938), who for several
+years kept records of birds striking the Washington Monument,
+stated that the record number of 576 individuals killed on the night
+of September 12, 1937, all came down between 10:30 P.&nbsp;M. and midnight.
+His report of the mortality on other nights fails to mention
+the time factor, but I am recently informed by Frederick C. Lincoln
+(<i>in litt.</i>) that it is typical for birds to strike the monument in
+greatest numbers between ten and twelve o'clock at night. At the
+latter time the lights illuminating the shaft are extinguished, thus
+resulting in few or no casualties after midnight. The recent report
+by Spofford (1949) of over 300 birds killed or incapacitated at the
+Nashville airport on the night of September 9-10, 1948, after flying
+into the light beam from a ceilometer, is of interest in this connection
+even though the cause of the fatality is shrouded in mystery.
+It may be noted, however, that "most of the birds fell in the first
+hour," which, according to the account, was between 12:30 A.&nbsp;M. and
+1:30 A.&nbsp;M. Furthermore, birds killed at the Empire State Building in
+New York on the night of September 10-11, 1948, began to strike
+the tower "shortly after midnight" (Pough, 1948). Also it will be
+recalled that the observations of Stone (<i>loc. cit.</i>), already referred
+to in this paper (page 410), show a situation where the flight in the
+early part of the night was negligible but mounted to a peak between
+ten and eleven o'clock, with continuing activity at least until
+midnight.</p>
+
+<p>All of these observations are of significance in connection with
+the conclusions herein advanced, but by far the most striking correlation
+between these present results and other evidences is found
+in the highly important work of various European investigators
+studying the activity of caged migratory birds. This work was
+recently reviewed and extended by Palmgren (1944) in the most
+comprehensive treatise on the subject yet published. Palmgren recorded,
+by an electrically operated apparatus, the seasonal, daily,
+and hourly activity patterns in caged examples of two typical
+European migrants, <i>Turdus ericetorum <ins title="TN: philomelas => philomelos">philomelos</ins></i> Brehm and <i>Erithacus
+rubecula</i> (Linnaeus). Four rather distinct seasonal phases
+in activity of the birds were discerned: <i>winter non-migratory</i>,
+<i>spring migratory</i>, <i>summer non-migratory</i>, and <i>autumn migratory</i>.
+The first of these is distinguished by morning and evening maxima
+<span class="pagenum"><a name="Page_421" id="Page_421">[Pg_421]</a></span>
+of activity, the latter being better developed but the former being
+more prolonged. Toward the beginning of migration, these two periods
+of activity decline somewhat. The second, or spring migratory
+phase, which is of special interest in connection with the present
+problem, is characterized by what Palmgren describes as nightly
+migratory restlessness (<i>Zugunruhe</i>). The morning maximum, when
+present, is weaker and the evening maximum often disappears altogether.
+Although variations are described, the migratory restlessness
+begins ordinarily after a period of sleep ("sleeping pause") in
+the evening and reaches a maximum and declines before midnight.</p>
+
+<p>This pattern agrees closely with the rhythm of activity indicated
+by the time curves emerging from the present research. Combining
+the two studies, we may postulate that most migrants go to sleep for
+a period following twilight, thereby accounting for the low densities
+in the early part of the night. On awakening later, they begin to
+exhibit migratory restlessness. The first hour finds a certain number
+of birds sufficiently stimulated so that they rise forthwith into
+the air. In the next hour still others respond to this urge and they
+too mount into the air. This continues until the "restlessness" begins
+to abate, after which fewer and fewer birds take wing. By this
+time, the birds that began to fly early are commencing to descend,
+and since their place is not being filled by others leaving the ground,
+the density curve starts its decline. Farner (1947) has called attention
+to the basic importance of the work by Palmgren and the many
+experimental problems it suggests. Of particular interest would be
+studies comparing the activity of caged American migrant species
+and the nightly variations in the flight rates.</p>
+
+
+<div class="caption3nci">The Baton Rouge Drop-off</div>
+
+<p>As already stated, the present study was initiated at Baton Rouge,
+Louisiana, in 1945, and from the outset a very peculiar density time
+pattern was manifest. I soon found that birds virtually disappeared
+from the sky after midnight. Within an hour after the termination
+of twilight, the density would start to ascend toward a peak which
+was usually reached before ten o'clock, and then would begin, surprisingly
+enough, a rapid decline, reaching a point where the migratory
+flow was negligible. In <a href="#Fig_30">Figure 30</a> the density curves are shown for
+five nights that demonstrate this characteristically early decline in
+the volume of migration at this station. Since, in the early stages
+of the work, coördinates of apparent pathways of all the birds seen
+were not recorded, I am unable now to ascertain the direction of
+flight and thereby arrive at a density figure based on the dimension
+of the cone and the length of the front presented to birds flying in
+certain directions. It is feasible, nevertheless, to compute what I
+have termed a "plus or minus" flight density figure stating the rate
+of passage of birds in terms of the maximum and minimum corrections
+which all possible directions of flight would impose. In other
+words, density is here computed, first, as if all the birds were flying
+perpendicular to the long axis of the ellipse, and, secondly, as if all
+the birds were flying across the short axis of the ellipse. Since the
+actual directions of flight were somewhere between these two extremes,
+the "plus or minus" density figure is highly useful.</p>
+<br />
+<br />
+
+<a name="Fig_30"></a>
+<p><span class="pagenum"><a name="Page_422" id="Page_422">[Pg_422]</a></span></p>
+<div class="center">
+<img src="images/fig_30.png" width="383" height="562" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 30.</span> Density-time curves on various nights at Baton Rouge. (A) April
+25, 1945; (B) April 15, 1946; (C) May 10, 1946; (D) May 15, 1946; (E) April
+22-23, 1948. These curves are plotted on a "plus or minus" basis as described in
+the text, with the bottom of the curve representing the minimum density and
+the top of the curve the maximum.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_423" id="Page_423">[Pg_423]</a></span>
+The well-marked decline before midnight in the migration rates
+at Baton Rouge may be regarded as one of the outstanding results
+emerging from this study. Many years of ornithological investigation
+in this general region failed to suggest even remotely that a
+situation of this sort obtained. Now, in the light of this new fact,
+it is possible for the first time to rationalize certain previously incongruous
+data. Ornithologists in this area long have noted that
+local storms and cold-front phenomena at night in spring sometimes
+precipitate great numbers of birds, whereupon the woods are filled
+the following day with migrants. On other occasions, sudden storms
+at night have produced no visible results in terms of bird densities
+the following day. For every situation such as described by Gates
+(1933) in which hordes of birds were forced down at night by inclement
+weather, there are just as many instances, even at the height
+of spring migration, when similar weather conditions yielded no
+birds on the ground. However, the explanation of these facts is
+simple; for we discover that storms that produced birds occurred
+before midnight and those that failed to produce birds occurred after
+that time (the storm described by Gates occurred between 8:30 and
+9:00 P.&nbsp;M.).</p>
+
+<p>The early hour decline in density at Baton Rouge at first did not
+seem surprising in view of the small amount of land area between
+this station and the Gulf of Mexico. Since the majority of the
+birds destined to pass Baton Rouge on a certain night come in
+general from the area to the south of that place, and since the
+distances to various points on the coast are slight, we inferred
+that a three-hour flight from even the more remote points would
+probably take the bulk of the birds northward past Baton Rouge.
+In short, the coastal plain would be emptied well before midnight
+of its migrant bird life, or at least that part of the population destined
+to migrate on any particular night in question. Although data
+<span class="pagenum"><a name="Page_424" id="Page_424">[Pg_424]</a></span>
+in quantity are not available from stations on the coastal plain other
+than Baton Rouge, it may be pointed out that such observations as
+we do have, from Lafayette and New Orleans, Louisiana, and from
+Thomasville, Georgia, are in agreement with this hypothesis.</p>
+
+<p>A hundred and seventy miles northward in the Mississippi Valley,
+at Oak Grove, Louisiana, a somewhat more normal density pattern
+is manifested. There, in four nights of careful observation, a pronounced
+early peak resulted on the night of May 21-22 (<a href="#Fig_29">Figure 29E</a>),
+but on the other three nights significant densities held up until
+near twelve o'clock, thereby demonstrating the probable effect of
+the increased amount of land to the south of the station.</p>
+
+<p>Subsequent studies, revealing the evident existence of an underlying
+density time pattern, cast serious doubt on the explanations
+just advanced of the early decline in the volume of migration at
+Baton Rouge. It has as yet been impossible to reconcile the early
+drop-off at this station with the idea that birds are still mounting
+into the air at eleven o'clock, as is implied by the ideal time curves.</p>
+<br />
+<br />
+
+<a name="Migration_in_Relation_to_Topography"></a>
+<div class="caption3 smcap">C. MIGRATION IN RELATION TO TOPOGRAPHY</div>
+
+<p>To this point we have considered the horizontal distribution of
+birds in the sky only on a very narrow scale and mainly in terms
+of the chance element in observations. Various considerations have
+supported the premise that the spread of nocturnal migration is
+rather even, at least within restricted spacial limits and short intervals
+of time. This means that in general the flow of birds from
+hour to hour at a single station exhibits a smooth continuity. It
+does not mean that it is a uniform flow in the sense that approximately
+the same numbers of birds are passing at all hours, or at all
+localities, or even on all one-mile fronts in the same locality. On the
+contrary, there is evidence of a pronounced but orderly change
+through the night in the intensity of the flight, corresponding to a
+basic and definitely timed cycle of activity. Other influences may
+interfere with the direct expression of this temporal rhythm as it
+is exhibited by observations at a particular geographical location.
+Among these, as we have just seen, is the disposition of the areas
+that offer suitable resting places for transient birds and hence contribute
+directly and immediately to the flight overhead. A second
+possible geographical effect is linked with the question of the tendency
+of night migrants to follow topographical features.</p>
+
+
+<div class="caption3nci">General Aspects of the Topographical Problem</div>
+
+<p>That many diurnal migrants tend to fly along shorelines, rivers,
+<span class="pagenum"><a name="Page_425" id="Page_425">[Pg_425]</a></span>
+and mountain ridges is well known, but this fact provides no assurance
+that night migrants do the same thing. Many of the obvious
+advantages of specialized routes in daylight, such as feeding opportunities,
+the lift provided by thermal updrafts, and the possible
+aid of certain landmarks in navigation, assume less importance after
+night falls. Therefore, it would not be safe to conclude that <i>all</i>
+nocturnal migrants operate as do <i>some</i> diurnal migrants. For instance,
+the passage of great numbers of certain species of birds along
+the Texas coast in daylight hours cannot be regarded as certain
+proof that the larger part of the nocturnal flight uses the same route.
+Neither can we assume that birds follow the <ins title="TN: Mississippii => Mississippi">Mississippi</ins> River at
+night simply because we frequently find migrants concentrated along
+its course in the day. Fortunately we shall not need to speculate
+indefinitely on this problem; for the telescopic method offers a means
+of study based on what night migrants are doing <i>at night</i>. Two
+lines of attack may be pursued. First we may compare flight
+densities obtained when the field of the telescope lies over some outstanding
+topographical feature, such <ins title="TN: a => as">as</ins> a river, with the recorded
+volume of flight when the cone of observation is directed away from
+that feature. Secondly, we may inquire how the major flight directions
+at a certain station are oriented with respect to the terrain.
+If the flight is concentrated along a river, for instance, the flight
+density curve should climb upward as the cone of observation swings
+over the river, <i>regardless of the hour at which it does so</i>. The effect
+should be most pronounced if the observer were situated on the river
+bank, so that the cone would eventually come to a position directly
+along the watercourse. Though in that event birds coming up the
+river route would be flying across the short axis of an elliptical section
+of the cone, the fact that the whole field of observation would be
+in their path should insure their being seen in maximum proportions.
+If, on the other hand, the telescope were set up some distance away
+from the river so that the cone merely moved <i>across</i> its course, only
+a section of the observation field would be interposed on the main
+flight lane.</p>
+
+<p>The interaction of these possibilities with the activity rhythm
+should have a variety of effects on the flight density curves. If the
+cone comes to lie over the favored topographical feature in the hour
+of greatest migrational activity, the results would be a simple sharp
+peak of doubtful meaning. However, since the moon rises at a different
+time each evening, the cone likewise would reach the immediate
+<span class="pagenum"><a name="Page_426" id="Page_426">[Pg_426]</a></span>
+vicinity of the terrain feature at a different time each night.
+As a result, the terrain peak would move away from its position of
+coincidence with the time peak on successive dates, producing first,
+perhaps, a sustention of peak and later a definitely bimodal curve.
+Since other hypotheses explain double peaks equally well, their
+mere existence does not necessarily imply that migrants actually
+do travel along narrow topographical lanes. Real proof requires
+that we demonstrate a moving peak, based on properly corrected
+density computations, corresponding always with the position of the
+cone over the most favored terrain, and that the flight vectors be
+consistent with the picture thus engendered.</p>
+
+
+<div class="caption3nci">The Work of Winkenwerder</div>
+
+<p>To date, none of the evidence in favor of the topographical hypothesis
+completely fills these requirements. Winkenwerder (<i>loc. cit.</i>),
+in analyzing the results of telescopic counts of birds at Madison and
+Beloit, Wisconsin, Detroit and Ann Arbor, Michigan, and at Lake
+Forest, Illinois, between 1898 and 1900, plotted the number of birds
+seen at fifteen-minute intervals as a function of the time of the night.
+He believed that the high points in the resulting frequency histograms
+represented intervals when the field of the telescope was
+moving over certain topographically determined flight lanes, though
+he did not specify in all cases just what he assumed the critical
+physiographic features to be. Especially convincing to him were
+results obtained at Beloit, where the telescope was situated on the
+east bank of the Rock River, on the south side of the city. Immediately
+below Beloit the river turns southwestward and continues
+in this direction about five miles before turning again to flow in a
+southeastward course for approximately another five miles. In this
+setting, on two consecutive nights of observation in May, the number
+of birds observed increased tremendously in the 2 to 3 A.&nbsp;M. interval,
+when, according to Winkenwerder's interpretation of the data (he
+did not make the original observations at Beloit himself), the telescope
+was pointing directly down the course of the river. This conclusion
+is weakened, however, by notable inconsistencies. Since
+the moon rises later each evening, it could not have reached the
+same position over the Rock River at the same time on both May
+12-13 and May 13-14, and therefore, if the peaks in the graph were
+really due to a greater volume of migration along the watercourse,
+they should not have so nearly coincided. As a matter of fact the
+incidence of the peak on May 12-13 should have preceded that of
+<span class="pagenum"><a name="Page_427" id="Page_427">[Pg_427]</a></span>
+the peak on May 13-14; whereas his figure shows the reverse to
+have been true. Singularly enough, Winkenwerder recognized this
+difficulty in his treatment of the data from Madison, Wisconsin.
+Unable to correlate the peak period with the Madison terrain by the
+approach used for Beloit, he plotted the observations in terms of
+hours after moonrise instead of standard time. This procedure was
+entirely correct; the moon does reach approximately the same position
+at each hour after its rise on successive nights. The surprising
+thing is that Winkenwerder did not seem to realize the incompatibility
+of his two approaches or to realize that he was simply choosing
+the method to suit the desired results.</p>
+
+<p>Furthermore, as shown in Part I of this paper, the number of birds
+seen through the telescope often has only an indirect connection with
+the actual number of birds passing over. My computations reveal
+that the highest counts of birds at Beloit on May 12-13 were recorded
+when the moon was at an altitude of only 8&deg; to 15&deg; and, that
+when appropriate allowance is made for the immense size of the field
+of observation at this time, the partially corrected flight density for
+the period is not materially greater than at some other intervals
+in the night when the telescope was not directed over the course of
+the Rock River. These allowances do not take the direction factor
+into consideration. Had the birds been flying at right angles to the
+short axis of an elliptical section of the cone throughout the night, the
+flight density in the period Winkenwerder considered the peak would
+have been about twice as high as in any previous interval. On the
+other hand, if they had been flying across the long axis at all times,
+the supposed peak would be decidedly inferior to the flight density
+at 10 to 11:00 P.&nbsp;M., before the cone came near the river.</p>
+
+<p>Admittedly, these considerations contain a tremendous element of
+uncertainty. They are of value only because they expose the equal
+uncertainty in Winkenwerder's basic evidence. Since the coördinates
+of the birds' apparent pathways at Beloit were given, I at
+first entertained the hope of computing the flight densities rigorously,
+by the method herein employed. Unfortunately, Winkenwerder
+was apparently dealing with telescopes that gave inverted images,
+and he used a system for recording coördinates so ambiguously described
+that I am not certain I have deciphered its true meaning.
+When, however, his birds are plotted according to the instructions
+as he stated them, the prevailing direction of flight indicated by the
+projection formula falls close to west-northwest, not along the course
+of the Rock River, but <i>at direct right angles to it</i>.</p>
+<br />
+<br />
+
+<a name="Fig_31"></a>
+<p><span class="pagenum"><a name="Page_428" id="Page_428">[Pg_428]</a></span></p>
+<div class="center">
+<img src="images/fig_31.png" width="470" height="483" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 31.</span> Directional components in the flight at Tampico on three nights in 1948. The lengths of the sector vectors are determined by their respective
+densities expressed as a percentage of the station density for that night; the
+vector resultants are plotted from them by standard procedure. Thus, the
+nightly diagrams are not on the same scale with respect to the actual number
+of birds involved.</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_32"></a>
+<div class="center">
+<img src="images/fig_32.png" width="488" height="240" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 32.</span> Hourly station density curve at Tampico, Tamaulipas, on the night of April 21-22, 1948 (CST).</div>
+</div>
+<br />
+<br />
+
+<div class="caption3nci">Interpretation of Recent Data</div>
+
+<p>I am in a position to establish more exact correlations between
+flight density and terrain features in the case of current sets of
+observations. Some of these data seem at first glance to fit the idea
+of narrow topographically-oriented flight lanes rather nicely. At
+Tampico, where six excellent sets of observations were made in
+March and April, 1948, the telescope was set up on the beach within
+a few yards of the Gulf of Mexico. As can be seen from <a href="#Fig_25">Figure 25</a>
+(<i>ante</i>), the slant of the coastline at this point is definitely west of
+north, as is also the general trend of the entire coast from southern
+Veracruz to southern Tamaulipas (see <a href="#Fig_34">Figure 34</a>, beyond). The
+<span class="pagenum"><a name="Page_429" id="Page_429">[Pg_429]</a></span>
+over-all vector resultant of all bird flights at this station was
+N 11&deg; W, and, as will be seen from <a href="#Fig_31">Figure 31</a>, none of the nightly
+vector resultants in April deviates more than one degree from this
+average. Thus the prevailing direction of flight, as computed, agrees
+with the trend of the coast at the precise point of the observations,
+at least to the extent that both are west of north. To be sure, the
+individual sector vectors indicate that not all birds were following
+this course; indeed, some appear to have been flying east of north,
+heading for a landfall in the region of Brownsville, Texas, and a
+very few to have been traveling northeastward toward the central
+Gulf coast. But it must be remembered that a certain amount of
+computational deviation and of localized zigzagging in flight must be
+anticipated. Perhaps none of these eastward vectors represents an
+actual extended flight path. The nightly vector resultants, on the
+other hand, are so consistent that they have the appearance of remarkable
+accuracy and tempt one to draw close correlations with the
+terrain. When this is done, it is found that, while the prevailing
+flight direction is 11&deg; west of north, the exact slant of the coastline at
+the location of the station is about 30&deg; west of north, a difference of
+around 19&deg;. It appears, therefore, that the birds were not following
+the shoreline precisely but cutting a chord about ten miles long across
+an indentation of the coast. If it be argued that the method of
+calculation is not accurate enough to make a 19&deg; difference significant,
+and that most of the birds might have been traveling along
+the beach after all, it can be pointed out with equal justification
+that, if this be so, the 11&deg; divergence from north does not mean
+anything either and that perhaps the majority of the birds were
+<span class="pagenum"><a name="Page_430" id="Page_430">[Pg_430]</a></span>
+going due north. We are obliged to conclude either that the main
+avenue of flight paralleled the disposition of the major topographical
+features only in a general way or that the angle between the line of
+the coast and true north is not great enough to warrant any inference
+at all.</p>
+
+<p>Consideration of the Tampico density curves leads to similarly
+ambiguous results. On the night of April 21-22, as is evident from
+a comparison of Figures <a href="#Fig_25">25</a> and <a href="#Fig_32">32</a>, the highest flight density occurred
+when the projection of the cone on the terrain was wholly
+included within the beach. This is very nearly the case on the night
+of April 23-24 also, the positions of the cone during the peak period
+of density being only about 16&deg; apart. (On the intervening date,
+clouds prevented continuous observation during the critical part of
+the night.) These correlations would seem to be good evidence that
+most of these night migrants were following the coastline of the
+Gulf of Mexico. However, the problem is much more complicated.
+The estimated point of maximum flight density fell at 10:45 P.&nbsp;M.
+on April 21-22 <ins title="TN: at => and">and</ins> 11:00 P.&nbsp;M. on April 23-24, both less than an hour
+from the peak in the ideal time curve (<a href="#Fig_26">Figure 26</a>, <i>ante</i>). We cannot
+be sure, therefore, that the increase in density coinciding with the
+position of the moon over the beach is not an increase which would
+have occurred anyway. Observations conducted several nights
+before or after the second quarter, when the moon is not on or near
+its zenith at the time of the predictable peak in the density curve,
+would be of considerable value in the study of this particular
+problem.</p>
+
+<p>The situation at Tampico has been dealt with at length because,
+among all the locations for which data are available, it is the one
+that most strongly supports the topographical hypothesis. In none
+of the other cases have I been able to find a definite relation between
+the direction of migration and the features of the terrain. Studies
+of data from some of these stations disclose directional patterns that
+vary from night to night only slightly more than does the flight at
+Tampico. In three nights of observation at Lawrence, Kansas,
+marked by very high densities, the directional trend was north by
+north-northeast with a variation of less than 8&deg;, yet Lawrence is so
+situated that there seems to be no feature of the landscape locally
+or in the whole of eastern Kansas or of western Missouri that coincides
+with this heading. At Mansfield, Louisiana, in twelve nights
+of observation, the strong east by northeast trend varied less than
+15&deg;, but again there appears to be no correlation over a wide area
+<span class="pagenum"><a name="Page_431" id="Page_431">[Pg_431]</a></span>
+between this direction and any landmarks. And, at Progreso, Yucatán,
+where the vector resultants were 21&deg; and 27&deg; on successive
+nights, most of the birds seen had left the land and were beginning
+their flight northward over the trackless waters of the Gulf of Mexico.
+Furthermore, as I have elsewhere pointed out (1946: 205), the
+whole northern part of the Yucatán Peninsula itself is a flat terrain,
+unmarked by rivers, mountains, or any other strong physiographic
+features that conceivably might be followed by birds.</p>
+<br />
+<br />
+
+<a name="Fig_33"></a>
+<div class="center">
+<img src="images/fig_33.png" width="485" height="373" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 33.</span> The nightly net trend of migrations at three stations in 1948.
+Each arrow is the vector resultant for a particular night, its length expressing the
+nightly density as a percentage of the total station density for the nights represented.
+Thus, the various station diagrams are not to the same scale.</div>
+</div>
+<br />
+<br />
+
+<p>In <a href="#Fig_33">Figure 33</a> I have shown the directional patterns at certain stations
+where, unlike the cases noted above, there is considerable
+change on successive nights. Each vector shown is the vector resultant
+for one particular night. The lengths of the vectors have been
+determined by their respective percentages of the total computed
+density, or total station magnitude, for all the nights in question.
+In other words, the lengths of the individual vectors denote the percentile
+rôle that each night played in the total density. From the
+directional spread at these stations it becomes apparent that if most
+of the birds were traveling along <ins title='TN: Added "a"'>a</ins> certain topographic feature on one
+<span class="pagenum"><a name="Page_432" id="Page_432">[Pg_432]</a></span>
+night, they could not have been traveling along the same feature on
+other nights.</p>
+
+<p>The possibility should be borne in mind, however, that there may
+be more than one potential topographic feature for birds to follow
+at some stations. Moreover, it is conceivable that certain species
+might follow one feature that would lead them in the direction of
+their ultimate goal, whereas other species, wishing to go in an entirely
+different direction, might follow another feature that would
+lead them toward their respective destination. It would seem unlikely,
+however, that the species composition of the nocturnal flights
+would change materially from night to night, although there is a
+strong likelihood that it might do so from week to week and certainly
+from month to month.</p>
+
+<p>By amassing such data as records of flight direction along the
+same coast from points where the local slant of the shoreline is
+materially different, and comparisons of the volume of migration at
+night along specialized routes favored during the day with the flight
+densities at progressive distances from the critical terrain feature
+involved, we shall eventually be able to decide definitely the rôle
+topography plays in bird migration. We cannot say on the basis of
+the present ambiguous evidence that it is not a factor in determining
+which way birds fly, but, if I had to hazard a guess one way or the
+other, I would be inclined to discount the likelihood of its proving
+a major factor.</p>
+
+<a name="Geographical_Factors"></a>
+<div class="caption3 smcap">D. Geographical Factors and the Continental Density
+Pattern</div>
+
+<p>A study of the total nightly or seasonal densities at the various
+stations brings forth some extremely interesting factors, many of
+which, however, cannot be fully interpreted at this time. A complete
+picture of the magnitude of migration at a given station cannot
+be obtained from the number of birds that pass the station on
+only a few nights in one spring. Many years of study may be required
+before hard and fast principles are justifiable. Nevertheless,
+certain salient features stand out in the continental density pattern
+in the spring of 1948. (The general results are summarized in
+Tables 2-5; the location of the stations is shown in <a href="#Fig_34">Figure 34</a>.)
+These features will be discussed now on a geographical basis.</p>
+
+<a name="Tbl_2"></a>
+<p><span class="pagenum"><a name="Page_433" id="Page_433">[Pg_433]</a></span></p>
+<div class="center">
+<div class="caption3nb"><span class="smcap">Table 2.</span>&mdash;Extent of Observations and Seasonal Station Densities at Major Stations in 1948</div>
+<br />
+<table cellpadding="4" cellspacing="0" class="center data" summary="observation data">
+<tr>
+  <td class="bt bb smcap" rowspan="2">Observation Station</td>
+  <td class="bt bl bb center" colspan="4">Nights of observation</td>
+  <td class="bt bl bb center" colspan="4">Hours of observation</td>
+  <td class="bt bl bb" rowspan="2">Season<br />density</td>
+</tr>
+<tr>
+  <td class="bl bb">March</td>
+  <td class="bl bb">April</td>
+  <td class="bl bb">May</td>
+  <td class="bl bb">Total</td>
+  <td class="bl bb">March</td>
+  <td class="bl bb">April</td>
+  <td class="bl bb">May</td>
+  <td class="bl bb">Total</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Canada</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Pt. Pelee</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6</td>
+  <td class="bl">6</td>
+  <td class="bl">2,500</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Mexico</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;S. L. P.: Ebano</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">1,300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tamps.: Tampico</td>
+  <td class="bl">3</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6</td>
+  <td class="bl">20</td>
+  <td class="bl">20</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">40</td>
+  <td class="bl">140,300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Yuc.: Progreso</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">18</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">18</td>
+  <td class="bl">60,500</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">United States</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Fla.: Pensacola</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">2</td>
+  <td class="bl">4</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">8</td>
+  <td class="bl">7</td>
+  <td class="bl">15</td>
+  <td class="bl">1,500</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Winter Park</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">5</td>
+  <td class="bl">6</td>
+  <td class="bl">11</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">39</td>
+  <td class="bl">38</td>
+  <td class="bl">77</td>
+  <td class="bl">21,700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ga.: Athens</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">10</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">10</td>
+  <td class="bl">4,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Thomasville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">8</td>
+  <td class="bl">8</td>
+  <td class="bl">16</td>
+  <td class="bl">4,700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Iowa: Ottumwa</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">5</td>
+  <td class="bl">5</td>
+  <td class="bl">10</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">16</td>
+  <td class="bl">28</td>
+  <td class="bl">44</td>
+  <td class="bl">134,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Kans.: Lawrence</td>
+  <td class="bl">2</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">16</td>
+  <td class="bl">4</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">20</td>
+  <td class="bl">68,700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ky.: Louisville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">2</td>
+  <td class="bl">5</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">20</td>
+  <td class="bl">14</td>
+  <td class="bl">34</td>
+  <td class="bl">49,300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Murray</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">13</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">13</td>
+  <td class="bl">26,200</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;La.: Baton Rouge</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">15</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">15</td>
+  <td class="bl">11,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Lafayette</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">5</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">5</td>
+  <td class="bl">2,800</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mansfield</td>
+  <td class="bl">1</td>
+  <td class="bl">5</td>
+  <td class="bl">4</td>
+  <td class="bl">10</td>
+  <td class="bl">2</td>
+  <td class="bl">16</td>
+  <td class="bl">22</td>
+  <td class="bl">40</td>
+  <td class="bl">22,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;New Orleans</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">5</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">7</td>
+  <td class="bl">1,900</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Oak Grove</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">2</td>
+  <td class="bl">4</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">16</td>
+  <td class="bl">15</td>
+  <td class="bl">31</td>
+  <td class="bl">33,900</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mich.: Albion</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">1,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Minn.: Hopkins</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">4</td>
+  <td class="bl">4</td>
+  <td class="bl">2,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Miss.: Rosedale</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6</td>
+  <td class="bl">8</td>
+  <td class="bl">14</td>
+  <td class="bl">12,600</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mo.: Columbia</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">1</td>
+  <td class="bl">3</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">8</td>
+  <td class="bl">6</td>
+  <td class="bl">14</td>
+  <td class="bl">13,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Liberty</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">2</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">7</td>
+  <td class="bl">7</td>
+  <td class="bl">14</td>
+  <td class="bl">4,800</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Okla.: Stillwater</td>
+  <td class="bl">1</td>
+  <td class="bl">2</td>
+  <td class="bl">1</td>
+  <td class="bl">4</td>
+  <td class="bl">5</td>
+  <td class="bl">11</td>
+  <td class="bl">3</td>
+  <td class="bl">19</td>
+  <td class="bl">8,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;S. Car.: Charleston</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">1</td>
+  <td class="bl">3</td>
+  <td class="bl">5</td>
+  <td class="bl">8</td>
+  <td class="bl">9</td>
+  <td class="bl">22</td>
+  <td class="bl">3,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tenn.: Knoxville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2</td>
+  <td class="bl">2</td>
+  <td class="bl">4</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">18</td>
+  <td class="bl">14</td>
+  <td class="bl">32</td>
+  <td class="bl">35,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Memphis</td>
+  <td class="bl">2</td>
+  <td class="bl">3</td>
+  <td class="bl">2</td>
+  <td class="bl">7</td>
+  <td class="bl">13</td>
+  <td class="bl">20</td>
+  <td class="bl">12</td>
+  <td class="bl">45</td>
+  <td class="bl">29,700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tex.: College Station</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3</td>
+  <td class="bl">1</td>
+  <td class="bl">4</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">19</td>
+  <td class="bl">8</td>
+  <td class="bl">27</td>
+  <td class="bl">32,200</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Rockport</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">4</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">4</td>
+  <td class="bl">6,200</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Tbl_3"></a>
+<p><span class="pagenum"><a name="Page_434" id="Page_434">[Pg_434]</a></span></p>
+<div class="center">
+<div class="caption3nb"><span class="smcap">Table 3.</span>&mdash;Average Hourly Station Densities in 1948</div>
+<br />
+<table cellpadding="4" cellspacing="0" class="data center" summary="observation data">
+<tr>
+  <td class="bt bb smcap">Observation Station</td>
+  <td class="bt bl bb">March</td>
+  <td class="bt bl bb">April</td>
+  <td class="bt bl bb">May</td>
+  <td class="bt bl bb">Season</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Canada</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Pt. Pelee</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+  <td class="bl">400</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Mexico</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;S. L. P.: Ebano</td>
+  <td class="bl">400</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tamps.: Tampico</td>
+  <td class="bl">700</td>
+  <td class="bl">6,300</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,500</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Yuc.: Progreso</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,800</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,800</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">United States</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Fla.: Pensacola</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">0+</td>
+  <td class="bl">200</td>
+  <td class="bl">100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Winter Park</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">300</td>
+  <td class="bl">200</td>
+  <td class="bl">300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ga.: Athens</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Thomasville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">500</td>
+  <td class="bl">100</td>
+  <td class="bl">300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Iowa: Ottumwa</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,700</td>
+  <td class="bl">3,800</td>
+  <td class="bl">3,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Kans.: Lawrence</td>
+  <td class="bl">4,000</td>
+  <td class="bl">1,400</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ky.: Louisville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,000</td>
+  <td class="bl">700</td>
+  <td class="bl">1,500</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Murray</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,000</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;La.: Baton Rouge</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">700</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Lafayette</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">600</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">600</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mansfield</td>
+  <td class="bl">0</td>
+  <td class="bl">700</td>
+  <td class="bl">800</td>
+  <td class="bl">600</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;New Orleans</td>
+  <td class="bl">60</td>
+  <td class="bl">800</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Oak Grove</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,400</td>
+  <td class="bl">800</td>
+  <td class="bl">1,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mich.: Albion</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Minn.: Hopkins</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">500</td>
+  <td class="bl">500</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Miss.: Rosedale</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,100</td>
+  <td class="bl">700</td>
+  <td class="bl">900</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mo.: Columbia</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">400</td>
+  <td class="bl">1,700</td>
+  <td class="bl">900</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Liberty</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">500</td>
+  <td class="bl">200</td>
+  <td class="bl">300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Okla.: Stillwater</td>
+  <td class="bl">500</td>
+  <td class="bl">200</td>
+  <td class="bl">1,000</td>
+  <td class="bl">400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;S. Car.: Charleston</td>
+  <td class="bl">200</td>
+  <td class="bl">200</td>
+  <td class="bl">0+</td>
+  <td class="bl">100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tenn.: Knoxville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,300</td>
+  <td class="bl">800</td>
+  <td class="bl">1,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Memphis</td>
+  <td class="bl">300</td>
+  <td class="bl">800</td>
+  <td class="bl">900</td>
+  <td class="bl">700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tex.: College Station</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,100</td>
+  <td class="bl">1,500</td>
+  <td class="bl">1,200</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Rockport</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,600</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,600</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Tbl_4"></a>
+<p><span class="pagenum"><a name="Page_435" id="Page_435">[Pg_435]</a></span></p>
+<div class="center">
+<div class="caption3nb"><span class="smcap">Table 4.</span>&mdash;Maximum Hourly Station Densities in 1948</div>
+<br />
+<table class="data center" summary="observation data">
+<tr>
+  <td class="bt bb smcap">Observation Station</td>
+  <td class="bt bl bb">March</td>
+  <td class="bt bl bb">April</td>
+  <td class="bt bl bb">May</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Canada</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Pt. Pelee</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,400</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Mexico</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;S. L. P.: Ebano</td>
+  <td class="bl">600</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tamps.: Tampico</td>
+  <td class="bl">3,100</td>
+  <td class="bl">21,200</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Yuc.: Progreso</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">11,900</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">United States</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Fla.: Pensacola</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">100</td>
+  <td class="bl">700</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Winter Park</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,300</td>
+  <td class="bl">1,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ga.: Athens</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">900</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Thomasville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,500</td>
+  <td class="bl">200</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Iowa: Ottumwa</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,800</td>
+  <td class="bl">12,500</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Kans.: Lawrence</td>
+  <td class="bl">14,500</td>
+  <td class="bl">2,200</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ky.: Louisville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">5,000</td>
+  <td class="bl">1,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Murray</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,700</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;La.: Baton Rouge</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,400</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Lafayette</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,800</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mansfield</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,100</td>
+  <td class="bl">1,600</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;New Orleans</td>
+  <td class="bl">200</td>
+  <td class="bl">1,100</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Oak Grove</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,700</td>
+  <td class="bl">2,500</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mich.: Albion</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">700</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Minn.: Hopkins</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Miss.: Rosedale</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,200</td>
+  <td class="bl">1,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mo.: Columbia</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">800</td>
+  <td class="bl">3,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Liberty</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">800</td>
+  <td class="bl">800</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Okla.: Stillwater</td>
+  <td class="bl">900</td>
+  <td class="bl">700</td>
+  <td class="bl">1,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;S. Car.: Charleston</td>
+  <td class="bl">400</td>
+  <td class="bl">600</td>
+  <td class="bl">200</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tenn.: Knoxville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">5,800</td>
+  <td class="bl">1,900</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Memphis</td>
+  <td class="bl">1,200</td>
+  <td class="bl">3,400</td>
+  <td class="bl">2,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tex.: College Station</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,400</td>
+  <td class="bl">3,100</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Rockport</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,400</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Tbl_5"></a>
+<p><span class="pagenum"><a name="Page_436" id="Page_436">[Pg_436]</a></span></p>
+<div class="center">
+<div class="caption3nb"><span class="smcap">Table 5.</span>&mdash;Maximum Nightly Densities at Stations with More Than One Night of Observation</div>
+<br />
+<table cellpadding="4" cellspacing="0" class="data center" summary="observation data">
+<tr>
+  <td class="bt bb smcap">Observation Station</td>
+  <td class="bt bl bb">March</td>
+  <td class="bt bl bb">April</td>
+  <td class="bt bl bb">May</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">Mexico</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tamps.: Tampico</td>
+  <td class="bl">5,500</td>
+  <td class="bl">63,600</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Yuc.: Progreso</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">31,600</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf smcap">United States</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Fla.: Winter Park</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6,200</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ga.: Athens</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">2,600</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Thomasville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">3,900</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Iowa: Ottumwa</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">15,300</td>
+  <td class="bl">54,600</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Kans.: Lawrence</td>
+  <td class="bl">51,600</td>
+  <td class="bl">5,400</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Ky.: Louisville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">17,000</td>
+  <td class="bl">8,400</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Murray</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">16,400</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;La.: Baton Rouge</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6,200</td>
+  <td class="bl">&nbsp;</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mansfield</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">4,900</td>
+  <td class="bl">5,200</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Oak Grove</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">13,600</td>
+  <td class="bl">5,800</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Miss.: Rosedale</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6,800</td>
+  <td class="bl">5,800</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Mo.: Columbia</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">1,400</td>
+  <td class="bl">10,300</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Okla.: Stillwater</td>
+  <td class="bl">2,700</td>
+  <td class="bl">1,900</td>
+  <td class="bl">3,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tenn.: Knoxville</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">15,200</td>
+  <td class="bl">9,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Memphis</td>
+  <td class="bl">3,600</td>
+  <td class="bl">7,900</td>
+  <td class="bl">7,000</td>
+</tr>
+<tr>
+  <td class="text_lf">&nbsp;&nbsp;&nbsp;&nbsp;Tex.: College Station</td>
+  <td class="bl">&nbsp;</td>
+  <td class="bl">6,200</td>
+  <td class="bl">13,200</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_437" id="Page_437">[Pg_437]</a></span></p>
+<a name="Fig_34"></a>
+<div class="center">
+<img src="images/fig_34.png" width="406" height="594" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 34.</span> Stations at which telescopic observations were made in 1948.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_438" id="Page_438">[Pg_438]</a></span></p>
+<div class="caption3nci">Gulf Migration: A Review of the Problem</div>
+
+<p>In view of the controversy in recent years pertaining to migration
+routes in the region of the Gulf of Mexico (Williams, 1945 and 1947;
+Lowery, 1945 and 1946), the bearing of the new data on the problem
+is of especial interest. While recent investigations have lent
+further support to many of the ideas expressed in my previous
+papers on the subject, they have suggested alternative explanations
+in the case of others. In the three years that have elapsed since my
+last paper dealing with Gulf migration, some confusion seems to
+have arisen regarding the concepts therein set forth. Therefore, I
+shall briefly re-state them.</p>
+
+<p>It was my opinion that evidence then available proved conclusively
+that birds traverse the Gulf frequently and intentionally;
+that the same evidence suggested trans-Gulf flights of sufficient magnitude
+to come within the meaning of migration; that great numbers
+of birds move overland around the eastern and western edges of the
+Gulf; that it was too early to say whether the coastal or trans-Gulf
+route was the more important, but that enough birds cross the
+water from Yucatán to account for transient migration in the extreme
+lower Mississippi Valley; and, that, in fair weather, most
+trans-Gulf migrants continue on inland for some distance before
+coming to land, creating an area of "hiatus" that is usually devoid
+of transient species. I tried to make it emphatically clear that I
+realized that many birds come into Texas from Mexico overland,
+that I did not think the hordes of migrants normally seen on the
+Texas coast in spring were by any means all trans-Gulf migrants.
+I stated (1946: 206): "Proving that birds migrate in numbers
+across the Gulf does not prove that others do not make the journey
+by the coastal routes. But that is exactly the point. No one has
+ever pretended that it does." Although some ornithologists seem
+to have gained the impression that I endorse only the trans-Gulf
+route, this is far from the truth. I have long held that the migrations
+overland through eastern Mexico and southern Texas on
+one hand, and the over-water flights on the other, are each part of
+the broad movement of transients northward into the United States.
+There are three avenues of approach by which birds making up the
+tremendous concentrations on the Texas coast may have reached
+there: by a continental pathway from a wintering ground in eastern
+and southern Mexico; by the over-water route from Yucatán and
+points to the southward; and, finally, by an overland route from
+Central America via the western edge of the Gulf. As a result of
+Louisiana State University's four-year study of the avifauna in
+<span class="pagenum"><a name="Page_439" id="Page_439">[Pg_439]</a></span>
+eastern Mexico, I know that migrants reach Texas from the first
+source. As a consequence of my studies in Yucatán of nocturnal
+flight densities and their directional trends, I strongly believe that
+migrants reach Texas from this second source. As for the third
+source, I have never expressed an opinion. I am not prepared to do
+so now, for the reason that today, as three years ago, there is no dependable
+evidence on which to base a judgment one way or another.</p>
+
+
+<div class="caption3nci">Western Gulf Area</div>
+
+<p>Among the present flight density data bearing on the above issues,
+are the six sets of observations from the vicinity of Tampico, Tamaulipas,
+already referred to. These were secured in the spring of 1948
+by a telescope set up on the Gulf beach just north of the Miramar
+pavilion and only a hundred feet from the surf (see <a href="#Fig_25">Figure 25</a>, <i>ante</i>).
+The beach here is approximately 400 feet wide and is backed by
+scrub-covered dunes, which rapidly give way toward the west to a
+rather dense growth of low shrubs and trees. One might have expected
+that station densities at Tampico in March would be rather
+high. Actually, though they are the second highest recorded for
+the month, they are not impressive and afford a striking contrast
+with the record flights there in April (<a href="#Tbl_6">Table 6</a>). Unfortunately, only</p>
+
+<a name="Tbl_6"></a>
+<div class="center">
+<div class="caption3nb"><span class="smcap">Table 6.</span>&mdash;Computed Hourly Densities at Tampico, Tamps., in Spring of 1948</div>
+<br />
+<table width="100%" class="center" summary="Observational Data">
+<tr>
+  <td class="bt bb smcap" rowspan="2">Date</td>
+  <td class="bt bl" colspan="9">Average hour of observation</td>
+</tr>
+<tr>
+  <td class="bt bl bb">8:30</td>
+  <td class="bt bl bb">9:30</td>
+  <td class="bt bl bb">10:30</td>
+  <td class="bt bl bb">11:30</td>
+  <td class="bt bl bb">12:30</td>
+  <td class="bt bl bb">1:30</td>
+  <td class="bt bl bb">2:30</td>
+  <td class="bt bl bb">3:30</td>
+  <td class="bt bl bb">4:30</td>
+</tr>
+<tr>
+  <td>22-23 March</td>
+  <td class="bl">600</td>
+  <td class="bl">700</td>
+  <td class="bl"> 1,000</td>
+  <td class="bl">800</td>
+  <td class="bl">100</td>
+  <td class="bl">100</td>
+  <td class="bl">0</td>
+  <td class="bl">100</td>
+  <td class="bl">..</td>
+</tr>
+<tr>
+  <td>23-24 March</td>
+  <td class="bl">0</td>
+  <td class="bl">400</td>
+  <td class="bl"> 1,200</td>
+  <td class="bl">3,100</td>
+  <td class="bl">800</td>
+  <td class="bl">.. </td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+</tr>
+<tr>
+  <td>24-25 March</td>
+  <td class="bl">300</td>
+  <td class="bl">700</td>
+  <td class="bl">800</td>
+  <td class="bl">1,600</td>
+  <td class="bl">1,100</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">  ..</td>
+</tr>
+<tr>
+  <td>21-22 April</td>
+  <td class="bl">1,100</td>
+  <td class="bl">7,000</td>
+  <td class="bl">14,900</td>
+  <td class="bl">12,900</td>
+  <td class="bl"> 8,100</td>
+  <td class="bl"> 3,800</td>
+  <td class="bl">3,500</td>
+  <td class="bl">200</td>
+  <td class="bl">..</td>
+</tr>
+<tr>
+  <td>22-23 April</td>
+  <td class="bl">700</td>
+  <td class="bl">2,900</td>
+  <td class="bl">7,500</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+  <td class="bl">..</td>
+</tr>
+<tr>
+  <td class="bb">23-24 April</td>
+  <td class="bl bb">600</td>
+  <td class="bl bb">4,700</td>
+  <td class="bl bb">19,100</td>
+  <td class="bl bb">21,200</td>
+  <td class="bl bb">5,500</td>
+  <td class="bl bb">5,900</td>
+  <td class="bl bb">4,000</td>
+  <td class="bl bb">2,000</td>
+  <td class="bl bb">200</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+
+<p>a few stations were operating in March and thus adequate comparisons
+are impossible; but the indications are that, in March, migration
+activity on the western edges of the Gulf is slight. It fails
+even to approach the volume that may be observed elsewhere at the
+same time, as for example, in eastern Kansas where, however, the
+migration is not necessarily correlated with the migration in the
+<span class="pagenum"><a name="Page_440" id="Page_440">[Pg_440]</a></span>
+lower Gulf area. Strangely enough, on the night of March 22-23, at
+Tampico, approximately 85 per cent of the birds were flying from
+north of an east-west line to south of it, opposite to the normal
+trend of spring migration. This phenomenon, inexplicable in the
+present instance, will be discussed below. On the other two nights
+in March, the directional trend at Tampico was northward with
+few or no aberrant components. Observations made approximately
+thirty-five miles inland from the Gulf, at Ebano, San Luis Potosí,
+on the night of March 25-26, show lower station densities than the
+poorest night at Tampico, but since they cover only a three-hour
+watch, they reveal little or nothing concerning the breadth of the
+so-called coastal flyway.</p>
+
+<p>April flight densities at Tampico are the highest recorded in the
+course of this study. The maximum hourly density of 21,200 birds
+is 46 per cent higher than the maximum hourly density anywhere
+else. The average hourly density of 6,300 in April is more than
+twice as great as the next highest average for that month. These
+figures would seem to satisfy certain hypotheses regarding a coastwise
+flight of birds around the western edge of the Gulf. Other
+aspects of the observations made at that time do not satisfy these
+hypotheses. Texas ornithologists have found that in periods of
+heavy spring migration, great numbers of birds are invariably precipitated
+by rainy weather. On April 23, in the midst of the record-breaking
+telescopic studies at Tampico, Mr. Robert J. Newman made
+a daytime census immediately following four hours of rain. He
+made an intensive search of a small area of brush and low growth
+back of the beach for traces of North American migrants. In his
+best hour, only thirteen individual birds out of seventy-five seen
+were of species that do not breed there. The transient species were
+the Ruby-throated Hummingbird (1), Scissor-tailed Flycatcher (1),
+Western Wood Pewee (1), Black-throated Green Warbler (2)
+Orchard Oriole (7), and Baltimore Oriole (1), all of which winter
+extensively in southern Mexico. Perhaps, however, the apparent
+scarcity of transients on this occasion is not surprising in the light
+of the analysis of flight density in terms of bird density on the
+ground which I shall develop beyond. My only point here is to
+demonstrate that rain along the coast does not always produce
+birds.</p>
+
+<p>As large as the nocturnal flights at Tampico have so far proved to
+be, they are not commensurate with the idea that nearly all birds
+follow a narrow coastwise route around the Gulf. To establish the
+<span class="pagenum"><a name="Page_441" id="Page_441">[Pg_441]</a></span>
+latter idea, one must be prepared to show that the migrant species
+returning to the United States pass along two flyways a few miles
+wide in the immense volume necessary to account for their later
+abundance on a 1500-mile front extending across eastern North
+America. One might expect at least ten to twenty fold the number
+observable at any point in the interior of the United States. In actuality,
+the highest nightly density of 63,600 birds at Tampico is
+barely sufficient to account for the highest nightly density of 54,600
+at Ottumwa, Iowa, alone.</p>
+
+<p>Of course, there is no way of knowing how closely a ratio of anywhere
+from ten to one through twenty to one, employed in this comparison,
+expresses the true situation. It may be too high. It could
+be too low, particularly considering that preliminary studies of flight
+density in Florida indicate that the western shores of the Gulf of
+Mexico must carry the major part of the traffic if migratory flights
+back to the United States in spring take place only along coastwise
+routes. Consideration of the data obtained in Florida in 1948 will
+serve to emphasize the point.</p>
+
+
+<div class="caption3nci">Eastern Gulf Area</div>
+
+<p>At Winter Park, Florida, seventy-seven hours were spent at the
+telescope in April and May. This was 71 per cent more hours of
+actual observation than at the next highest station. Nevertheless,
+the total seasonal density amounted to only 21,700 birds. The average
+hourly density was only 300 birds, with the maximum for any
+one hour being 2,300 birds. In contrast, forty-five hours of observation
+at Tampico, Tamaulipas, in March and April, yielded a total
+station density of 140,300 birds. At the latter place, on the night of
+April 23-24, almost as many birds passed <i>in a single hour</i> as passed
+Winter Park in all of its seventy-seven hours of observation.</p>
+
+<p>Should future telescopic studies at Florida stations fail to produce
+densities appreciably higher than did Winter Park in 1948, the currently-held
+ideas that the Florida Peninsula is a major flyway will
+be seriously shaken. But one consideration must be kept in mind
+regarding the present picture. No observations were made at
+Winter Park in March, when it is conceivable that densities may
+have been materially higher. We know, for instance, that many of
+the early migrants to the southern United States are species whose
+winter homes are in the West Indies. Numbers of Vireonidae and
+Parulidae (notably the genera <i>Vireo</i>, <i>Parula</i>, <i>Protonotaria</i>, <i>Mniotilta</i>,
+<i>Seiurus</i>, <i>Geothlypis</i>, <i>Setophaga</i>, and certain <i>Dendroica</i> and
+<i>Vermivora</i>) winter extensively in this region and are among the first
+<span class="pagenum"><a name="Page_442" id="Page_442">[Pg_442]</a></span>
+birds to return to the southern states in the spring. Many of them
+often reach Louisiana and other states on the Gulf coastal plain by
+mid-March. In the same connection, it may be mentioned that
+many of the outstanding instances of birds striking lighthouses in
+southern Florida occurred in March and early April (Howell, 1932).</p>
+
+
+<div class="caption3nci">Yucatán Area</div>
+
+<p>I have long felt that the answers to many of the questions which
+beset us in our study of Gulf migration are to be found on the open
+waters of the Gulf of Mexico itself or on the northern tip of the
+Yucatán Peninsula. Accordingly, in the spring of 1945 I crossed
+the Gulf by slow freighter for the purpose of determining how many
+and what kinds of birds might be seen between the mouth of the
+Mississippi River and the Yucatán Peninsula in fair weather, when
+it could not be argued that the birds had been blown there by inclement
+weather. To my own observations I was able to add those
+of other ornithologists who likewise had been aboard ship in the
+Gulf.</p>
+
+<p>The summary of results proved that birds of many species cross
+the Gulf and do so frequently. It failed to demonstrate beyond
+all doubt that they do so in large numbers. Nor had I expected it
+to do so. The <ins title="TN: concensus => consensus">consensus</ins> of Gulf coast ornithologists seemed to be
+that transient migration in their respective regions is often performed
+at too high an elevation to be detected unless the birds are
+forced to earth by bad weather. I saw no reason to anticipate that
+the results would be otherwise over the waters of the Gulf of Mexico.</p>
+
+<p>The application of the telescopic method held promise of supplying
+definite data on the numbers of trans-Gulf migrants, however
+high their flight levels. The roll and vibration of the ship had prevented
+me in 1945 from making telescopic observations at sea. Since
+no immediate solution to the technical difficulties involved presented
+itself, I undertook to reach one of the small cays in Alacrán Reef,
+lying seventy-five miles north of Yucatán and in line with the coast
+of southern Louisiana. Because of transportation difficulties, my
+plans to place a telescopic station in this strategic location failed.
+Consequently, I returned in 1948 by freighter to Progreso, Yucatán,
+where telescopic counts were made for three nights, one of which was
+rendered almost valueless by the cloud cover.</p>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_443" id="Page_443">[Pg_443]</a></span></p>
+<a name="Fig_35"></a>
+<div class="center">
+<img src="images/fig_35.png" width="490" height="490" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 35.</span> Positions of the cone of observation at Progreso, Yucatán, on the
+night of April 23-24, 1948, from 8:53 P.&nbsp;M. to 3:53 A.&nbsp;M. Essential features of
+this map are drawn to scale. The telescope was set up on the end of a one-mile
+long wharf that extends northward from the shore over the waters of the
+Gulf of Mexico. The triangular (white) lines represent the projections of the
+cone of visibility on the earth at the mid-point of each hour of observation.
+Only briefly, in the first two hours, did the cone lie even in part over the adjacent
+mainland. Hence, nearly all of the birds seen in the course of the night
+had actually left the land behind.</div>
+</div>
+<br />
+<br />
+
+<p>The observation station at Progreso was situated on the northern
+end of the new wharf which projects northward from the beach to
+a point one mile over the Gulf. As will be seen from <a href="#Fig_35">Figure 35</a>, the
+entire cone of observation lay at nearly all times over the intervening
+ing water between the telescope on the end of the wharf and the
+beach. Therefore, nearly all of the birds seen were actually observed
+leaving the coast and passing out over the open waters of the
+Gulf. The hourly station densities are shown in <a href="#Tbl_7">Table 7</a> and Figures
+<a href="#Fig_24">24</a> and <a href="#Fig_36">36</a>. In the seventeen hours of observation on the nights of
+April 23-24 and April 24-25, a total computed density of 59,200 birds
+passed within one-half mile of each side of Progreso. This is the
+third highest density recorded in the course of this study. The
+<span class="pagenum"><a name="Page_444" id="Page_444">[Pg_444]</a></span>
+maximum for one hour was a computed density of 11,900 birds. This
+is the fourth highest hourly density recorded in 1948.</p>
+<br />
+<br />
+
+<a name="Fig_36"></a>
+<div class="center">
+<img src="images/fig_36.png" width="484" height="368" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 36.</span> Hourly station density curve for night of April 23-24, 1948, at Progreso, Yucatán.</div>
+</div>
+<br />
+<br />
+
+<a name="Tbl_7"></a>
+<div class="center">
+<div class="caption3nb"><span class="smcap">Table 7.</span>&mdash;Computed Hourly Densities at Progreso, Yuc., in Spring of 1948</div>
+<br />
+<table width="100%" class="center" summary="Observational Data">
+<tr>
+  <td class="bt bb smcap" rowspan="2">Date</td>
+  <td class="bt bl" colspan="9">Average hour of observation</td>
+</tr>
+<tr>
+  <td class="bt bl bb">8:30</td>
+  <td class="bt bl bb">9:30</td>
+  <td class="bt bl bb">10:30</td>
+  <td class="bt bl bb">11:30</td>
+  <td class="bt bl bb">12:30</td>
+  <td class="bt bl bb">1:30</td>
+  <td class="bt bl bb">2:30</td>
+  <td class="bt bl bb">3:30</td>
+  <td class="bt bl bb">4:30</td>
+</tr>
+<tr>
+  <td>23-24 April</td>
+  <td class="bl">400</td>
+  <td class="bl"> 3,000</td>
+  <td class="bl"> 5,100</td>
+  <td class="bl">10,000</td>
+  <td class="bl"> 9,000</td>
+  <td class="bl"> 2,800</td>
+  <td class="bl">900</td>
+  <td class="bl">400</td>
+  <td class="bl"> ....</td>
+</tr>
+<tr>
+  <td class="bb">24-25 April</td>
+  <td class="bl bb">0</td>
+  <td class="bl bb">500</td>
+  <td class="bl bb">3,700</td>
+  <td class="bl bb">11,900</td>
+  <td class="bl bb">7,900</td>
+  <td class="bl bb">1,900</td>
+  <td class="bl bb">1,100</td>
+  <td class="bl bb">400</td>
+  <td class="bl bb">200</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+
+<p>It is not my contention that this many birds leave the northern
+coast of Yucatán every night in spring. Indeed, further studies may
+show negligible flight densities on some nights and even greater densities
+on others. As a matter of fact several hours of observation on
+the night of April 25-26, at Mérida, Yucatán, approximately twenty-five
+miles inland from Progreso, indicated that on this night the
+density overhead was notably low, a condition possibly accounted
+for by a north wind of 10 mph blowing at 2,000 feet. I merely submit
+<span class="pagenum"><a name="Page_445" id="Page_445">[Pg_445]</a></span>
+that on the nights of April 23-24 and 24-25, birds were leaving
+the coast of Yucatán <i>at Progreso</i> at the rate indicated. But, as I
+have emphasized in this paper and elsewhere (1946: 205-206), the
+northern part of the Yucatán Peninsula is notably unmarked by
+streams or any other physiographic features which birds might follow.
+The uniformity of the topography for many miles on either
+side of Progreso, if not indeed for the entire breadth of the Peninsula,
+makes it probable that Progreso is not a particularly favored
+spot for observing migration, and that it is not the only point along
+the northern coast of Yucatán where high flight densities can be
+recorded. This probability must be considered when comparisons
+are made between Progreso densities and those at Tampico. The
+argument could be advanced that the present densities from Tampico
+do not sufficiently exceed those at Progreso to establish the
+coastal route as the main avenue of traffic in spring, since there is
+every reason to suspect topography of exerting some influence to
+produce a channeling effect in eastern Mexico. Here the coast parallels
+the directional trend of the migratory movement for more than
+600 miles. Likewise the Sierra Madre Oriental of eastern Mexico,
+situated approximately 100 miles inland (sometimes less), lies
+roughly parallel to the coast. Because of the slant of the Mexican
+land mass, many winter residents in southern Mexico, by short
+northward movements, would sooner or later filter into the coastal
+plain. Once birds are shunted into this lowland area, it would seem
+unlikely that they would again ascend to the top of the Sierra Madre
+to the west. In this way the great north-south cordillera of mountains
+may act as a western barrier to the horizontal dispersion of
+transients bound for eastern North America. Similarly, the Gulf
+itself may serve as an eastern barrier; for, as long as migrants may
+progress northward in the seasonal direction of migration and still
+remain over land, I believe they would do so.</p>
+
+<p>To put the matter in a slightly different way, the idea of a very
+narrow flight lane is inherent in the idea of coastwise migration.
+For, as soon as we begin to visualize flights of great volume over
+fronts extending back more than fifty miles from the shore line, we
+are approaching, if indeed we have not already passed, the point
+where the phenomenon is no longer coastwise in essence, but merely
+overland (as indeed my own unprocessed, telescopic data for 1949
+indicate may be the case). In actuality, those who have reported on
+the migration along the western edge of the Gulf of Mexico have
+never estimated the width of the main flight at more than fifty miles
+<span class="pagenum"><a name="Page_446" id="Page_446">[Pg_446]</a></span>
+and have intimated that under some circumstances it may be as
+narrow as two miles. No evidence of such restrictions can be discerned
+in the case of the trans-Gulf flights. If it cannot be said
+that they may be assumed to be as wide as the Gulf itself, they at
+least have the potential breadth of the whole 260-mile northern
+coast of the Yucatán Peninsula. On these premises, to be merely
+equal in total magnitude, the coastwise flights must exhibit, depending
+on the particular situation, from five to 130 times the concentrations
+observable among trans-Gulf migrants. This point
+seems almost too elementary to mention, but I have yet to find anyone
+who, in comparing the two situations, takes it into consideration.</p>
+
+<p>Judged in this light, the average hourly density of 2,800 birds at
+Progreso in April would appear to be indicative of many more migrants
+on the entire potential front than the 6,300 birds representing
+the average hourly density for the same month at Tampico.</p>
+
+<p>That the Progreso birds were actually beginning a trans-Gulf
+flight seems inevitable. The Yucatán Peninsula projects 200 miles
+or more northward into the vast open expanses of the Gulf of Mexico
+and the Caribbean Sea, with wide stretches of water on either side.
+The great majority of the birds were observed <i>after</i> they had proceeded
+beyond the northern edge of this land mass. Had they later
+veered either to the east or the west, they would have been obliged
+to travel several hundred miles before again reaching land, almost
+as far as the distance straight across the Gulf. Had they turned
+southward, some individuals should have been detected flying in
+that direction. As can be seen from Figures <a href="#Fig_23">23</a>, <a href="#Fig_42">42</a>, and <a href="#Fig_44">44</a>, not one
+bird observed was heading south of east or south of west on either
+night. No other single piece of evidence so conclusively demonstrates
+that birds cross the Gulf of Mexico in spring in considerable
+numbers as do flight density data recorded from Progreso in 1948.</p>
+
+<div class="caption3nci">Northern Gulf Area</div>
+
+<p>Unfortunately only a few data on flight density are available from
+critical localities on the northern shores of the Gulf in spring. As
+the density curves in <a href="#Fig_30">Figure 30</a> demonstrate, several sets of observation,
+including some phenomenal flights, have been recorded at
+Baton Rouge. This locality, however, lies sixty-four miles from the
+closest point on the Gulf coast, and the point due southward on the
+coast is eighty-four miles distant. Since all of the birds seen at
+Baton Rouge on any one night may have come from the heavily
+forested area between Baton Rouge and the coast of the Gulf, we
+cannot use data from Baton Rouge as certainly representative of
+<span class="pagenum"><a name="Page_447" id="Page_447">[Pg_447]</a></span>
+incoming trans-Gulf flights. Data from repeated observations at
+stations on the coast itself are needed to judge the degree of trans-Gulf
+migration northward. On the few nights of observation at
+such localities (Cameron and Grand Isle, Louisiana, and Pensacola,
+Florida), flight densities have been zero or negligible. To be sure,
+negative results have been obtained at stations in the interior of
+the United States, and flights of low density have been recorded on
+occasion at stations where the flight densities are otherwise high.
+Nevertheless, in view of the volume of migration departing from
+Progreso, Yucatán, it would appear, upon first consideration, that
+we should at times record on the coast of Louisiana enough birds
+arriving in a night of continuous observation to yield a high density
+figure.</p>
+
+<p>Upon further consideration, however, there are factors mitigating
+against heavy densities of birds in northern flight on the northern
+coast of the Gulf. In the first place, presuming the main trans-Gulf
+flight to originate from northern Yucatán, and that there is a directional
+fanning to the northward, the birds leave on a 260-mile
+front, and arrive on a front 400 miles or more wide. Consequently,
+other factors remaining the same, there would be only approximately
+half the number of birds on the coast of arrival, at a given
+time and place, as there was on the coast of departure. Secondly,
+we may now presume on the basis of the telescopic studies at Progreso,
+that most migrants leaving northern Yucatán do so in the
+few hours centering about midnight. The varying speeds of the
+birds making the 580-mile flight across the Gulf distribute them still
+more sparsely on the north coast of the Gulf both in time and in
+space. Also we can see only that segment of the flight, which
+arrives in that part of a twenty-four hour period when the moon
+is up. This circumstance further reduces the interceptive potential
+because the hours after dark, to which the present telescopic studies
+have been restricted, comprise the period in which the fewest migrants
+arrive from over the water. To illustrate: it is a mathematical
+certainty that <i>none</i> of the birds leaving Yucatán in the hours
+of heaviest flight, before 12 P.&nbsp;M., and flying on a straight course at a
+speed of approximately 33 mph will reach the northern Gulf coast
+after nightfall; they arrive in the daytime. It will be useful to
+devise a technique for employing the sun as a background for telescopic
+observation of birds, thereby making observations possible
+on a twenty-four hour basis, so as to test these inferences by objective
+data.</p>
+
+<p><span class="pagenum"><a name="Page_448" id="Page_448">[Pg_448]</a></span></p>
+<p>When a whole night's observation (1949 data not yet processed)
+at Port Aransas, on the southern coast of Texas, on the great overland
+route from eastern Mexico, yields in one night in April only
+seven birds, the recording of no birds at a station near the mouth of
+the Mississippi River becomes less significant.</p>
+
+<p>As I have previously remarked in this paper, the new data obtained
+since 1946, when I last wrote on the subject of migration in the region
+of Gulf of Mexico, requires that I alter materially some of my
+previously held views. As more and more facts come to light, I
+may be compelled to alter them still further. For one thing, I
+have come to doubt seriously the rigidity of the coastal hiatus as I
+envisioned it in 1945. I believe instead that the scarcity of records
+of transient migrants on the Gulf coastal plain in fair weather is to
+a very large extent the result of a wide dispersion of birds in the
+dense cover that characterizes this general region. I now question
+if appreciable bird densities on the ground ever materialize anywhere
+except when the sparseness of suitable habitat for resting
+or feeding tends to concentrate birds in one place, or when certain
+meteorological conditions erect a barrier in the path of an oncoming
+migratory flight, precipitating many birds in one place.</p>
+
+<p>This retrenchment of ideas is a direct consequence of the present
+study, for time and again, as discussed in the case of Tampico densities,
+maximal nightly flights have failed to produce a visible abundance
+of transients on land the following day. A simple example
+may serve to illustrate why. The highest one-hour density recorded
+in the course of this study is 21,200 birds. That means that this
+many birds crossed a line one mile long on the earth's surface and
+at right angles to the direction of flight. Let us further assume that
+the average flight speed of all birds comprising this flight was 30
+mph. Had the entire flight descended simultaneously, it would have
+been dispersed over an area one mile wide and thirty miles long,
+and the precipitated density on the ground would have been only
+1.1 birds per acre. Moreover, if as many as ten species had been
+involved in the flight, this would have meant an average per species
+of less than one bird per nine acres. This would have failed, of
+course, to show appreciable concentrations to the observer in the
+field the following day. If, however, on the other hand, the same
+flight of 21,200 birds had encountered at one point a weather barrier,
+such as a cold-front storm, all 21,200 birds might have been precipitated
+in one place and the field observer would have recorded an
+"inundation of migrants." This would be especially true if the
+<span class="pagenum"><a name="Page_449" id="Page_449">[Pg_449]</a></span>
+locality were one with a high percentage of open fields or prairies
+and if the flight were mainly of woodland dwelling species, or conversely,
+if the locality were densely forested with few open situations
+and the flight consisted mainly of open-country birds. As explained
+on page 389, the density formula may be too conservative in its
+expression of actual bird densities. Even if the densities computed
+for birds in the air are only half as high as the actual densities in the
+air, the corresponding ground density of 2.2 birds per acre that
+results if all the birds descended simultaneously would hardly be
+any more impressive than the 1.1 bird per acre.</p>
+
+<p>This consideration is doubtless highly modified by local circumstances,
+but, in general, it seems to suggest a working hypothesis
+that provides an explanation for many of the facts that we now
+have. For example, on the coast of Texas there are great expanses
+of terrain unattractive to such birds as warblers, vireos, tanagers, and
+thrushes. The precipitation there by bad weather of even a mediocre
+nightly flight composed of birds of the kinds mentioned would
+surely produce an overwhelming concentration of birds in the
+scattered woods and shrubs.</p>
+
+<p>In spite of all that has been written about the great concentrations
+of transient migrants on the coast of Texas in spring, I am not convinced
+that they are of a different order of magnitude than those concentrations
+that sometimes occur along the cheniers and coastal islands
+of Louisiana and Mississippi. I have read over and over the
+highly informative accounts of Professor Williams (<i>loci cit.</i>) and the
+seasonal summaries by Davis (1936-1940) and Williams (1941-1945).
+I have conversed at length with Mrs. Jack Hagar, whom I
+regard as one of the leading authorities on the bird life of the
+Texas coast, and she has even permitted me access to her voluminous
+records covering a period of fifteen years residence at Rockport.
+Finally, I have spent a limited amount of time myself on the Texas
+coast studying first-hand the situation that obtains there in order
+that I might be in a position to compare it with what I have learned
+from observations elsewhere in the region of the Gulf of Mexico,
+Louisiana, Florida, Yucatán, and eastern Mexico.</p>
+
+<p>Although the concentrations of birds on some days near the mouth
+of the Mississippi River are almost incalculable, the fact remains
+that in Texas the densities of transient species on the ground are
+more consistently high from day to day. The reason for this may
+be simple. As birds move up daily from Mexico overland, a certain
+percentage would be destined to come down at all points along the
+<span class="pagenum"><a name="Page_450" id="Page_450">[Pg_450]</a></span>
+route but so dispersed in the inland forest that they might pass unnoticed.
+However, that part of the same flight settling down in
+coastal areas, where trees are scarce, would produce visible concentrations
+of woodland species. With the advent of a cold-front
+storm, two diametrically opposite effects of the same meteorological
+phenomenon would tend to pile up great concentrations of migrants
+of two classes&mdash;the overland and the trans-Gulf flights. During the
+prepolar-front weather the strong southerly (from the south) and
+southeasterly winds would tend to displace much of the trans-Gulf
+segment to the western part of the Gulf. With the shift of the winds
+to the north and northwest, which always occurs as the front passes,
+the overland flight still in the air would tend to be banked up against
+the coast, and the incoming trans-Gulf flight would be confronted
+with a barrier, resulting in the precipitation of birds on the first
+available land.</p>
+
+<p>These postulated conditions are duplicated in part in autumn
+along the Atlantic coast of the eastern United States. There, as a
+result of the excellent work of Allen and Peterson (1936) and Stone
+(1937), a similar effect has been demonstrated when northwest
+winds shove the south-bound flights up against the coast of New
+Jersey and concentrate large aggregations of migrants there.</p>
+
+
+<div class="caption3nci">Interior of the United States</div>
+
+<p>Attention has been drawn already to the nature of the nightly
+flights at stations immediately inland from the Gulf coast, where
+densities decline abruptly well before midnight. I have suggested
+that this early drop-off is mainly a result of the small amount of
+terrain south of these stations from which birds may be contributed
+to a night's flight. At Oak Grove, Louisiana, the flight exhibited a
+strong directional trend with no significant aberrant components.
+Therefore, one may infer that a considerable part of the flight was
+derived from regions to the south of the station.</p>
+
+<p>At Mansfield, Louisiana, thirty-eight hours of observation in
+April and May resulted in flight densities that are surprisingly low&mdash;much
+lower, in fact, than at Oak Grove. In eleven of the hours
+of observation no birds at all were seen. A possible explanation for
+these low densities lies in the fact that eastern Texas and western
+Louisiana, where, probably, the Mansfield flights originated, is not
+an especially attractive region to migrants because of the great
+amount of deforested and second growth pine land. Oak Grove, in
+contrast, is in the great Tensas-Mississippi River flood plain, characterized
+by an almost solid stand of deciduous forest extending
+over thousands of square miles in the lower Mississippi valley.</p>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_451" id="Page_451">[Pg_451]</a></span></p>
+<a name="Fig_37"></a>
+<div class="center">
+<img src="images/fig_37.png" width="480" height="356" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 37.</span> Sector density representation on two nights at Rosedale, Mississippi, in 1948. The white lines are the vector resultants.</div>
+</div>
+<br />
+<br />
+
+<p>In further contrast to the considerable flight densities and pronounced
+directional trend at Oak Grove, we have the results from
+Rosedale, Mississippi, only seventy miles to the north and slightly
+to the east. At Rosedale the densities were mediocre and the flight
+directions were extremely divergent. Many of the nights of observation
+at this locality were seriously interrupted by clouds, but such
+counts as were made on those dates indicated little migration taking
+place. On two nights, however, April 21-22 and May 20-21, visibility
+was almost continuous and densities were moderately high.
+In <a href="#Fig_37">Figure 37</a> I have shown the flight directions for these two nights.
+The lengths of the individual sector vectors are plotted as a percentage
+of the total station density for each of the two nights (5,800
+and 6,800 birds, respectively). Although the vector resultants show
+a net movement of birds to the northeast, there are important divergent
+components of the flights. This "round-the-compass" pattern
+is characteristic of stations on the edge of meteorological
+disturbances, as was Rosedale on April 21-22, but not on the night
+of May 20-21. If bats are presumed to have played a rôle in
+these latter observations, their random flights would tend to cancel
+<span class="pagenum"><a name="Page_452" id="Page_452">[Pg_452]</a></span>
+out and the vector resultant would emerge as a graphic representation
+of the actual net trend density of the birds and its prevailing
+direction of flow. Although I do not believe that bats are the real
+reason for the diverse directional patterns at Rosedale, I can offer
+no alternative explanation consistent with data from other stations.</p>
+
+<p>Moving northward in the valley of the Mississippi and its tributaries,
+we find a number of stations that yielded significantly high
+densities on most nights when weather conditions were favorable for
+migration. Louisville and Murray, Kentucky, and Knoxville, Tennessee,
+each show several nights with many birds flying, but only
+Lawrence, Kansas, and Ottumwa, Iowa, had migrations that approach
+in magnitude the record station densities at Tampico. Indeed,
+these were the only two stations in the United States that produced
+flights exceeding the densities at Progreso, Yucatán. The
+densities at Lawrence are unique in one respect, in that they were
+extremely high in the month of March. Since there were very few
+stations in operation then, these high densities would be of little
+significance were it not for the fact that at no time in the course of
+this study from 1945 to the present have comparable densities been
+obtained this early in the migration period. Examination of the
+"Remarks" section of the original data sheets from Lawrence show
+frequent mention of "duck-like" birds passing before the moon.
+We may infer from these notations that a considerable part of the
+overhead flight was composed of ducks and other aquatic birds that
+normally leave the southern United States before the main body of
+transient species reach there. The heavy flight densities at Lawrence
+may likewise have contained certain Fringillidae, Motacillidae,
+Sylviidae, and other passerine birds that winter mainly in the
+southern United States and which are known to begin their return
+northward in March or even earlier. Observations in 1948 at Lawrence
+in April were hindered by clouds, and in May no studies were
+attempted. However, we do have at hand two excellent sets of data
+recorded at Lawrence on the nights of May 3-4 and May 5-6, 1947,
+when the density was also extremely high.</p>
+
+<p>At Ottumwa, Iowa, where a splendid cooperative effort on the
+part of the local ornithologists resulted in forty-four hours of
+observation in April and May, densities were near the maximum
+for all stations. Considering this fact along with results at Lawrence
+and other mid-western stations where cloud cover did not
+interfere at the critical periods of observation, we have here evidence
+supporting the generally held thesis that eastern Kansas, Missouri,
+and Iowa lie on a principal migratory flyway.</p>
+
+<span class="pagenum"><a name="Page_453" id="Page_453">[Pg_453]</a></span>
+<p>Stations in Minnesota, Illinois, Michigan, Massachusetts, and
+Ontario were either operated for only parts of one or two nights, or
+else clouds seriously interfered with observations, resulting in discontinuous
+counts. It may be hoped that future studies will include
+an adequate representation of stations in these states and that
+observations will be extensive enough to permit conclusions regarding
+the density and direction of migration.</p>
+
+<p>Charleston, South Carolina, which does not conveniently fall in
+any of the geographic regions so far discussed, had, to me, a surprisingly
+low flight density; twenty-two hours of observation there
+in March, April, and May yielded a total flight density of only
+3,000 birds. This is less, for example, than the number of birds
+computed to have passed Lawrence, Kansas, in one hour, or to have
+passed Progreso, Yucatán, in one twenty-minute interval! Possibly
+observations at Charleston merely chanced to fall on nights of inexplicably
+low densities; further observations will be required to
+clear up this uncertainty.</p>
+<br />
+<br />
+
+<a name="Migration_and_Meteorological_Conditions"></a>
+<div class="caption3 smcap">E. Migration And Meteorological Conditions</div>
+
+<p>The belief that winds affect the migration of birds is an old one.
+The extent to which winds do so, and the precise manner in which
+they operate, have not until rather recently been the subject of real
+investigation. With modern advances in aerodynamics and the development
+of the pressure-pattern system of flying in aviation, attention
+of ornithologists has been directed anew to the part that air
+currents may play in the normal migrations of birds. In America,
+a brief article by Bagg (1948), correlating the observed abundance
+of migrants in New England with the pressure pattern obtaining at
+the time, has been supplemented by the unpublished work of Winnifred
+Smith. Also Landsberg (1948) has pointed out the close correspondence
+between the routes of certain long-distance migrants
+and prevailing wind trajectories. All of this is basis for the hypothesis
+that most birds travel along definite air currents, riding with the
+wind. Since the flow of the air moves clockwise around a high pressure
+area and counterclockwise around a low pressure area, the birds
+are directed away from the "high" and toward the center of the
+"low." The arrival of birds in a particular area can be predicted
+from a study of the surrounding meteorological conditions, and the
+evidence in support of the hypothesis rests mainly upon the success
+of these predictions in terms of observations in the field.</p>
+
+<p>From some points of view, this hypothesis is an attractive one. It
+explains how long distances involved in many migrations may be
+<span class="pagenum"><a name="Page_454" id="Page_454">[Pg_454]</a></span>
+accomplished with a minimum of effort. But the ways in which
+winds affect migration need analysis on a broader scale than can be
+made from purely local vantage points. Studies of the problem
+must be implemented by data accumulated from a study of the process
+in action, not merely from evidence inferred from the visible
+results that follow it. Although several hundred stations operating
+simultaneously would surely yield more definite results, the telescopic
+observations in 1948 offer a splendid opportunity to test the
+theory on a continental scale.</p>
+
+<p>The approach employed has been to plot on maps sector vectors
+and vector resultants that express the directional trends of migration
+in the eastern United States and the Gulf region, and to compare
+the data on these maps with data supplied by the U. S. Weather
+Bureau regarding the directions and velocities of the winds, the location
+of high and low pressure areas, the movement of cold and warm
+fronts, and the disposition of isobars or lines of equal pressure. It
+should be borne in mind when interpreting these vectors that they
+are intended to represent the directions of flight only at the proximal
+ends, or junction points, of the arrows. The tendency of the eye to
+follow a vector to its distal extremity should not be allowed to create
+the misapprehension that the actual flight is supposed to have continued
+on in a straight line to the map location occupied by the
+arrowhead.</p>
+
+<p>A fundamental difficulty in the pressure-pattern theory of migration
+has no doubt already suggested itself to the reader. The difficulty
+to which I refer is made clear by asking two questions. How
+can the birds ever get where they are going if they are dependent
+upon the whim of the winds? How can pressure-pattern flying be
+reconciled with the precision birds are supposed to show in returning
+year after year to the same nesting area? The answer is, in
+part, that, if the wind is a major controlling influence on the routes
+birds follow, there must be a rather stable pattern of air currents
+prevailing from year to year. Such a situation does in fact exist.
+There are maps showing wind roses at 750 and 1,500 meters above
+mean sea level during April and May (Stevens, 1933, figs. 13-14,
+17-18). Similarly, the "Airway Meteorological Atlas for the United
+States" (Anonymous, 1941) gives surface wind roses for April
+(Chart 6) and upper wind roses at 500 and 1,000 meters above mean
+sea level for the combined months of March, April, and May
+(Charts 81 and 82). The same publication shows wind resultants
+at 500 and 1,000 meters above mean sea level (Charts 108 and 109).
+Further information permitting a description in general terms of
+conditions prevailing in April and May is found in the "Monthly
+Weather Review" covering these months (<i>cf.</i> Anonymous, 1948 <i>a</i>,
+Charts 6 and 8; 1948 <i>b</i>, Charts 6 and 8).</p>
+<br />
+<br />
+
+<a name="Fig_38"></a>
+<p><span class="pagenum"><a name="Page_455" id="Page_455">[Pg_455]</a></span></p>
+<div class="center">
+<img src="images/fig_38.png" width="396" height="577" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 38.</span> Over-all sector vectors at major stations in the spring 1948. See text for explanation of system used in determining the length of vectors. For identification of stations, see <a href="#Fig_34">Figure 34</a>.</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_39"></a>
+<span class="pagenum"><a name="Page_456" id="Page_456">[Pg_456]</a></span>
+<div class="center">
+<img src="images/fig_39.png" width="393" height="575" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 39.</span> Over-all net trend of flight directions at stations shown in <a href="#Fig_38">Figure 38</a>. The arrows indicate direction only and their slants were obtained by
+vector analysis of the over-all sector densities.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_457" id="Page_457">[Pg_457]</a></span>
+First, however, it is helpful as a starting point to consider the
+over-all picture created by the flight trends computed from this
+study. In <a href="#Fig_38">Figure 38</a>, the individual sector vectors are mapped for
+the season for all stations with sufficient data. The length of each
+sector vector is determined as follows: the over-all seasonal density
+for the station is regarded as 100 percent, and the total for the
+season of the densities in each individual sector is then expressed as
+a percentage. The results show the directional spread at each station.
+In <a href="#Fig_39">Figure 39</a>, the direction of the over-all vector resultant,
+obtained from the sector vectors on the preceding map, is plotted
+to show the net trend at each station.</p>
+
+<p>As is evident from the latter figure, the direction of the net trend
+at Progreso, Yucatán, is decidedly west of north (N 26&deg; W). At
+Tampico this trend is west of north (N 11&deg; W), but not nearly so
+much so as at Progreso. In Texas, Louisiana, Georgia, Tennessee,
+and Kentucky, it is decidedly east of north. In the upper Mississippi
+Valley and in the eastern part of the Great Plains, the flow
+appears to be northward or slightly west of north. At Winter Park,
+Florida, migration follows in general the slant of the Florida
+Peninsula, but, the meager data from Thomasville, Georgia, do not
+indicate a continuation of this trend.</p>
+
+<p>It might appear, on the basis of the foregoing data, that birds
+migrate along or parallel to the southeast-northwest extension of
+the land masses of Central America and southern Mexico. This
+would carry many of them west of the meridian of their ultimate
+goal, obliging them to turn back eastward along the lines of net
+trend in the Gulf states and beyond. This curved trajectory is undoubtedly
+one of the factors&mdash;but certainly not the only factor&mdash;contributing
+to the effect known as the "coastal hiatus." The question
+arises as to whether this northwestward trend in the southern
+part of the hemisphere is a consequence of birds following the land
+masses or whether instead it is the result of some other natural
+cause such as a response to prevailing winds. I am inclined to the
+opinion that both factors are important. Facts pertinent to this
+opinion are given below.</p>
+
+<p>In April and May a high pressure area prevails over the region
+of the Gulf of Mexico. As the season progresses, fewer and fewer
+<span class="pagenum"><a name="Page_458" id="Page_458">[Pg_458]</a></span>
+cold-front storms reach the Gulf area, and as a result the high
+pressure area over the Gulf is more stable. Since the winds move
+clockwise around a "high," this gives a general northwesterly trajectory
+to the air currents in the vicinity of the Yucatán Peninsula.
+In the western area of the Gulf, the movement of the air mass is
+in general only slightly west of north, but in the central Gulf states
+and lower Mississippi Valley the trend is on the average northeasterly.
+In the eastern part of the Great Plains, however, the average
+circulation veers again slightly west of north. The over-all vector
+resultants of bird migration at stations in 1948, as mapped in
+<a href="#Fig_39">Figure 39</a>, correspond closely to this general pattern.</p>
+
+<p>Meteorological data are available for drawing a visual comparison
+between the weather pattern and the fight pattern on individual
+nights. I have plotted the directional results of four nights
+of observation on the Daily Weather Maps for those dates, showing
+surface conditions (Figures <a href="#Fig_40">40</a>, <a href="#Fig_42">42</a>, <a href="#Fig_44">44</a> and <a href="#Fig_46">46</a>). Each sector vector
+is drawn in proportion to its percentage of the corresponding nightly
+station density; hence the vectors at each station are on an independent
+scale. The vector resultants, distinguished by the large
+arrowheads, are all assigned the same length, but the nightly and
+average hourly station densities are tabulated in the legends under
+each figure. For each map showing the directions of flight, there
+is on the facing page another map showing the directions of winds
+aloft at 2,000 and 4,000 feet above mean sea level on the same
+date (see Figures <a href="#Fig_41">41</a>-<a href="#Fig_47">47</a>). The maps of the wind direction show
+also the velocities.</p>
+
+<p>Unfortunately, since there is no way of analyzing the sector trends
+in terms of the elevations of the birds involved, we have no certain
+way of deciding whether to compare a given trend with the winds at
+2,000, 1,000, or 0 feet. Nor do we know exactly what wind corresponds
+to the average or median flight level, which would otherwise
+be a good altitude at which to study the net trend or vector resultant.
+Furthermore, the Daily Weather Map illustrates conditions that
+obtained at 12:30 A.&nbsp;M. (CST); the winds aloft are based on observations
+made at 10:00 P.&nbsp;M. (CST); and the data on birds covers in
+most cases the better part of the whole night. Add to all this the fact
+that the flight vectors, their resultants, and the wind representations
+themselves are all approximations, and it becomes apparent that
+only the roughest sort of correlations are to be expected.</p>
+
+<p>However, as will be seen from a study of the accompanying maps
+(Figures <a href="#Fig_40">40</a>-<a href="#Fig_47">47</a>), the shifts in wind direction from the surface up to
+4,000 feet above sea level are not pronounced in most of the instances
+<span class="pagenum"><a name="Page_459" id="Page_459">[Pg_459]</a></span>
+at issue, and such variations as do occur are usually in a
+clockwise direction. All in all, except for regions where frontal
+activity is occurring, the weather maps give a workable approximation
+to the average meteorological conditions on a given night.</p>
+
+<p>The maps (Figures <a href="#Fig_40">40</a>-<a href="#Fig_47">47</a>) permit, first, study of the number of
+instances in which the main trend of flight, as shown by the vector
+resultant, parallels the direction of wind at a reasonable potential
+mean flight elevation, and, second, comparison of the larger individual
+sector vectors and the wind currents at any elevation below
+the tenable flight ceiling&mdash;one mile.</p>
+
+<p>On the whole, inspection of the trend of bird-flight and wind direction
+on specific nights supports the principle that the flow of
+migration is in general coincident with the flow of air. It might be
+argued that when the flow of air is toward the north, and when
+birds in spring are proceeding normally in that direction, no significance
+can be attached to the agreement of the two trends. However,
+the same coincidence of wind directions and bird flights seems to be
+maintained when the wind currents deviate markedly from a northward
+trajectory. Figures <a href="#Fig_46">46</a> and <a href="#Fig_47">47</a>, particularly in regard to the
+unusual slants of the flight vectors at Ottumwa, Knoxville, and
+Memphis, illustrate that this coincidence holds even when the wind is
+proceeding obliquely eastward or westward. On the night of May
+22-23, when a high pressure area prevailed from southern Iowa to
+the Atlantic coast, and the trajectory of the winds was northward,
+migration activity at Knoxville and Ottumwa was greatly increased
+and the flow of birds was again northward in the normal seasonal
+direction of migration.</p>
+
+<p>Further study of the data shows fairly conclusively that maximum
+migration activity occurs in the regions of high barometric
+pressure and that the volume of migration is either low or negligible
+in regions of low pressure. The passage of a cold-front storm may
+almost halt migration in spring. This was demonstrated first to me
+by the telescopic method at Baton Rouge, on April 12, 1946, following
+a strong cold front that pushed southeastward across the Gulf
+coastal plain and over the eastern Gulf of Mexico. The winds, as
+usual, shifted and became strong northerly. On this night, following
+the shift of the wind, only three birds were seen in seven hours of
+continuous observation. Three nights later, however, on April 15,
+when the warm air of the Gulf was again flowing from the south, I
+saw 104 birds through the telescope in two hours. Apropos of this
+consideration in the 1948 data are the nights of May 21-22 and 22-23.</p>
+
+
+<p><span class="pagenum"><a name="Page_460" id="Page_460">[Pg_460]</a></span></p>
+<a name="Fig_40"></a>
+<div class="center">
+<img src="images/fig_40.png" width="406" height="506" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 40.</span> Comparison of flight trends and surface weather conditions on April 22-23, 1948. The meteorological data were taken from the U. S. Weather
+Bureau Daily Weather Map for 12:30 A.&nbsp;M. (CST) on April 23. The nightly
+station densities and the average hourly station density (shown in parentheses)
+are as follows:<br />
+
+<table width="100%" summary="frame">
+<tr>
+  <td align="center">
+<table width="80%" summary="stations">
+<tr>
+  <td>&nbsp;5. Louisville: 9,100 (1,100)</td>
+  <td>16. College Station: 13,300 (1,900)</td>
+</tr>
+<tr>
+  <td>&nbsp;6. Murray: 16,300 (2,700)</td>
+  <td>17. Baton Rouge: 6,200 (1,000)</td>
+</tr>
+<tr>
+  <td>&nbsp;8. Stillwater: 1,900 (500)</td>
+  <td>19. Lafayette: 2,800 (600)</td>
+</tr>
+<tr>
+  <td>&nbsp;9. Knoxville: 15,200 (1,700)</td>
+  <td>21. Winter Park: 6,200 (700)</td>
+</tr>
+<tr>
+  <td>13. Oak Grove: 13,600 (1,700)</td>
+  <td>23. Tampico: 11,100 (3,700)</td>
+</tr>
+</table>
+</td>
+</tr>
+</table>
+<br />
+<br />
+</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_461" id="Page_461">[Pg_461]</a></span></p>
+<a name="Fig_41"></a>
+<div class="center">
+<img src="images/fig_45.png" width="482" height="600" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 41.</span> <ins title="TN: correct image inserted above">Winds aloft at 10:00 P.&nbsp;M. on April 22 (CST).</ins> Winds at 2,000 feet
+above mean sea level are shown in black; those at 4,000 feet, in white. Velocities
+are indicated by standard Beaufort Scale of Wind Force. The numbers
+in circles refer to the stations shown in <a href="#Fig_40">Figure 40</a>.</div>
+</div>
+<ins title="TN: correct image inserted above">&nbsp;</ins>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_462" id="Page_462">[Pg_462]</a></span></p>
+<a name="Fig_42"></a>
+<div class="center">
+<img src="images/fig_42.png" width="384" height="478" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 42.</span> Comparison of flight trends and surface weather conditions on April 23-24, 1948. The meteorological data were taken from the U. S. Weather
+Bureau Daily Weather Map for 12:30 A.&nbsp;M. (CST) on April 24. The nightly
+station densities and the average hourly station density (shown in parentheses)
+are as follows:<br />
+
+<table width="100%" summary="frame">
+<tr>
+  <td align="center">
+<table width="80%" summary="stations">
+<tr>
+  <td>&nbsp;1. Albion: 1,100 (300)</td>
+  <td>14. Mansfield: 4,900 (1,200)</td>
+</tr>
+<tr>
+  <td>&nbsp;2. Ottumwa: 5,500 (900)</td>
+  <td>16. College Station: 700 (100)</td>
+</tr>
+<tr>
+  <td>&nbsp;4. Lawrence: 5,400 (1,400)</td>
+  <td>17. Baton Rouge: 1,700 (400)</td>
+</tr>
+<tr>
+  <td>&nbsp;5. Louisville: 13,300 (2,700)</td>
+  <td>18. Pensacola: migration negligible</td>
+</tr>
+<tr>
+  <td>&nbsp;6. Murray: 9,800 (1,400)</td>
+  <td>20. New Orleans: 1,600 (800)</td>
+</tr>
+<tr>
+  <td>&nbsp;8. Stillwater: 800 (100)</td>
+  <td>21. Winter Park: 2,700 (300)</td>
+</tr>
+<tr>
+  <td>&nbsp;9. Knoxville: 8,000 (900)</td>
+  <td>23. Tampico: 63,600 (6,300)</td>
+</tr>
+<tr>
+  <td>10. Memphis: 7,900 (1,000)</td>
+  <td>24. Progreso: 31,300 (3,900)</td>
+</tr>
+</table>
+</td>
+</tr>
+</table>
+<br />
+<br />
+</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_463" id="Page_463">[Pg_463]</a></span></p>
+<a name="Fig_43"></a>
+<div class="center">
+<img src="images/fig_43.png" width="474" height="598" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 43.</span> Winds aloft at 10:00 P.&nbsp;M. on April 23 (CST). Winds at 2,000 feet
+above mean sea level are shown in black; those at 4,000 feet, in white. Velocities
+are indicated by standard Beaufort Scale of Wind Force. The numbers in
+circles refer to the stations shown in <a href="#Fig_42">Figure 42</a>.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_464" id="Page_464">[Pg_464]</a></span></p>
+<a name="Fig_44"></a>
+<div class="center">
+<img src="images/fig_44.png" width="490" height="614" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 44.</span> Comparison of flight trends and surface weather conditions on April 24-25, 1948. The meteorological data were taken from the U. S. Weather
+Bureau Daily Weather Map for 12:30 A.&nbsp;M. (CST) on April 25. The nightly
+station densities and the average hourly station density (shown in parentheses)
+are as follows:<br />
+
+<table width="100%" summary="frame">
+<tr>
+  <td align="center">
+<table width="80%" summary="stations">
+<tr>
+  <td>&nbsp;1. Albion: migration negligible</td>
+  <td>12. Rosedale: 1,100 (100)</td>
+</tr>
+<tr>
+  <td>&nbsp;2. Ottumwa: 4,600 (1,500)</td>
+  <td>14. Mansfield: 1,700 (400)</td>
+</tr>
+<tr>
+  <td>&nbsp;3. Columbia: 1,400 (400)</td>
+  <td>18. Pensacola: migration negligible</td>
+</tr>
+<tr>
+  <td>&nbsp;5. Louisville: 1,700 (200)</td>
+  <td>21. Winter Park: 600 (100)</td>
+</tr>
+<tr>
+  <td>10. Memphis: 6,600 (900)</td>
+  <td>24. Progreso: 27,300 (3,000)</td>
+</tr>
+</table>
+</td>
+</tr>
+</table>
+<br />
+<br />
+</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_465" id="Page_465">[Pg_465]</a></span></p>
+<a name="Fig_45"></a>
+<div class="center">
+<img src="images/fig_41.png" width="405" height="514" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 45.</span> <ins title="TN: correct image inserted above">Winds aloft at 10:00 P.&nbsp;M. on April 24 (CST).</ins> Winds at 2,000 feet
+above mean sea level are shown in black; those at 4,000 feet, in white. Velocities
+are indicated by standard Beaufort Scale of Wind Force. The numbers
+in circles refer to the stations shown in <a href="#Fig_44">Figure 44</a>.</div>
+</div>
+<br />
+<br />
+
+
+<a name="Fig_46"></a>
+<p><span class="pagenum"><a name="Page_466" id="Page_466">[Pg_466]</a></span></p>
+<div class="center">
+<img src="images/fig_46.png" width="474" height="590" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 46.</span> Comparison of flight trends and surface weather conditions on
+May 21-22, 1948. The meteorological data were taken from the U. S. Weather
+Bureau Daily Weather Map for 12:30 A.&nbsp;M. (CST) on May 22. The nightly
+station densities and the average hourly station density (shown in parentheses)
+are as follows:<br />
+
+<table width="100%" summary="frame">
+<tr>
+  <td align="center">
+<table width="80%" summary="stations">
+<tr>
+  <td>&nbsp;2. Ottumwa: 6,900 (1,400)</td>
+  <td>13. Oak Grove: 5,800 (800)</td>
+</tr>
+<tr>
+  <td>&nbsp;5. Louisville: 1,500 (200)</td>
+  <td>14. Mansfield: 2,500 (800)</td>
+</tr>
+<tr>
+  <td>&nbsp;9. Knoxville: 3,200 (500)</td>
+  <td>18. Pensacola: migration negligible</td>
+</tr>
+<tr>
+  <td>10. Memphis: 7,000 (1,200)</td>
+  <td>21. Winter Park: 1,200 (200)</td>
+</tr>
+</table>
+</td>
+</tr>
+</table>
+<br />
+<br />
+</div>
+</div>
+<br />
+<br />
+
+<a name="Fig_47"></a>
+<p><span class="pagenum"><a name="Page_467" id="Page_467">[Pg_467]</a></span></p>
+<div class="center">
+<img src="images/fig_47.png" width="468" height="594" alt="" title="" /><br /><br />
+<div class="fig_text"><span class="bold smcap">Fig. 47.</span> Winds aloft at 10:00 P.&nbsp;M. on May 21 (CST). Winds at 2,000 feet
+above mean sea level are shown. Velocities are indicated by standard Beaufort
+Scale of Wind Force. The numbers in circles refer to the stations shown in
+<a href="#Fig_46">Figure 46</a>.</div>
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_468" id="Page_468">[Pg_468]</a></span>
+On the first night, following the passage of a cold front, migration
+at Ottumwa was comparatively low (6,900 birds in five hours). On
+the following night, when the trajectory of the winds was toward the
+north, the volume of migration was roughly twice as high (22,300
+birds in eight hours). At Louisville, on May 21-22, the nightly
+station density was only 1,500 birds in seven hours, whereas on
+the following night, it was 8,400 birds in the same length of time, or
+about six times greater.</p>
+
+<p>The evidence adduced from the present study gives support to the
+hypothesis that the continental pattern of spring migration in
+eastern North America is regulated by the movement of air masses.
+The clockwise circulation of warm air around an area of high pressure
+provides, on its western edge, tail winds which are apparently
+favorable to northward migration. High pressure areas exhibit a
+centrifugal force outward from the center, which may tend to disperse
+the migratory flight originating at any given point. In contrast,
+the circulation of air in the vicinity of a low pressure area is
+counterclockwise with the force tending to be directed inward toward
+the center. Since the general movement of the air is from the high
+pressure area toward a low pressure area, birds starting their migrations
+with favorable tail winds, are often ultimately carried to
+a region where conditions are decidedly less favorable. In the
+vicinity of an area of low pressure the greater turbulence and high
+wind velocities, combined with the possibly slightly less buoyant
+property of the air, cause birds to descend. Since low pressure areas
+in spring generally precede cold fronts, with an attending shift of
+the wind to the north, an additional barrier to the northward migration
+of birds is imposed. The extreme manifestation of low
+pressure conditions and the manner in which they operate against
+bird flight, are associated with tropical hurricanes. There, the centripetal
+force of the wind is so great that it appears to draw birds
+into the "eye" of the hurricane. A classic example of this effect is
+seen in the case of the birds that came aboard the "West Quechee"
+when this vessel passed through the "eye" of a hurricane in the Gulf
+of Mexico in August, 1927. I have already discussed the details of
+this incident in a previous paper (1946:192). There is also the
+interesting observation of Mayhew (1949), in which a similar
+observation was made of large numbers of birds aboard a ship
+passing through one of these intense low-pressure areas.</p>
+
+<p>Although the forces associated with an ordinary low-pressure area
+are by no means as intense as those associated with a tropical hurricane,
+<span class="pagenum"><a name="Page_469" id="Page_469">[Pg_469]</a></span>
+the forces operating are much the same. Consequently birds
+conceivably might tend to be drawn toward a focal point near the
+center of the low, where the other factors already mentioned would
+tend to precipitate the entire overhead flight. Visible evidence of
+migration would then manifest itself to the field ornithologists.</p>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Conclusions" id="Conclusions"></a>
+<div class="caption2">CONCLUSIONS</div>
+
+<table width="100%" summary="Conclusions">
+<tr>
+  <td class="vtop">&nbsp;1.</td>
+  <td>Telescopic counts of birds passing before the moon may be used
+to determine reliable statistical expressions of the volume of
+migration in terms of direction and of definite units of time
+and space.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;2.</td>
+  <td>Night migrants fly singly more often than in flocks, creating a
+remarkably uniform dispersion on a local scale throughout the
+sky, quite unlike the scattered distributions observable in the
+daytime.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;3.</td>
+  <td>The nocturnal migration of birds is apparently preceded by a
+resting or feeding pause during which there are few migrants in
+the air. It is not to an important degree a non-stop continuation
+of flights begun in the daylight.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;4.</td>
+  <td>Nightly migrational activity in North America varies from
+hour to hour according to a definite temporal pattern, corresponding
+to the <i>Zugunruhe</i> of caged European birds, and expressed
+by increasingly heavy flights up until the hour before
+midnight, followed by a pronounced decline.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;5.</td>
+  <td>The visible effects of the time pattern are subject to modification
+at a particular station by its location with respect to the
+resting areas from which the night's flight originates.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;6.</td>
+  <td>Quantitative and directional studies have so far failed to prove
+that nocturnal migrants favor narrow, topographically-determined
+flight lanes to an important degree.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;7.</td>
+  <td>Flight densities on the east coast of Mexico, though of first
+magnitude, have not yet been demonstrated in the volume demanded
+by the premise that almost all migrants returning to the
+United States from regions to the south do so by coastal routes.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;8.</td>
+  <td>Heavy flights have been recorded from the northern coast of
+Yucatán under circumstances leading inevitably to the conclusion
+that birds migrate across the Gulf of Mexico in considerable
+numbers.</td>
+</tr>
+
+<tr>
+  <td class="vtop">&nbsp;9.</td>
+  <td>There is reason to believe that the importance of the Florida
+Peninsula as an April and May flyway has been over-estimated,
+as regards the numbers of birds using it in comparison with the
+numbers of birds using the Mexican and Gulf routes.</td>
+</tr>
+
+<tr>
+  <td class="vtop">10.</td>
+  <td>The amount of migration is apparently seldom sufficient to produce
+<span class="pagenum"><a name="Page_470" id="Page_470">[Pg_470]</a></span>
+heavy densities of transient species on the ground without
+the operation of concentrative factors such as ecological patterns
+and meteorological forces.</td>
+</tr>
+
+<tr>
+  <td class="vtop">11.</td>
+  <td>The absence or scarcity of transients in some areas in fine
+weather may be explained by this consideration.</td>
+</tr>
+
+<tr>
+  <td class="vtop">12.</td>
+  <td>A striking correlation exists between air currents and the directional
+flight trends of birds, suggesting that most night migrants
+travel by a system of pressure-pattern flying.</td>
+</tr>
+</table>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Literature_Cited" id="Literature_Cited"></a>
+<div class="caption2">LITERATURE CITED</div>
+<br />
+
+<div class="smcap">Allen, R. P., and R. T. Peterson</div>
+<div class="reference">1936.   The hawk migrations at Cape May Point, New Jersey. Auk, 53:393-404.</div>
+<br />
+
+<div class="smcap">Anonymous</div>
+<div class="reference">1936-1941. Tables of computed altitude and azimuth. U. S. Navy Department
+Hydrographic Office. U. S. Govt. Printing Office, Washington,
+D. C., vols. 3-5.</div>
+<div class="reference">1941. Airway meteorological atlas for the United States. Weather Bureau Publ. 1314. U. S. Dept. Commerce, Washington, D. C.</div>
+<div class="reference">1945-1948. The American air almanac. U. S. Naval Observatory. U. S. Govt. Printing Office, Washington, D. C., 3 vols., issued annually.</div>
+<div class="reference">1948<i>a</i>. Meteorological and climatological data for April 1948. Monthly Weather Review, April 1948, 76:65-84, 10 charts.</div>
+<div class="reference">1948<i>b</i>. Meteorological and climatological data for May 1948. Monthly Weather Review, May 1948, 76:85-103, 11 charts.</div>
+<br />
+
+<div class="smcap">Bagg, A. M.</div>
+<div class="reference">1948. Barometric pressure-patterns and spring migration. Auk, 65:147.</div>
+<br />
+
+<div class="smcap">Bergman, G.</div>
+<div class="reference">1941. Der Fruhlingszug von <i>Clangula hyemalis</i> (L.) und <i>Oidemia nigra</i> (L.) bei Helsingfors. Eine Studie über Zugverlauf und Witterung sowie Tagesrhythmus und Flughöhe. Ornis Fennica, 18:1-26.</div>
+<br />
+
+<div class="smcap">Bray, R. A.</div>
+<div class="reference">1895. A remarkable flight of birds. Nature (London), 52:415.</div>
+<br />
+
+<div class="smcap">Carpenter, F. W.</div>
+<div class="reference">1906. An astronomical determination of the height of birds during nocturnal migration. Auk, 23:210-217.</div>
+<br />
+
+<div class="smcap">Chapman, F. M.</div>
+<div class="reference">1888. Observations on the nocturnal migration of birds. Auk, 5:37-39.</div>
+<br />
+
+<div class="smcap">Davis, L. I.</div>
+<div class="reference">1936-1940. The season: lower Rio Grande Valley region. Bird-Lore (now Audubon Mag.), 38-42.</div>
+<br />
+
+<div class="smcap">F. [arner], D. [onald] S.<span class="pagenum"><a name="Page_471" id="Page_471">[Pg_471]</a></span></div>
+<div class="reference">1947. Studies on daily rhythm of caged migrant birds (review of Palmgren article). Bird-Banding, 18:83-84.</div>
+<br />
+
+<div class="smcap">Gates, W. H.</div>
+<div class="reference">1933. Hailstone damage to birds. Science, 78:263-264.</div>
+<br />
+
+<div class="smcap">Howell, A. H.</div>
+<div class="reference">1932. Florida bird life. Florida Department Game and Fresh Water Fish, Tallahassee, 1-579 + 14 pp., 58 pls., 72 text figs.</div>
+<br />
+
+<div class="smcap">Lansberg, H.</div>
+<div class="reference">1948. Bird migration and pressure patterns. Science, 108:708-709.</div>
+<br />
+
+<div class="smcap">Libby, O. G.</div>
+<div class="reference">1899. The nocturnal flight of migratory birds. Auk, 16:140-146.</div>
+<br />
+
+<div class="smcap">Lowery, G. H., Jr.</div>
+<div class="reference">1945. Trans-Gulf spring migration of birds and the coastal hiatus. Wilson Bull., 57:92-121.</div>
+<div class="reference">1946. Evidence of trans-Gulf migration. Auk, 63:175-211.</div>
+<br />
+
+<div class="smcap">Mayhew, D. F.</div>
+<div class="reference">1949. Atmospheric pressure and bird flight. Science, 109:403.</div>
+<br />
+
+<div class="smcap">Overing, R.</div>
+<div class="reference">1938. High mortality at the Washington Monument. Auk, 55:679.</div>
+<br />
+
+<div class="smcap">Palmgren, P.</div>
+<div class="reference">1944. Studien über die Tagesrhythmik gekäfigter Zugvögel. Zeitschrift für Tierpsychologie, 6:44-86.</div>
+<br />
+
+<div class="smcap">Pough, R. H.</div>
+<div class="reference">1948. Out of the night sky. Audubon Mag., 50:354-355.</div>
+<br />
+
+<div class="smcap">Putkonen, T. A.</div>
+<div class="reference">1942. Kevätmuutosta Viipurinlakdella. Ornis Fennica, 19:33-44.</div>
+<br />
+
+<div class="smcap">Rense, W. A.</div>
+<div class="reference">1946. Astronomy and ornithology. Popular Astronomy, 54:55-73.</div>
+<br />
+
+<div class="smcap">Scott, W. E. D.</div>
+<div class="reference">1881<i>a.</i> Some observations on the migration of birds. Bull. Nuttall Orni. Club, 6:97-100.</div>
+<div class="reference">1881<i>b.</i> Migration of birds at night. Bull. Nuttall Orni. Club, 6:188.</div>
+<br />
+
+<div class="smcap">Siivonen, L.</div>
+<div class="reference">1936. Die Stärkevariation des Nächtlichen Zuges bei <i>Turdus ph. philomelos</i> Brehn und <i>T. musicus</i> L. auf Grund der Zuglaute geschätz und mit der Zugunruhe einer gekäfigten Singdrossel Verglichen. Ornis Fennica, 13:59-63.</div>
+<br />
+
+<div class="smcap">Spofford, W. R.</div>
+<div class="reference">1949. Mortality of birds at the ceilometer of the Nashville airport. Wilson Bull., 61:86-90.</div>
+<br />
+
+<div class="smcap">Stebbins, J.</div>
+<div class="reference">1906. A method of determining height of migrating birds. Popular Astronomy,
+14:65-70.</div>
+<br />
+
+<div class="smcap"><ins title="TN: Stephens => Stevens">Stevens</ins>, Loyd A.<span class="pagenum"><a name="Page_472" id="Page_472">[Pg_472]</a></span></div>
+<div class="reference">1933. Upper-air wind roses and resultant winds for the eastern United States.
+Monthly Weather Review, Supplement No. 35, November 13, pp. 1-3,
+65 figs.</div>
+<br />
+
+<div class="smcap">Stone, W.</div>
+<div class="reference">1906. Some light on night migration. Auk, 23:249-252.</div>
+<div class="reference">1937. Bird studies at Old Cape May. Delaware Valley Orni. Club, Philadelphia,
+Vol. 1, 1-520 + 15 pp., pls. 1-46 and frontis.</div>
+<br />
+
+<div class="smcap">Thomson, A. L.</div>
+<div class="reference">1926. Problems of bird migration. Houghton Mifflin Company, Boston.</div>
+<br />
+
+<div class="smcap">Van Oordt, G.</div>
+<div class="reference">1943. Vogeltrek. E. J. Brill, Leiden, xii + 145 pp.</div>
+<br />
+
+<div class="smcap">Very, F. W.</div>
+<div class="reference">1897. Observations of the passage of migrating birds across the lunar disc on the nights of September 23 and 24, 1896. Science, 6:409-411.</div>
+<br />
+
+<div class="smcap">Walters, W.</div>
+<div class="reference">1927. Migration and the telescope. Emu, 26:220-222.</div>
+<br />
+
+<div class="smcap">West, R. H.</div>
+<div class="reference">1896. Flight of birds across the moon's disc. Nature (London), 53:131.</div>
+<br />
+
+<div class="smcap">Williams, G. G.</div>
+<div class="reference">1941-1948. The season: Texas coastal region. Audubon Mag., 43-50.</div>
+<div class="reference">1945. Do birds cross the Gulf of Mexico in spring? Auk, 62:98-111.</div>
+<div class="reference">1947. Lowery on trans-Gulf migration. Auk, 64:217-238.</div>
+<br />
+
+<div class="smcap">Winkenwerder, H. A.</div>
+<div class="reference">1902<i>a</i>. The migration of birds with special reference to nocturnal flight. Bull.
+Wisconsin Nat. Hist. Soc., 2:177-263.</div>
+<div class="reference">1902<i>b</i>. Some recent observations on the migration of birds. Bull. Wisconsin
+Nat. Hist. Soc., 2:97-107.</div>
+<br />
+
+<br />
+<br />
+<i>Transmitted June 1, 1949.</i><br />
+<br />
+<br />
+
+<div class="center">
+23-1020<br />
+<br />
+<img src="images/square.png" width="16" height="17" alt="square" title="square" />
+</div>
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Page_i" id="Page_i">[Pg_i]</a></span></p>
+
+<div class="caption2">UNIVERSITY OF KANSAS PUBLICATIONS</div>
+
+
+<p>The University of Kansas Publications, Museum of Natural History,
+are offered in exchange for the publications of learned societies
+and institutions, universities and libraries. For exchanges and
+information, address the <span class="smcap">Exchange Desk, University of Kansas
+Library, Lawrence, Kansas</span>, U. S. A.</p>
+
+<span class="smcap">Museum Of Natural History.</span>&mdash;E. Raymond Hall, Chairman, Editorial Committee.<br />
+<br />
+<div style="margin-left:3em; text-indent:-1.5em">This series contains contributions from the Museum of Natural History.<br />
+Cited as Univ. Kans. Publ., Mus. Nat. Hist.</div>
+
+<table width="100%" summary="Publication List">
+<tr>
+  <td class="vtop">Vol.&nbsp;1.</td>
+  <td colspan="2">(Complete) Nos. 1-26. Pp. 1-638. August 15, 1946-January 20, 1951.</td>
+</tr>
+<tr>
+  <td class="vtop">Vol. 2.</td>
+  <td colspan="2">(Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444, 140 figures in text. April 9, 1948.</td>
+</tr>
+<tr>
+  <td class="vtop">Vol. 3.</td>
+  <td class="vtop">1.</td>
+  <td>The avifauna of Micronesia, its origin, evolution, and distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text. June 12, 1951.</td>
+</tr>
+<tr>
+  <td>&nbsp;</td>
+  <td class="vtop">2.</td>
+  <td>A quantitative study of the nocturnal migration of birds. By George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29, 1951.</td>
+</tr>
+</table>
+<br />
+<br />
+<br />
+<br />
+
+<p><span class="pagenum"><a name="Notes" id="Notes">[Notes]</a></span></p>
+
+<div class="trans_notes">
+<div class="caption2">Transcriber's Notes</div>
+
+<p>With the exception of the typographical corrections detailed below
+and some minor corrections for missing periods or extra punctuation
+(item 28 in List of Figures), the text presented here is that
+contained in the original printed version. A transcription of the
+Data presented in <a href="#Fig_12">Figure 12</a> was added (see <a href="#Fig_12_Trans">below</a>) to illustrate the information
+contained on that sheet. Some text was moved to rejoin paragraphs.</p>
+
+<p>There are two notes in the original text indicating that the images
+for Figures <a href="#Fig_41">41</a> and <a href="#Fig_45">45</a> were transposed. The correct images have been
+placed with the captions and the two notes were removed. Lastly, the
+cover image was compiled from a copy of the original cover with two of the
+graphics contained in the article added and the list of UK pulications
+was moved to the end of the document.</p>
+
+<div class="caption2">Typographical Corrections</div>
+<br />
+<div  style="margin-left: 30%">
+<table summary="typos">
+<tr>
+  <td>Page</td>
+  <td>Correction</td>
+</tr>
+<tr>
+  <td><a href="#Page_385">385</a></td>
+  <td>flght &#8658; flight</td>
+</tr>
+<tr>
+  <td><a href="#Page_394">394</a></td>
+  <td>diargrams &#8658; diagrams</td>
+</tr>
+<tr>
+  <td><a href="#Page_404">404</a></td>
+  <td>Determinaton &#8658; Determination</td>
+</tr>
+<tr>
+  <td><a href="#Page_411">411</a></td>
+  <td>obsever &#8658; observer</td>
+</tr>
+<tr>
+  <td><a href="#Page_419">419</a></td>
+  <td>Morover &#8658; Moreover</td>
+</tr>
+<tr>
+  <td><a href="#Page_425">425</a></td>
+  <td>Mississippii &#8658; Mississippi</td>
+</tr>
+<tr>
+  <td><a href="#Page_425">425</a></td>
+  <td>a &#8658; as</td>
+</tr>
+<tr>
+  <td><a href="#Page_430">430</a></td>
+  <td>at &#8658; and</td>
+</tr>
+<tr>
+  <td class="vtop"><a href="#Page_431">431</a></td>
+  <td>inserted "a"<br />("&hellip;traveling along a certain topographic feature&hellip;")</td>
+</tr>
+<tr>
+  <td><a href="#Page_442">442</a></td>
+  <td>concensus &#8658; consensus</td>
+</tr>
+<tr>
+  <td><a href="#Page_472">472</a></td>
+  <td>Stephens, Loyd A. &#8658; Stevens, Lloyd A.</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+</div>
+<br />
+<br />
+<br />
+<br />
+
+<a name="Fig_12_Trans"></a>
+<div style="margin-left: 20%">
+<table style="padding:12px; width: 38em; border: solid 1px #000;" summary="Fig. 12 Transcription">
+<tr>
+  <td>
+<div class="caption3">Transcription of the Data in <a href="#Fig_12">Figure 12</a></div>
+
+<div class="text_lf">
+<pre>
+                  ORIGINAL DATA SHEET
+
+ DATE        <span class="undrln">24-25 April 1948</span>   LOCALITY  <span class="undrln">Progreso, Yucatán</span>
+
+ OBSERVERS   <span class="undrln">Harold Harry; George H. Lowery</span>
+
+ WEATHER     <span class="undrln">Moderate to strong "trade" winds along coast, slightly N of E.</span>
+             <span class="undrln">Moon emerged above low cloud bank at 8:26.                   &nbsp;</span>
+
+ INSTRUMENT  <span class="undrln">B. &amp; L. 19.5 Spotting Scope; image erect                     &nbsp;</span>
+
+ REMARKS     <span class="undrln">Observation station located 1 mile from land, over Gulf of   &nbsp;</span>
+             <span class="undrln">Mexico, at end of new Progreso wharf                         &nbsp;</span>
+
+ -----------+------+-------+--------------------------------------------
+    TIME    |  IN  |  OUT  |                   REMARKS
+ -----------+------+-------+--------------------------------------------
+   C.S.T    |      |       |
+   8:26     |  --  |  --   | observations begin; H.H. observing
+     50     | 4:30 |  9    | slow; small
+     56     | 3    | 10    | medium size
+   9:00     | 2    | 10:30 | very small
+     11     | 5    |  9:30 | moderately fast
+     25     | 5    | 10    | very small; rather slow
+     26     | 3    | 11    |     "            "
+     36     | 5    | 10    | medium size
+     40     | 3    | 10    |   "     "
+     43     | 5:30 |  9    |   "     "
+     46     | 3:30 | 10    | small
+     56     | 4:30 | 10    | medium size
+ 9:58-10:00 |  --  |  --   | time out to change observers; G.L. at scope
+  10:05     | 4:30 | 11:30 | small
+     06     | 3    | 11    |
+     12     | 5    |  8    | very small
+     25     | 5    | 12    | very fast; small
+     30     | 4    | 10    | small
+     32     | 4    | 11    |   "
+     32     | 2    | 11    |   "
+     33     | 5    | 11    |   "
+     33     | 4    |  1    |   "
+     33     | 5:30 | 11    |   "
+     35     | 4:30 | 10    | swallow-like
+     36     | 5    |  1:30 |
+</pre>
+</div>
+</td>
+</tr>
+</table>
+</div>
+<br />
+<br />
+</div><!-- End Book -->
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of A Quantitative Study of the Nocturnal
+Migration of Birds., by George H. Lowery.
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@@ -0,0 +1,4466 @@
+The Project Gutenberg EBook of A Quantitative Study of the Nocturnal
+Migration of Birds., by George H. Lowery.
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: A Quantitative Study of the Nocturnal Migration of Birds.
+       Vol.3 No.2
+
+Author: George H. Lowery.
+
+Editor: E. Raymond Hall
+
+Release Date: October 31, 2011 [EBook #37894]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK A QUANTITATIVE STUDY OF THE ***
+
+
+
+
+Produced by Chris Curnow, Tom Cosmas, Joseph Cooper, The
+Internet Archive for some images and the Online Distributed
+Proofreading Team at http://www.pgdp.net
+
+
+
+
+
+
+
+
+
+  A Quantitative Study of the Nocturnal
+  Migration of Birds
+
+  BY
+
+  GEORGE H. LOWERY, JR.
+
+  University of Kansas Publications
+  Museum of Natural History
+
+  Volume 3, No. 2, pp. 361-472, 47 figures in text
+  June 29, 1951
+
+  University of Kansas
+  LAWRENCE
+  1951
+
+
+
+
+  UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
+
+  Editors: E. Raymond Hall, Chairman; A. Byron Leonard,
+  Edward H. Taylor, Robert W. Wilson
+
+  UNIVERSITY OF KANSAS
+  Lawrence, Kansas
+
+  PRINTED BY
+  FERD VOILAND, JR., STATE PRINTER
+  TOPEKA, KANSAS
+  1951
+
+  [Union Label]
+
+  23-1020
+
+
+
+
+  A Quantitative Study of the Nocturnal
+  Migration of Birds
+
+  By
+
+  GEORGE H. LOWERY, JR.
+
+
+
+
+CONTENTS
+
+
+                                                                   Page
+
+  INTRODUCTION                                                      365
+
+  ACKNOWLEDGMENTS                                                   367
+
+  PART I. FLIGHT DENSITIES AND THEIR DETERMINATION                  370
+
+     Lunar Observations of Birds and the Flight Density Concept     370
+
+     Observational Procedure and the Processing of Data             390
+
+  PART II. THE NATURE OF NOCTURNAL MIGRATION                        408
+
+     Horizontal Distribution of Birds on Narrow Fronts              409
+
+     Density as a Function of the Hour of the Night                 413
+
+     Migration in Relation to Topography                            424
+
+     Geographical Factors and the Continental Density Pattern       432
+
+     Migration and Meteorological Conditions                        453
+
+  CONCLUSIONS                                                       469
+
+  LITERATURE CITED                                                  470
+
+
+
+
+LIST OF FIGURES
+
+
+   Figure                                                          Page
+
+   1. The field of observation as it appears to the observer        374
+
+   2. Determination of diameter of cone at any point                375
+
+   3. Temporal change in size of the field of observation           376
+
+   4. Migration at Ottumwa, Iowa                                    377
+
+   5. Geographic variation in size of cone of observation           378
+
+   6. The problem of sampling migrating birds                       380
+
+   7. The sampling effect of a square                               381
+
+   8. Rectangular samples of square areas                           382
+
+   9. The effect of vertical components in bird flight              383
+
+  10. The interceptory potential of slanting lines                  384
+
+  11. Theoretical possibilities of vertical distribution            388
+
+  12. Facsimile of form used to record data in the field            391
+
+  13. The identification of co-ordinates                            392
+
+  14. The apparent pathways of birds seen in one hour               393
+
+  15. Standard form for plotting the apparent paths of flight       395
+
+  16. Standard sectors for designating flight trends                398
+
+  17. The meaning of symbols used in the direction formula          399
+
+  18. Form used to compute zenith distance and azimuth of the moon  400
+
+  19. Plotting sector boundaries on diagrammatic plots              402
+
+  20. Form to compute sector densities                              403
+
+  21. Determination of the angle [alpha]                            404
+
+  22. Facsimile of form summarizing sector densities                405
+
+  23. Determination of net trend density                            406
+
+  24. Nightly station density curve at Progreso, Yucatan            407
+
+  25. Positions of the cone of observation at Tampico, Tamps        411
+
+  26. Average hourly station densities in spring of 1948            414
+
+  27. Hourly station densities plotted as a percentage of peak      415
+
+  28. Incidence of maximum peak at the various hours of the
+        night in 1948                                               416
+
+  29. Various types of density-time curves                          418
+
+  30. Density-time curves on various nights at Baton Rouge          422
+
+  31. Directional components in the flight at Tampico, Tamps        428
+
+  32. Hourly station density curve at Tampico, Tamps                429
+
+  33. The nightly net trend of migrations at three stations in 1948 431
+
+  34. Stations at which telescopic observations were made in 1948   437
+
+  35. Positions of the cone of observation at Progreso, Yucatan     443
+
+  36. Hourly station density curve at Progreso, Yucatan             444
+
+  37. Sector density representation on two nights at
+        Rosedale, Miss.                                             451
+
+  38. Over-all sector vectors at major stations in spring of 1948   455
+
+  39. Over-all net trend of flight directions shown in Figure 38    456
+
+  40. Comparison of flight trends and surface weather conditions
+        on April 22-23, 1948                                        460
+
+  41. Winds aloft at 10:00 P. M. on April 22 (CST)                  461
+
+  42. Comparison of flight trends and surface weather conditions
+        on April 23-24, 1948                                        462
+
+ 43. Winds aloft at 10:00 P. M. on April 23 (CST)                   463
+
+  44. Comparison of flight trends and surface weather conditions
+        on April 24-25, 1948                                        464
+
+  45. Winds aloft at 10:00 P. M. on April 24 (CST)                  465
+
+  46. Comparison of flight trends and surface weather conditions
+        on May 21-22, 1948                                          466
+
+  47. Winds aloft at 10:00 P. M. on May 21 (CST)                    467
+
+
+
+
+INTRODUCTION
+
+
+The nocturnal migration of birds is a phenomenon that long has
+intrigued zoologists the world over. Yet, despite this universal
+interest, most of the fundamental aspects of the problem remain
+shrouded in uncertainty and conjecture.
+
+Bird migration for the most part, whether it be by day or by night, is
+an unseen movement. That night migrations occur at all is a conclusion
+derived from evidence that is more often circumstantial than it is
+direct. During one day in the field we may discover hundreds of
+transients, whereas, on the succeeding day, in the same situation, we
+may find few or none of the same species present. On cloudy nights we
+hear the call notes of birds, presumably passing overhead in the
+seasonal direction of migration. And on stormy nights birds strike
+lighthouses, towers, and other tall obstructions. Facts such as these
+are indisputable evidences that migration is taking place, but they
+provide little basis for evaluating the flights in terms of magnitude
+or direction.
+
+Many of the resulting uncertainties surrounding the nocturnal
+migration of birds have a quantitative aspect; their resolution hinges
+on how many birds do one thing and how many do another. If we knew,
+for instance, how many birds are usually flying between 2 and 3 A. M.
+and how this number compares with other one-hour intervals in the
+night, we would be in a position to judge to what extent night flight
+is sustained from dusk to dawn. If we could measure the number of
+birds passing selected points of observation, we could find out
+whether such migration in general proceeds more or less uniformly on a
+broad front or whether it follows certain favored channels or flyways.
+This in turn might give us a clearer insight into the nature of the
+orienting mechanism and the extent to which it depends on visual
+clues. And, if we had some valid way of estimating the number of birds
+on the wing under varying weather conditions, we might be able to
+understand better the nature and development of migration waves so
+familiar to field ornithologists. These are just random examples
+suggesting some of the results that may be achieved in a broad field
+of inquiry that is still virtually untouched--the quantitative study
+of migratory flights.
+
+This paper is a venture into that field. It seeks to evaluate on a
+more factual basis the traditional ideas regarding these and similar
+problems, that have been developed largely from circumstantial
+criteria. It is primarily, therefore, a study of comparative
+quantities or volumes of migration--or what may be conveniently called
+flight densities, if this term be understood to mean simply the number
+of birds passing through a given space in a given interval of time.
+
+In the present study, the basic data permitting the numerical
+expression of such migration rates from many localities under many
+different sets of circumstances were obtained by a simple method. When
+a small telescope, mounted on a tripod, is focused on the moon, the
+birds that pass before the moon's disc may be seen and counted, and
+their apparent pathways recorded in terms of cooerdinates. In bare
+outline, this approach to the problem is by no means new.
+Ornithologists and astronomers alike have recorded the numbers of
+birds seen against the moon in stated periods of time (Scott, 1881a
+and 1881b; Chapman, 1888; Libby, 1889; West, 1896; Very, 1897;
+Winkenwerder, 1902a and 1902b; Stebbins, 1906; Carpenter, 1906).
+Unfortunately, as interesting as these observations are, they furnish
+almost no basis for important generalizations. Most of them lack
+entirely the standardization of method and the continuity that would
+make meaningful comparisons possible. Of all these men, Winkenwerder
+appears to have been the only one to follow up an initial one or two
+nights of observation with anything approaching an organized program,
+capable of leading to broad conclusions. And even he was content
+merely to reproduce most of his original data without correlation or
+comment and without making clear whether he fully grasped the
+technical difficulties that must be overcome in order to estimate the
+important flight direction factor accurately.
+
+The present study was begun in 1945, and early results obtained were
+used briefly in a paper dealing with the trans-Gulf migration of birds
+(Lowery, 1946). Since that time the volume of field data, as well as
+the methods by which they can be analyzed, has been greatly expanded.
+In the spring of 1948, through the cooperation and collaboration of a
+large number of ornithologists and astronomers, the work was placed on
+a continent-wide basis. At more than thirty stations (Figure 34, page
+437) on the North American continent, from Yucatan to Ontario, and
+from California to South Carolina, observers trained telescopes
+simultaneously on the moon and counted the birds they saw passing
+before its disc.
+
+Most of the stations were in operation for several nights in the full
+moon periods of March, April, and May, keeping the moon under constant
+watch from twilight to dawn when conditions permitted. They have
+provided counts representing more than one thousand hours of
+observation, at many places in an area of more than a million square
+miles. But, as impressive as the figures on the record sheets are,
+they, like the published observations referred to above, have dubious
+meaning as they stand. Were we to compare them directly, station for
+station, or hour for hour, we would be almost certain to fall into
+serious errors. The reasons for this are not simple, and the measures
+that must be taken to obtain true comparisons are even less so. When I
+first presented this problem to my colleague, Professor William A.
+Rense, of the Department of Physics and Astronomy at Louisiana State
+University, I was told that mathematical means exist for reducing the
+data and for ascertaining the desired facts. Rense's scholarly insight
+into the mathematics of the problem resulted in his derivation of
+formulae that have enabled me to analyze on a comparable basis data
+obtained from different stations on the same night, and from the same
+station at different hours and on different nights. Astronomical and
+technical aspects of the problem are covered by Rense in his paper
+(1946), but the underlying principles are discussed at somewhat
+greater length in this paper.
+
+Part I of the present paper, dealing with the means by which the data
+were obtained and processed, will explore the general nature of the
+problem and show by specific example how a set of observations is
+prepared for analysis. Part II will deal with the results obtained and
+their interpretation.
+
+
+
+
+ACKNOWLEDGMENTS
+
+
+In the pursuit of this research I have received a tremendous amount of
+help from my colleagues, students, and other friends. In the first
+place, in order to obtain much of the data on which the study was
+based, it was necessary to enlist the aid of many persons in various
+parts of the country and to draw heavily on their time and patience to
+get all-night telescopic counts of migrating birds. Secondly, the
+processing of the primary data and its subsequent analysis demanded
+that I delve into the fields of astronomy and mathematics. Here, from
+the outset, I have enjoyed the constant and untiring help of Professor
+W. A. Rense of the Department of Physics and Astronomy at Louisiana
+State University. Without his collaboration, I would not have been
+able to do this work, for he not only supplied formulae whereby I was
+able to make desired computations, but time and again he maneuvered me
+through my difficulties in the mathematical procedures. Moreover,
+Professor Rense has manifested a great interest in the ornithological
+aspect of the problem, and his trenchant advice has been of
+inestimable value to me. No less am I indebted to my associate, Robert
+J. Newman, with whom I have spent untold hours discussing the various
+aspects of the problem. Indeed, most of the concepts that have evolved
+in the course of this study have grown out of discussions over a
+four-year period with both Rense and Newman. Whatever merit this work
+may have may be attributable in no small part to the help these two
+men have given me. In the preparation of many of the illustrations, I
+am further obligated to Newman for his excellent creative ideas as
+well as draftsmanship, and to Miss Helen Behrnes and A. Lowell Wood
+for their assistance.
+
+The mathematical computations required in this study have been
+laborious and time-consuming. It is estimated that more than two
+thousand man-hours have gone into this phase of the work alone.
+Whereas I have necessarily done most of this work, I have received a
+tremendous amount of help from A. Lowell Wood. Further assistance in
+this regard came from Herman Fox, Donald Norwood, and Lewis Kelly.
+
+The recording of the original field data in the spring of 1948 from
+the thirty-odd stations in North America involved the participation of
+more than 200 ornithologists and astronomers. This collaboration
+attests to the splendid cooperative spirit that exists among
+scientists. Many of these persons stayed at the telescope, either as
+observer or as recorder, hours on end in order to get sets of data
+extending through a whole night.
+
+The following were responsible for much of the field data herein used:
+J. R. Andrews, S. A. Arny, M. Dale Arvey, H. V. Autrey, Charles C.
+Ayres, Mr. and Mrs. Roy Bailey, Irwin L. Baird, Maurice F. Baker,
+Rollin H. Baker, Bedortha and Edna Baldwin, Mrs. A. Marguerite
+Baumgartner, T. A. Becket, Paul Bellington, Donald Bird, Carl Black,
+Jr., Lea Black, Lytle Blankenship, Mr. and Mrs. J. Stewart Boswell,
+Bruce Boudreaux, Frank Bray, Mr. and Mrs. Leonard Brecher, Homer
+Brewer, Mrs. Harvey Broome, Heyward Brown, Floyd Browning, Cyril
+Broussard, Paul Buress, Ralph M. Burress, Robert Cain, Don Carlos,
+Mrs. Reba Campbell, Mr. and Mrs. E. Burnham Chamberlain, Laura Chaney,
+Van B. Chaney, Jr., Edward Clebsch, Mr. and Mrs. Ben B. Coffey,
+William Cook, Dr. Jack Craven, Hugh C. and William Davis, Katherine
+Davis, Richard Davis, Richard DeArment, Robert E. Delphia, J. C.
+Dickinson, Mr. and Mrs. Otto Dietrich, John Dietrich, Clara Dixon,
+Nina Driven, John J. Duffy, Mr. and Mrs. R. J. Dunbar, Betty Dupre,
+Bernard E. Eble, Jr., Robert G. Eble, Dr. and Mrs. William H. Elder,
+C. C. Emory, Davis Emory, Alice H. Farnsworth, James Fielding, William
+R. Fish, Mr. and Mrs. Myron Ford, W. G. Fuller, Louis Gainey, Dr. Mary
+E. Gaulden, Mr. and Mrs. John J. Giudice, Lt. L. E. Goodnight, Earl R.
+Greene, Max Grilkey, W. W. H. Gunn, Noel Maxwell Hall, Jr., A. J.
+Hanna, Paul Hansen, Harold W. Harry, Joseph Healy, Dorothy Helmer, Mr.
+and Mrs. John H. Helmer, Philip E. Hoberecht, William D. Hogan, Dr.
+and Mrs. Joseph C. Howell, E. J. Huggins, Mrs. Walter Huxford, Hugh
+Iltis, W. S. Jennings, William M. Johnson, William Kasler, Luther F.
+Keeton, Lawrence C. Kent, W. H. Kiel, L. P. Kindler, Mr. and Mrs.
+Joseph E. King, Harriet Kirby, E. J. Koestner, Roy Komarek, Ann
+Knight, Mr. and Mrs. N. B. Langworthy, Mr. and Mrs. C. F. Lard,
+Prentiss D. Lewis, Ernest Liner, Dr. and Mrs. R. W. Lockwood, Dr.
+Harvey B. Lovell, William J. Lueck, Don Luethy, James Major, Mr. and
+Mrs. Russell L. Mannette, Mrs. John B. Mannix, Donald Mary, Dale E.
+McCollum, Stewart McConnell, Mr. and Mrs. M. L. McCroe, Robert L.
+McDaniel, Mr. and Mrs. Frank McGill, Thomas Merimer, Mr. and Mrs. I.
+S. H. Metcalf, Ann Michener, John Michener, T. H. Milby, D. S. Miller,
+Burt Monroe, Jr., Burt Monroe, Sr., Mrs. R. A. Monroe, Gordon
+Montague, Duryea Morton, James Mosimonn, Don L. Moyle, Grant Murphy,
+John T. Murphy, Mrs. H. F. Murphy, Mrs. Hill Myers, Mr. and Mrs.
+Robert J. Newman, William Nichols, R. A. Norris, Floyd Oaks, Eugene P.
+Odum, Mrs. E. E. Overton, Lennie E. Pate, Kenneth Patterson, Ralph
+Paxton, Louis Peiper, Marie Peiper, Mr. and Mrs. Harold S. Peters,
+Mary Peters, Mr. and Mrs. D. W. Pfitzer, Betty Plice, Max Plice,
+Lestar Porter, D. R. Power, Kenneth Price, George Rabb, Marge Reese,
+Wayne L. Reeve, C. L. Riecke, R. D. Ritchie, V. E. Robinson, Beverly
+J. Rose, Mary Jane Runyon, Roger Rusk, Bernd Safinsley, Mr. and Mrs.
+Glen C. Sanderson, Lewis L. Sandidge, John Sather, J. Benton Schaub,
+Evelyn Schneider, Henry W. Setzer, Mr. and Mrs. Walter Shackleton, Mr.
+and Mrs. Francis P. Shannon, Mr. and Mrs. Charles Shaw, Paul H.
+Shepard, Jr., Alan C. Sheppard, Mabel Slack, Alice Smith, R. Demett
+Smith, Jr., Nat Smith, Major and Mrs. Charles H. Snyder, Albert
+Springs, Dr. and Mrs. Fred W. Stamm, J. S. Steiner, Mrs. Paul
+Stephenson, Herbert Stern, Jr., Herbert Stoddard, Mr. and Mrs. F. W.
+Stomm, Charles Strull, Harold P. Strull, Mrs. Fan B. Tabler, Dr. and
+Mrs. James T. Tanner, S. M. H. Tate, David Taylor, Hall Tennin, Scott
+Terry, Mr. and Mrs. S. Charles Thacher, Olive Thomas, G. A. Thompson,
+Jr., Dr. and Mrs. S. R. Tipton, Robert Tucker, Tom Uzzel, Mr. and Mrs.
+M. G. Vaiden, Richard Vaught, Edward Violante, Brother I. Vincent,
+Marilyn L. Walker, Mr. and Mrs. Willis Weaver, Mr. and Mrs. W. L.
+Webb, Margaret M. L. Wehking, W. A. Welshans, Jr., Mrs. J. F.
+Wernicke, Francis M. Weston, Miss G. W. Weston, Dr. James W. White,
+John A. White, A. F. Wicke, Jr., Oren Williams, J. L. Wilson III, W.
+B. Wilson, Dr. and Mrs. Leonard Wing, Sherry Woo, Rodney Wuthnow,
+Grace Wyatt, Mr. and Mrs. Malcom Young, Mr. and Mrs. A. J. Zimmerman.
+To the scores of other people who assisted in making these
+observations I extend my hearty thanks.
+
+Drs. E. R. Hall, Edward H. Taylor, and H. B. Hungerford of the
+University of Kansas have read the manuscript and have made valuable
+suggestions, as have also Dr. W. H. Gates of Louisiana State
+University and Dr. Donald S. Farner of the State College of
+Washington. Dr. Farner has also been of great help, together with Drs.
+Ernst Mayr, J. Van Tyne, and Ernst Schuez, in suggesting source
+material bearing on the subject in foreign literature. Dr. N. Wyaman
+Storer, of the University of Kansas, pointed out a short-cut in the
+method for determining the altitude and azimuth of the moon, which
+resulted in much time being saved. For supplying climatological data
+and for guidance in the interpretation thereof, I am grateful to Dr.
+Richard Joel Russell, Louisiana State University; Commander F. W.
+Reichelderfer, Chief of the U. S. Weather Bureau, Washington, D. C.;
+Mr. Merrill Bernard, Chief of the Climatological and Hydrologic
+Services; and Mr. Ralph Sanders, U. S. Weather Bureau at New Orleans,
+Louisiana.
+
+Acknowledgment is made to Bausch and Lomb Optical Company for the loan
+of six telescopes for use in this project. Messrs. G. V. Cutler and
+George Duff of Smith and Johnson Steamship Company, operators of the
+Yucatan Line, are to be thanked for granting me free passage on the
+"S. S. Bertha Brovig" to Progreso, Yucatan, where I made observations
+in 1945 and 1948. I am also indebted to the Louisiana State University
+Committee on Faulty Research for a grant-in-aid.
+
+
+
+
+PART I. FLIGHT DENSITIES AND THEIR DETERMINATION
+
+
+A. LUNAR OBSERVATIONS OF BIRDS AND THE FLIGHT DENSITY CONCEPT
+
+The subject matter of this paper is wholly ornithological. It is
+written for the zoologist interested in the activities of birds. But
+its bases, the principles that make it possible, lie in other fields,
+including such rather advanced branches of mathematics as analytical
+geometry, spherical geometry, and differential calculus. No exhaustive
+exposition of the problem is practicable, that does not take for
+granted some previous knowledge of these disciplines on the part of
+all readers.
+
+There are, however, several levels of understanding. It is possible to
+appreciate _what_ is being done without knowing _how_ to do it; and it
+is possible to learn how to carry out the successive steps of a
+procedure without entirely comprehending _why_. Some familiarity with
+the concepts underlying the method is essential to a full
+understanding of the results achieved, and details of procedure must
+be made generally available if the full possibilities of the
+telescopic approach are to be realized. Without going into proof of
+underlying propositions or actual derivation of formulae, I shall
+accordingly present a discussion of the general nature of the problem,
+conveyed as much as possible in terms of physical visualization. The
+development begins with the impressions of the student when he first
+attempts to investigate the movements of birds by means of the moon.
+
+
+_What the Observer Sees_
+
+Watched through a 20-power telescope on a cloudless night, the full
+moon shines like a giant plaster hemisphere caught in the full glare
+of a floodlight. Inequalities of surface, the rims of its craters, the
+tips of its peaks, gleam with an almost incandescent whiteness; and
+even the darker areas, the so-called lunar seas, pale to a clear,
+glowing gray.
+
+Against this brilliant background, most birds passing in focus appear
+as coal-black miniatures, only 1/10 to 1/30 the apparent diameter of
+the moon. Small as these silhouettes are, details of form are often
+beautifully defined--the proportions of the body, the shape of the
+tail, the beat of the wings. Even when the images are so far away that
+they are pin-pointed as mere flecks of black against the illuminated
+area, the normal eye can follow their progress easily. In most cases
+the birds are invisible until the moment they "enter," or pass
+opposite, the rim of the moon and vanish the instant they reach the
+other side. The interval between is likely to be inestimably brief.
+Some birds seem fairly to flash by; others, to drift; yet seldom can
+their passing be counted in seconds, or even in measureable fractions
+of seconds. During these short glimpses, the flight paths tend to lie
+along straight lines, though occasionally a bird may be seen to
+undulate or even to veer off course.
+
+Now and again, in contrast to this typical picture, more eerie effects
+may be noted. Some of them are quite startling--a minute,
+inanimate-looking object drifting passively by like a corpuscle seen
+in the field of a microscope; a gigantic wing brushing across half the
+moon; a ghost-like suggestion of a bird so transparent it seems
+scarcely more than a product of the imagination; a bird that pauses in
+mid-flight to hang suspended in the sky; another that beats its way
+ineffectually forward while it moves steadily to the side; and flight
+paths that sweep across the vision in astonishingly geometric curves.
+All of these things have an explanation. The "corpuscle" is possibly a
+physical entity of some sort floating in the fluid of the observer's
+eye and projected into visibility against the whiteness of the moon.
+The winged transparency may be an insect unconsciously picked up by
+the unemployed eye and transferred by the _camera lucida_ principle to
+the field of the telescope. It may be a bird flying very close, so
+drastically out of focus that the observer sees right through it, as
+he would through a pencil held against his nose. The same cause,
+operating less effectively, gives a characteristic gray appearance
+with hazy edges to silhouettes passing just beneath the limits of
+sharp focus. Focal distortions doubtless also account for the precise
+curvature of some flight paths, for this peculiarity is seldom
+associated with distinct images. Suspended flight and contradictory
+directions of drift may sometimes be attributable to head winds or
+cross winds but more often are simply illusions growing out of a
+two-dimensional impression of a three-dimensional reality.
+
+Somewhat more commonplace are the changes that accompany clouds. The
+moon can be seen through a light haze and at times remains so clearly
+visible that the overcast appears to be behind, instead of in front
+of, it. Under these circumstances, birds can still be readily
+discerned. Light reflected from the clouds may cause the silhouettes
+to fade somewhat, but they retain sufficient definition to distinguish
+them from out-of-focus images. On occasion, when white cloud banks
+lie at a favorable level, they themselves provide a backdrop against
+which birds can be followed all the way across the field of the
+telescope, whether or not they directly traverse the main area of
+illumination.
+
+
+_Types of Data Obtained_
+
+The nature of the observations just described imposes certain
+limitations on the studies that can be made by means of the moon. The
+speed of the birds, for instance, is utterly beyond computation in any
+manner yet devised. Not only is the interval of visibility extremely
+short, but the rapidity with which the birds go by depends less on
+their real rate of motion than on their proximity to the observer. The
+identification of species taking part in the migration might appear to
+offer more promise, especially since some of the early students of the
+problem frequently attempted it, but there are so many deceptive
+elements to contend with that the results cannot be relied upon in any
+significant number of cases. Shorn of their bills by the diminution of
+image, foreshortened into unfamiliar shape by varying angles of
+perspective, and glimpsed for an instant only, large species at
+distant heights may closely resemble small species a few hundred feet
+away. A sandpiper may appear as large as a duck; or a hawk, as small
+as a sparrow. A goatsucker may be confused with a swallow, and a
+swallow may pass as a tern. Bats, however, can be consistently
+recognized, if clearly seen, by their tailless appearance and the
+forward tilt of their wings, as well as by their erratic flight. And
+separations of nocturnal migrants into broad categories, such as
+seabirds and passerine birds, are often both useful and feasible.
+
+It would be a wonderful convenience to be able to clock the speed of
+night-flying birds accurately and to classify them specifically, but
+neither of these things is indispensable to the general study of
+nocturnal migration, nor as important as the three kinds of basic data
+that _are_ provided by telescopes directed at the moon. These
+concern:--(1) the direction in which the birds are traveling; (2)
+their altitude above the earth; (3) the number per unit of space
+passing the observation station.
+
+Unfortunately none of these things can be perceived directly, except
+in a very haphazard manner. Direction is seen by the observer in terms
+of the slant of a bird's pathway across the face of the moon, and may
+be so recorded. But the meaning of every such slant in terms of its
+corresponding compass direction on the plane of the earth constantly
+changes with the position of the moon. Altitude is only vaguely
+revealed through a single telescope by the size and definition of
+images whose identity and consequent real dimensions are subject to
+serious misinterpretation, for reasons already explained. The number
+of birds per unit of space, seemingly the easiest of all the features
+of migration to ascertain, is actually the most difficult, requiring a
+prior knowledge of both direction and altitude. To understand why this
+is so, it will be necessary to consider carefully the true nature of
+the field of observation.
+
+
+_The Changing Field of Observation_
+
+Most of the observations used in this study were made in the week
+centering on the time of the full moon. During this period the lunar
+disc progresses from nearly round to round and back again with little
+change in essential aspect or apparent size. To the man behind the
+telescope, the passage of birds looks like a performance in two
+dimensions taking place in this area of seemingly constant
+diameter--not unlike the movement of insects scooting over a circle of
+paper on the ground. Actually, as an instant's reflection serves to
+show, the two situations are not at all the same. The insects are all
+moving in one plane. The birds only appear to do so. They may be
+flying at elevations of 500, 1000, or 2000 feet; and, though they give
+the illusion of crossing the same illuminated area, the actual breadth
+of the visible space is much greater at the higher, than at the lower,
+level. For this reason, other things being equal, birds nearby cross
+the moon much more swiftly than distant ones. The field of observation
+is not an area in the sky but a volume in space, bounded by the
+diverging field lines of the observer's vision. Specifically, it is an
+inverted cone with its base at the moon and its vertex at the
+telescope.
+
+Since the distance from the moon to the earth does not vary a great
+deal, the full dimensions of the Great Cone determined by the diameter
+of the moon and a point on the earth remain at all times fairly
+constant. Just what they are does not concern us here, except as
+regards the angle of the apex (roughly 1/2 deg.), because obviously the
+effective field of observation is limited to that portion of the Great
+Cone below the maximum ceiling at which birds fly, a much smaller
+cone, which I shall refer to as the Cone of Observation (Figure 1).
+
+    [Illustration: FIG. 1. The field of observation, showing
+       its two-dimensional aspect as it appears to the observer and
+       its three-dimensional actuality. The breadth of the cone is
+       greatly exaggerated.]
+
+    [Illustration: FIG. 2. Method for determining the diameter
+       of the cone at any point. The angular diameter of the moon
+       may be expressed in radians, or, in other words, in terms of
+       lengths of arc equivalent to the radius of a circle. In the
+       diagram, the arc between C and E, being equivalent to the
+       radius CO, represents a radian. If we allow the arc between A
+       and B to be the diameter of the moon, it is by astronomical
+       calculation about .009 radian, or .009 CO. This ratio will
+       hold for any smaller circle inscribed about the center O;
+       that is, the arc between A'B' equals .009 C'O. Thus the width
+       of the cone of observation at any point, expressed in degrees
+       of arc, is .009 of the axis of the cone up to that point. The
+       cone is so slender that the arc between A and B is
+       essentially equal to the chord AB. Exactly the same
+       consideration holds true for the smaller circle where the
+       chord A'B' represents part of the flight ceiling.]
+
+The problem of expressing the number of passing birds in terms of a
+definite quantity of space is fundamentally one of finding out the
+critical dimensions of this smaller cone. The diameter at any distance
+from the observer may be determined with enough accuracy for our
+purposes simply by multiplying the distance by .009, a convenient
+approximation of the diameter of the moon, expressed in radians (see
+Figure 2). One hundred feet away, it is approximately 11 inches; 1000
+feet away, nine feet; at one mile, 48 feet; at two miles, 95 feet.
+Estimating the effective length of the field of observation presents
+more formidable difficulties, aggravated by the fact that the lunar
+base of the Great Cone does not remain stationary. The moon rises in
+the general direction of east and sets somewhere in the west, the
+exact points where it appears and disappears on the horizon varying
+somewhat throughout the year. As it drifts across the sky it carries
+the cone of observation with it like the slim beam of an immense
+searchlight slowly probing space. This situation is ideal for the
+purpose of obtaining a random sample of the number of birds flying out
+in the darkness, yet it involves great complications; for the size of
+the sample is never at two consecutive instants the same. The nearer
+the ever-moving great cone of the moon moves toward a vertical
+position, the nearer its intersection with the flight ceiling
+approaches the observer, shortening, therefore, the cone of
+observation (Figure 3). The effect on the number of birds seen is
+profound. In extreme instances it may completely reverse the meaning
+of counts. Under the conditions visualized in Figure 3, the field of
+observation at midnight is only one-fourth as large as the field of
+observation earlier in the evening. Thus the twenty-four birds seen
+from 7 to 8 P. M., represent not twice as many birds actually flying
+per unit of space as the twelve observed from 11:30 to 12:30 A. M.,
+but only half the amount. Figure 4, based on observations at Ottumwa,
+Iowa, on the night of May 22-23, shows a similar effect graphically.
+Curve A represents the actual numbers of birds per hour seen; Curve B
+shows the same figures expressed as flight densities, that is,
+corrected to take into account the changing size of the field of
+observation. It will be noted that the trends are almost exactly
+opposite. While A descends, B rises, and _vice-versa_. In this case,
+inferences drawn from the unprocessed data lead to a complete
+misinterpretation of the real situation.
+
+    [Illustration: FIG. 3. Temporal change in the effective
+       size of the field of observation. The sample sections, A and
+       B, represent the theoretical densities of flight at 8:20 and
+       12:00 P. M., respectively. Though twice as many birds are
+       assumed to be in the air at midnight when the moon is on its
+       zenith (Z) as there were at the earlier hour, only half as
+       many are visible because of the decrease in size of the cone
+       of observation.]
+
+    [Illustration: FIG. 4. Migration at Ottumwa, Iowa, on the
+       night of May 22-23, 1948. Curve A is a graphic representation
+       of the actual numbers of birds seen hourly through the
+       telescope. Curve B represents the same figures corrected for
+       the variation in the size of the cone of observation. The
+       dissimilarity in the two curves illustrates the deceptive
+       nature of untreated telescopic counts.]
+
+Nor does the moon suit our convenience by behaving night after night
+in the same way. On one date we may find it high in the sky between 9
+and 10 P. M.; on another date, during the same interval of time, it
+may be near the horizon. Consequently, the size of the cone is
+different in each case, and the direct comparison of flights in the
+same hour on different dates is no more dependable than the misleading
+comparisons discussed in the preceding paragraph.
+
+The changes in the size of the cone have been illustrated in Figure 3
+as though the moon were traveling in a plane vertical to the earth's
+surface, as though it reached a point directly over the observer's
+head. In practice this least complicated condition seldom obtains in
+the regions concerned in this study. In most of the northern
+hemisphere, the path of the moon lies south of the observer so that
+the cone is tilted away from the vertical plane erected on the
+parallel of latitude where the observer is standing. In other words it
+never reaches the zenith, a point directly overhead. The farther north
+we go, the lower the moon drops toward the horizon and the more,
+therefore, the cone of observation leans away from us. Hence, at the
+same moment, stationed on the same meridian, two observers, one in the
+north and one in the south, will be looking into different effective
+volumes of space (Figure 5).
+
+    [Illustration: FIG. 5. Geographical variation in the size
+       of the cone of observation. The cones A and B represent the
+       effective fields of observation at two stations situated over
+       1,200 miles apart. The portions of the great cones included
+       here appear nearly parallel, but if extended far enough would
+       be found to have a common base on the moon. Because of the
+       continental scale of the drawing, the flight ceiling appears
+       as a curved surface, equidistant above each station. The
+       lines to the zenith appear to diverge, but they are both
+       perpendicular to the earth. Although the cones are shown at
+       the same instant in time, and have their origin on the same
+       meridian, the dimensions of B are less than one-half as great
+       as those of A, thus materially decreasing the opportunity to
+       see birds at the former station. This effect results from the
+       different slants at which the zenith distances cause the
+       cones to intersect the flight ceiling. The diagram
+       illustrates the principle that northern stations, on the
+       average, have a better chance to see birds passing in their
+       vicinity than do southern stations.]
+
+As a further result of its inclination, the cone of observation,
+seldom affords an equal opportunity of recording birds that are flying
+in two different directions. This may be most easily understood by
+considering what happens on a single flight level. The plane parallel
+to the earth representing any such flight level intersects the
+slanting cone, not in a circle, but in an ellipse. The proportions of
+this ellipse are very variable. When the moon is high, the
+intersection on the plane is nearly circular; when the moon is low,
+the ellipse becomes greatly elongated. Often the long axis may be more
+than twice the length of the short axis. It follows that, if the long
+axis happens to lie athwart the northward direction of flight and the
+short axis across the eastward direction, we will get on the average
+over twice as large a sample of birds flying toward the north as of
+birds flying toward the east.
+
+In summary, whether we wish to compare different stations, different
+hours of the night, or different directions during the same hour of
+the night, no conclusions regarding even the relative numbers of birds
+migrating are warranted, unless they take into account the
+ever-varying dimensions of the field of observation. Otherwise we are
+attempting to measure migration with a unit that is constantly
+expanding or contracting. Otherwise we may expect the same kind of
+meaningless results that we might obtain by combining measurements in
+millimeters with measurements in inches. Some method must be found by
+which we can reduce all data to a standard basis for comparison.
+
+
+_The Directional Element in Sampling_
+
+In seeking this end, we must immediately reject the simple logic of
+sampling that may be applied to density studies of animals on land. We
+must not assume that, since the field of observation is a volume in
+space, the number of birds therein can be directly expressed in terms
+of some standard volume--a cubic mile, let us say. Four birds counted
+in a cone of observation computed as 1/500 of a cubic mile are not the
+equivalent of 500 x 4, or 2000, birds per cubic mile. Nor do four
+birds flying over a sample 1/100 of a square mile mathematically
+represent 400 birds passing over the square mile. The reason is that
+we are not dealing with static bodies fixed in space but with moving
+objects, and the objects that pass through a cubic mile are not the
+sum of the objects moving through each of its 500 parts. If this fact
+is not immediately apparent, consider the circumstances in Figures 6
+and 7, illustrating the principle as it applies to areas. The relative
+capacity of the sample and the whole to intercept bodies in motion is
+more closely expressed by the ratio of their perimeters in the case of
+areas and the ratio of their surface areas in the case of volumes. But
+even these ratios lead to inaccurate results unless the objects are
+moving in all directions equally (see Figure 8). Since bird migration
+exhibits strong directional tendencies, I have come to the conclusion
+that no sampling procedure that can be applied to it is sufficiently
+reliable short of handling each directional trend separately.
+
+    [Illustration: FIG. 6. The problem of sampling migrating
+       birds. The large square in the diagram may be thought of as a
+       square mile on the earth's surface, divided into four equal
+       smaller squares. Birds are crossing over the area in three
+       directions, equally spaced, so that each of the subdivisions
+       is traversed by three of them. We might be tempted to
+       conclude that 4 x 3, or 12, would pass over the large square.
+       Actually there are only seven birds involved all told.
+       Obviously, the interceptive potential of a small square and a
+       larger square do not stand in the same ratio as their areas.]
+
+For this reason, the success of the whole quantitative study of
+migration depends upon our ability to make directional analyses of
+primary data. As I have already pointed out, the flight directions of
+birds may be recorded with convenience and a fair degree of
+objectivity by noting the slant of their apparent pathways across the
+disc of the moon. But these apparent pathways are seldom the real
+pathways. Usually they involve the transfer of the flight line from a
+horizontal plane of flight to a tilted plane represented by the face
+of the moon, and so take on the nature of a projection. They are
+clues to directions, but they are not the directions themselves. For
+each compass direction of birds flying horizontally above the earth,
+there is one, and only one, slant of the pathway across the moon at a
+given time. It is possible, therefore, knowing the path of a bird in
+relation to the lunar disc and the time of the observation, to compute
+the direction of its path in relation to the earth. The formula
+employed is not a complicated one, but, since the meaning of the lunar
+cooerdinates in terms of their corresponding flight paths parallel to
+the earth is constantly changing with the position of the moon, the
+calculation of each bird's flight separately would require a
+tremendous amount of time and effort.
+
+    [Illustration: FIG. 7. The sampling effect of a square. In
+       Diagram A eight evenly distributed birds are flying from
+       south to north, and another four are proceeding from east to
+       west. Three appear in each of the smaller squares. Thus, if
+       we were to treat any of these smaller sections as a directly
+       proportionate sample of the whole, we would be assuming that
+       3 x 16, or 48, birds had traversed the square mile--four
+       times the real total of 12. If we consider the paths
+       separately as in Diagram B, we see quite clearly what is
+       wrong. Every bird crosses four plots the size of the sample
+       and is being computed into the total over and over a
+       corresponding number of times. Patently, just as many
+       south-north birds cross the bottom tier of squares as cross
+       the four tiers comprising the whole area. Just as many
+       west-east birds traverse one side of the large square as
+       cross the whole square. In other words, the inclusion of
+       additional sections _athwart_ the direction of flight
+       involves the inclusion of additional birds proceeding in that
+       direction, while the inclusion of additional sections _along_
+       the direction does not. The correct ratio of the sample to
+       the whole would seem to be the ratio of their perimeters, in
+       this case the ratio of one to four. When this factor of four
+       is applied to the problem it proves correct: 4 x 3 (the
+       number of birds that have been seen in the sample square)
+       equals 12 (the exact number of birds that could be seen in
+       the square mile).]
+
+    [Illustration: FIG. 8. Rectangular samples of square areas.
+       In Diagram A, where as many birds are flying from west to
+       east as are flying from south to north, the perimeter ratio
+       (three to eight) correctly expresses the number of birds that
+       have traversed the whole area relative to the number that
+       have passed through the sample. But in Diagram B, where all
+       thirty-two birds are flying from south to north, the correct
+       ratio is the ratio of the base of the sample to the base of
+       the total area (one to four), and use of the perimeter ratio
+       would lead to an inaccurate result (forty-three instead of
+       thirty-two birds). Perimeter ratios do not correctly express
+       relative interceptory potential, unless the shape of the
+       sample is the same as the shape of the whole, or unless the
+       birds are flying in all directions equally.]
+
+Whatever we do, computed individual flight directions must be frankly
+recognized as approximations. Their anticipated inaccuracies are not
+the result of defects in the mathematical procedure employed. This is
+rigorous. The difficulty lies in the impossibility of reading the
+slants of the pathways on the moon precisely and in the
+three-dimensional nature of movement through space. The observed
+cooerdinates of birds' pathways across the moon are the projected
+product of two component angles--the compass direction of the flight
+and its slope off the horizontal, or gradient. These two factors
+cannot be dissociated by any technique yet developed. All we can do is
+to compute what a bird's course would be, if it were flying horizontal
+to the earth during the interval it passes before the moon. We cannot
+reasonably assume, of course, that all nocturnal migration takes place
+on level planes, even though the local distractions so often
+associated with sloping flight during the day are minimized in the
+case of migrating birds proceeding toward a distant destination in
+darkness. We may more safely suppose, however, that deviations from
+the horizontal are random in nature, that it is mainly a matter of
+chance whether the observer happens to see an ascending segment of
+flight or a descending one. Over a series of observations, we may
+expect a fairly even distribution of ups and downs. It follows that,
+although departures from the horizontal may distort individual
+directions, they tend to average out in the computed trend of the
+mean. The working of this principle applied to the undulating flight
+of the Goldfinch (_Spinus_) is illustrated in Figure 9.
+
+    [Illustration: FIG. 9. The effect of vertical components in
+       bird flight. The four diagrams illustrate various effects
+       that might result if a bird with an undulating flight, such
+       as a Goldfinch, flew before a moon 45 deg. above the horizon. In
+       each case the original profile of the pathways, illustrated
+       against the dark background, is flattened considerably as a
+       result of projection. In the situation shown in Diagram A,
+       where the high point of the flight line, GHJ, occurs within
+       the field of the telescope, it is not only obvious that a
+       deviation is involved, but the line GJ drawn between the
+       entry and departure points coincides with the normal
+       cooerdinates of a bird proceeding on a horizontal plane. In
+       Diagrams B and C, one which catches an upward segment of
+       flight, and the other, a downward segment, the nature of the
+       deviation would not be detectable, and an incorrect direction
+       would be computed from the cooerdinates. Over a series of
+       observations, including many Goldfinches, one would expect a
+       fairly even distribution of ups and downs. Since the average
+       between the cooerdinate angles in Diagrams B and C, +19 deg. and
+       -19 deg., is the angle of the true cooerdinate, we have here a
+       situation where the errors tend to compensate. In Diagram D,
+       where the bird is so far away that several undulations are
+       encompassed within the diameter of the field of view, the
+       cooerdinate readings do not differ materially from those of a
+       straight line.]
+
+Since _individually_ computed directions are not very reliable in any
+event, little is to be lost by treating the observed pathways in
+groups. Consequently, the courses of all the birds seen in a one-hour
+period may be computed according to the position of the moon at the
+middle of the interval and expressed in terms of their general
+positions on the compass, rather than their exact headings. For this
+latter purpose, the compass has been divided into twelve fixed
+sectors, 22-1/2 degrees wide. The trends of the flight paths are
+identified by the mid-direction of the sector into which they fall.
+The sectoring method is described in detail in the section on
+procedures.
+
+    [Illustration: Fig. 10. The interceptory potential of
+       slanting lines. The diagram deals with one direction of
+       flight and its incidence across lines of six different
+       slants, lines of identical length oriented in six different
+       ways. Obviously, the number of birds that cross a line
+       depends not only on the length of the line, but also on its
+       slant with respect to the flight paths.]
+
+The problem remains of converting the number of birds involved in each
+directional trend to a fixed standard of measurement. Figure 7A
+contains the partial elements of a solution. All of the west-east
+flight paths that cross the large square also cross one of its
+mile-long sides and suggest the practicability of expressing the
+amount of migration in any certain direction in terms of the assumed
+quantity passing over a one-mile line in a given interval of time.
+However, many lines of that length can be included within the same set
+of flight paths (Figure 10); and the number of birds intercepted
+depends in part upon the orientation of the line. The 90 deg. line is the
+only one that fully measures the amount of flight per linear unit of
+front; and so I have chosen as a standard an imaginary mile on the
+earth's surface lying at right angles to the direction in which the
+birds are traveling.
+
+
+_Definitions of Flight Density_
+
+When the count of birds in the cone of observation is used as a sample
+to determine the theoretical number in a sector passing over such a
+mile line, the resulting quantity represents what I shall call a
+Sector Density. It is one of several expressions of the more general
+concept of Flight Density, which may be defined as the passage of
+migration past an observation station stated in terms of the
+theoretical number of birds flying over a one-mile line on the earth's
+surface in a given interval of time. Note that a flight density is
+primarily a theoretical number, a statistical expression, a _rate_ of
+passage. It states merely that birds were moving through the effective
+field of observation at the _rate_ of so many per mile per unit of
+time. It may or may not closely express the amount of migration
+occurring over an actual mile or series of miles. The extent to which
+it does so is to be decided by other general criteria and by the
+circumstances surrounding a given instance. Its basic function is to
+take counts of birds made at different times and at different places,
+in fields of observation of different sizes, and to put them on the
+statistically equal footing that is the first requisite of any sound
+comparison.
+
+The idea of a one-mile line as a standard spacial measurement is an
+integral part of the basic concept, as herein propounded. But, within
+these limitations, flight density may be expressed in many different
+ways, distinguished chiefly by the directions included and the
+orientation of the one-mile line with respect to them. Three such
+kinds of density have been found extremely useful in subsequent
+analyses and are extensively employed in this paper: Sector, Net
+Trend, and Station Density, or Station Magnitude.
+
+Sector Density has already been referred to. It may be defined as the
+flight density within a 22-1/2 deg. directional spread, or sector,
+measured across a one-mile line lying at right angles to the
+mid-direction of the sector. It is the basic type of density from the
+point of view of the computer, the others being derived from it. In
+analysis it provides a means of comparing directional trends at the
+same station and of studying variation in directional fanning.
+
+Net Trend Density represents the maximum net flow of migration over a
+one-mile line. It is found by plotting the sector densities
+directionally as lines of thrust, proportioned according to the
+density in each sector, and using vector analysis to obtain a vector
+resultant, representing the density and direction of the net trend.
+The mile line defining the spacial limits lies at right angles to this
+vector resultant, but the density figure includes all of the birds
+crossing the line, not just those that do so at a specified angle.
+Much of the directional spread exhibited by sector densities
+undoubtedly has no basis in reality but results from inaccuracies in
+cooerdinate readings and from practical difficulties inherent in the
+method of computation. By reducing all directions to one major trend,
+net trend density has the advantage of balancing errors one against
+the other and may often give the truer index to the way in which the
+birds are actually going. On the other hand, if the basic directions
+are too widely spread or if the major sector vectors are widely
+separated with little or no representation between, the net trend
+density may become an abstraction, expressing the idea of a mean
+direction but pointing down an avenue along which no migrants are
+traveling. In such instances, little of importance can be learned from
+it. In others, it gives an idea of general trends indispensable in
+comparing station with station to test the existence of flyways and in
+mapping the continental distribution of flight on a given night to
+study the influence of weather factors.
+
+Station Density, or Station Magnitude, represents all of the migration
+activity in an hour in the vicinity of the observation point,
+regardless of direction. It expresses the sum of all sector densities.
+It includes, therefore, the birds flying at right angles over several
+one-mile lines. One way of picturing its physical meaning is to
+imagine a circle one-mile in diameter lying on the earth with the
+observation point in the center. Then all of the birds that fly over
+this circle in an hour's time constitute the hourly station density.
+While its visualization thus suggests the idea of an area, it is
+derived from linear expressions of density; and, while it involves no
+limitation with respect to direction, it could not be computed without
+taking every component direction into consideration. Station density
+is adapted to studies involving the total migration activity at
+various stations. So far it has been the most profitable of all the
+density concepts, throwing important light on nocturnal rhythm,
+seasonal increases in migration, and the vexing problem of the
+distribution of migrating birds in the region of the Gulf of Mexico.
+
+Details of procedure in arriving at these three types of flight
+density will be explained in Section B of this discussion. For the
+moment, it will suffice to review and amplify somewhat the general
+idea involved.
+
+
+_Altitude as a Factor in Flight Density_
+
+A flight density, as we have seen, may be defined as the number of
+birds passing over a line one mile long; and it may be calculated from
+the number of birds crossing the segment of that line included in an
+elliptical cross-section of the cone of observation. It may be thought
+of with equal correctness, without in any way contradicting the
+accuracy of the original definition, as the number of birds passing
+through a vertical plane one mile long whose upper limits are its
+intersection with the flight ceiling and whose base coincides with the
+one mile line of the previous visualization. From the second point of
+view, the sample becomes an area bounded by the triangular projection
+of the cone of observation on the density plane. The dimensions of two
+triangles thus determined from any two cones of observation stand in
+the same ratio as the dimensions of their elliptical sections on any
+one plane; so both approaches lead ultimately to the same result. The
+advantage of this alternative way of looking at things is that it
+enables us to consider the vertical aspects of migration--to
+comprehend the relation of altitude to bird density.
+
+If the field of observation were cylindrical in shape, if it had
+parallel sides, if its projection were a rectangle or a parallelogram,
+the height at which birds are flying would not be a factor in finding
+out their number. Then the sample would be of equal breadth
+throughout, with an equally wide representation of the flight at all
+levels. Since the field of observation is actually an inverted cone,
+triangular in section, with diverging sides, the opportunity to detect
+birds increases with their distance from the observer. The chances of
+seeing the birds passing below an elevation midway to the flight
+ceiling are only one-third as great as of seeing those passing above
+that elevation, simply because the area of that part of the triangle
+below the mid-elevation is only one-third as great as the area of that
+part above the mid-elevation. If we assume that the ratio of the
+visible number of birds to the number passing through the density
+plane is the same as the ratio of the triangular section of the cone
+to the total area of the plane, we are in effect assuming that the
+density plane is made up of a series of triangles the size of the
+sample, each intercepting approximately the same number of birds. We
+are assuming that the same number of birds pass through the inverted
+triangular sample as through the erect and uninvestigable triangle
+beside it (as in Figure 11, Diagram II). In reality, the assumption is
+sound only if the altitudinal distribution of migrants is uniform.
+
+    [Illustration: FIG. 11. Theoretical possibilities of
+       vertical distribution. Diagram I shows the effect of a
+       uniform vertical distribution of birds. The figures indicate
+       the number of birds in the respective areas. Here the sample
+       triangle, ABD, contains the same number of birds as the
+       upright triangle, ACD, adjacent to it; the density plane may
+       be conceived of as a series of such alternating triangles,
+       equal in their content of birds. Diagram II portrays, on an
+       exaggerated scale, the situation when many more birds are
+       flying below the median altitude than above it. In contrast
+       to the 152 birds occurring in the triangle A'C'D', only
+       seventy-two are seen in the triangle A'B'D'. Obviously, the
+       latter triangle does not provide a representative sample of
+       the total number of birds intersecting the density plane.
+       Diagram III illustrates one method by which this difficulty
+       may be overcome. By lowering the line F'G' to the median
+       altitude of bird density, F''G'' (the elevation above which
+       there are just as many birds as below), we are able to
+       determine a rectangular panel, HIJK, whose content of birds
+       provides a representative sample of the vertical
+       distribution.]
+
+The definite data on this subject are meagre. Nearly half a century
+ago, Stebbins worked out a way of measuring the altitude of migrating
+birds by the principle of parallax. In this method, the distance of a
+bird from the observers is calculated from its apparent displacement
+on the moon as seen through two telescopes. Stebbins and his
+colleague, Carpenter, published the results of two nights of
+observation at Urbana, Illinois (Stebbins, 1906; Carpenter, 1906); and
+then the idea was dropped until 1945, when Rense and I briefly applied
+an adaptation of it to migration studies at Baton Rouge. Results have
+been inconclusive. This is partly because sufficient work has not been
+done, partly because of limitations in the method itself. If the two
+telescopes are widely spaced, few birds are seen by both observers,
+and hence few parallaxes are obtained. If the instruments are brought
+close together, the displacement of the images is so reduced that
+extremely fine readings of their positions are required, and the
+margin of error is greatly increased. Neither alternative can provide
+an accurate representative sample of the altitudinal distribution of
+migrants at a station on a single night. New approaches currently
+under consideration have not yet been perfected.
+
+Meanwhile the idea of uniform vertical distribution of migrants must
+be dismissed from serious consideration on logical grounds. We know
+that bird flight cannot extend endlessly upward into the sky, and the
+notion that there might be a point to which bird density extends in
+considerable magnitude and then abruptly drops off to nothing is
+absurd. It is far more likely that the migrants gradually dwindle in
+number through the upper limits at which they fly, and the parallax
+observations we have seem to support this view.
+
+Under these conditions, there would be a lighter incidence of birds in
+the sample triangle than in the upright triangle beside it (Figure 11,
+Diagram III). Compensation can be made by deliberately scaling down
+the computed size of the sample area below its actual size. A
+procedure for doing this is explained in Figure 11. If it were applied
+to present altitudinal data, it would place the computational flight
+ceiling somewhere below 4000 feet. In arriving at the flight densities
+used in this paper, however, I have used an assumed ceiling of one
+mile. When the altitude factor is thus assigned a value of 1, it
+disappears from the formula, simplifying computations. Until the true
+situation with respect to the vertical distribution of flight is
+better understood, it seems hardly worthwhile to sacrifice the
+convenience of this approximation to a rigorous interpretation of
+scanty data. This particular uncertainty, however, does not
+necessarily impair the analytical value of the computations. Provided
+that the vertical pattern of migration is more or less constant,
+flight densities still afford a sound basis for comparisons, wherever
+we assume the upper flight limits to be. Raising or lowering the
+flight ceiling merely increases or reduces all sample cones or
+triangles proportionately.
+
+A more serious possibility is that the altitudinal pattern may vary
+according to time or place. This might upset comparisons. If the
+divergencies were severe enough and frequent enough, they could throw
+the study of flight densities into utter confusion.
+
+This consideration of possible variation in the altitudinal pattern
+combines with accidents of sampling and the concessions to perfect
+accuracy, explained on pages 379-385, to give to small quantities of
+data an equivocal quality. As large-scale as the present survey is
+from one point of view, it is only a beginning. Years of intensive
+work and development leading to a vast accumulation of data must
+elapse before the preliminary indications yet discernible assume the
+status of proved principles. As a result, much of the discussion in
+Part II of this paper is speculative in intent, and most of the
+conclusions suggested are of a provisional nature. Yet, compared with
+similar procedures in its field, flight density study is a highly
+objective method, and a relatively reliable one. In no other type of
+bird census has there ever been so near a certainty of recording _all_
+of the individuals in a specified space, so nearly independently of
+the subjective interpretations of the observer. The best assurance of
+the essential soundness of the flight density computations lies in the
+coherent results and the orderly patterns that already emerge from the
+analyses presented in Part II.
+
+
+B. OBSERVATIONAL PROCEDURE AND THE PROCESSING OF DATA
+
+At least two people are required to operate an observation
+station--one to observe, the other to record the results. They should
+exchange duties every hour to avoid undue eye fatigue. Additional
+personnel are desirable so that the night can be divided into shifts.
+
+Essential materials and equipment include: (1) a small telescope;
+(2) a tripod with pan-tilt or turret head and a mounting cradle;
+(3) data sheets similar to the one illustrated in Figure 12. Bausch
+and Lomb or Argus spotting scopes (19.5 x) and astronomical telescopes
+up to 30- or 40-power are ideal. Instruments of higher magnification
+are subject to vibration, unless very firmly mounted, and lead to
+difficulties in following the progress of the moon, unless powered by
+clockwork. Cradles usually have to be devised. An adjustable lawn chair
+is an important factor in comfort in latitudes where the moon reaches
+a point high overhead.
+
+    [Illustration: FIG. 12. Facsimile of form used to record
+       data in the field. One sheet of the actual observations
+       obtained at Progreso, Yucatan, on April 24-25, 1948, is
+       reproduced here. The remainder of this set of data, which is
+       to be used throughout the demonstration of procedures, is
+       shown in Table 1.]
+
+  [Transcription of Figure 12's Data]
+
+  ORIGINAL DATA SHEET
+
+  DATE  24-25 April 1948     LOCALITY  Progreso, Yucatan
+
+  OBSERVERS  Harold Harry; George H. Lowery
+
+  WEATHER  Moderate to strong "trade" winds along coast, slightly
+           N of E. Moon emerged above low cloud bank at 8:26.
+
+  INSTRUMENT  B. & L. 19.5 Spotting Scope; image erect
+
+  REMARKS  Observation station located 1 mile from land, over Gulf of
+           Mexico, at end of new Progreso wharf
+
+  -----------+------+-------+----------------------------------------
+     TIME    |  IN  |  OUT  |                   REMARKS
+  -----------+------+-------+----------------------------------------
+    C.S.T    |      |       |
+    8:26     |  --  |  --   | observations begin; H.H. observing
+      50     | 4:30 |  9    | slow; small
+      56     | 3    | 10    | medium size
+    9:00     | 2    | 10:30 | very small
+      11     | 5    |  9:30 | moderately fast
+      25     | 5    | 10    | very small; rather slow
+      26     | 3    | 11    |     "            "
+      36     | 5    | 10    | medium size
+      40     | 3    | 10    |   "     "
+      43     | 5:30 |  9    |   "     "
+      46     | 3:30 | 10    | small
+      56     | 4:30 | 10    | medium size
+  9:58-10:00 |  --  |  --   | time out to change observers; G.L. at
+   10:05     | 4:30 | 11:30 | scope small
+      06     | 3    | 11    |
+      12     | 5    |  8    | very small
+      25     | 5    | 12    | very fast; small
+      30     | 4    | 10    | small
+      32     | 4    | 11    |   "
+      32     | 2    | 11    |   "
+      33     | 5    | 11    |   "
+      33     | 4    |  1    |   "
+      33     | 5:30 | 11    |   "
+      35     | 4:30 | 10    | swallow-like
+      36     | 5    |  1:30 |
+
+
+As much detail as possible should be entered in the space provided at
+the top of the data sheet. Information on the weather should include
+temperature, description of cloud cover, if any, and the direction
+and apparent speed of surface winds. Care should be taken to specify
+whether the telescope used has an erect or inverted image. The entry
+under "Remarks" in the heading should describe the location of the
+observation station with respect to watercourses, habitations, and
+prominent terrain features.
+
+The starting time is noted at the top of the "Time" column, and the
+observer begins the watch for birds. He must keep the disc of the moon
+under unrelenting scrutiny all the while he is at the telescope. When
+interruptions do occur as a result of changing positions with the
+recorder, re-adjustments of the telescope, or the disappearance of the
+moon behind clouds, the exact duration of the "time out" must be set
+down.
+
+    [Illustration: FIG. 13. The identification of cooerdinates.
+       These diagrams illustrate how the moon may be envisioned as a
+       clockface, constantly oriented with six o'clock nearest the
+       horizon and completely independent of the rotation of the
+       moon's topographic features.]
+
+    [Illustration: FIG. 14. The apparent pathways of the birds
+       seen in one hour. The observations are those recorded in the
+       11:00-12:00 P. M. interval on April 24-25, 1948, at
+       Progreso, Yucatan (see Table 1).]
+
+Whenever a bird is seen, the exact time must be noted, together with
+its apparent pathway on the moon. These apparent pathways can be
+designated in a simple manner. The observer envisions the disc of the
+moon as the face of a clock, with twelve equally spaced points on the
+circumference marking the hours (Figure 13). He calls the bottommost
+point 6 o'clock and the topmost, 12. The intervals in between are
+numbered accordingly. As this lunar clockface moves across the sky, it
+remains oriented in such a way that 6 o'clock continues to be the
+point nearest the horizon, unless the moon reaches a position directly
+overhead. Then, all points along the circumference are equidistant
+from the horizon, and the previous definition of clock values ceases
+to have meaning. This situation is rarely encountered in the northern
+hemisphere during the seasons of migration, except in extreme
+southern latitudes. It is one that has never actually been dealt with
+in the course of this study. But, should the problem arise, it would
+probably be feasible to orient the clock during this interval with
+respect to the points of the compass, calling the south point
+6 o'clock.
+
+When a bird appears in front of the moon, the observer identifies its
+entry and departure points along the rim of the moon with respect to
+the nearest half hour on the imaginary clock and informs the recorder.
+In the case of the bird shown in Figure 13, he would simply call out,
+"5 to 10:30." The recorder would enter "5" in the "In" column on the
+data sheet (see Figure 12) and 10:30 in the "Out" column. Other
+comment, offered by the observer and added in the remarks column, may
+concern the size of the image, its speed, distinctness, and possible
+identity. Any deviation of the pathway from a straight line should be
+described. This information has no bearing on subsequent mathematical
+procedure, except as it helps to eliminate objects other than birds
+from computation.
+
+The first step in processing a set of data so obtained is to
+blue-pencil all entries that, judged by the accompanying remarks,
+relate to extraneous objects such as insects or bats. Next, horizontal
+lines are drawn across the data sheets marking the beginning and the
+end of each even hour of observation, as 8 P. M.-9 P. M., 9 P. M.-10
+P. M., etc. The cooerdinates of the birds in each one-hour interval may
+now be plotted on separate diagrammatic clockfaces, just as they
+appeared on the moon. Tick marks are added to each line to indicate
+the number of birds occurring along the same cooerdinate. The slant of
+the tick marks distinguishes the points of departure from the points
+of entry. Figure 14 shows the plot for the 11 P. M.-12 P. M.
+observations reproduced in Table 1. The standard form, illustrated in
+Figure 15, includes four such diagrams.
+
+Applying the self-evident principle that all pathways with the same
+slant represent the same direction, we may further consolidate the
+plots by shifting all cooerdinates to the corresponding lines passing
+through the center of the circle, as in Figure 15. To illustrate, the
+6 to 8, 5 to 9, 3 to 11, and 2 to 12 pathways all combine on the 4 to
+10 line. Experienced computers eliminate a step by directly plotting
+the pathways through center, using a transparent plastic straightedge
+ruled off in parallel lines.
+
+    [Illustration: FIG. 15. Standard form for plotting the
+       apparent paths of flight. On these diagrams the original
+       cooerdinates, exemplified by Figure 14, have been moved to
+       center. In practice the sector boundaries are drawn over the
+       circles in red pencil, as shown by the white lines in Figure
+       19, making it possible to count the number of birds falling
+       within each zone. These numbers are then tallied in the
+       columns at the lower right of each hourly diagram.]
+
+
+    TABLE 1.--Continuation of Data in Figure 12, Showing Time
+                and Readings of Observations on 24-25 April 1948,
+                Progreso, Yucatan
+
+  ==============================+==============================
+  Time          In       Out    |  Time          In       Out
+  ------------------------------+------------------------------
+  10:37-10:41  Time out         | 11:15        8          9:30
+  10:45        5:30      10     | 11:16        4         11
+               6          9     |              5          9
+               5:30      10     | 11:17        5         11:30
+  10:46        6          8     | 11:18        5         12
+               3:30      11     |              6         11:30
+               5         12     | 11:19        5:30      11:30
+  10:47        3:15       1     | 11:20        6         10
+               6          8:30  |              3         12
+               5:45      11:45  |              5         12
+               5         10     | 11:21        5:45      11
+  10:48        6          9:45  |              5         11
+  10:50        5:30      11     | 11:23        5         12
+  10:51        4         11     | 11:25        5         10:30
+  10:52        4          2     |              6         11
+               5:30      11     |              6         12
+  10:53        5:30      11:30  | 11:27        6         10
+               5         11     | 11:28        6         11:30
+  10:55        5         12     |              5:30      12:30
+               5         11     | 11:29        6         11:30
+  10:56        6         10     |              4         12
+  10:58        4:30      11:30  |              6:30      10:30
+               5:45      11:45  |              6         11
+  10:59        6:30      10:30  | 11:30        3         10
+  11:00        3:30      12     |       (2 birds at once)
+               6:30      11     | 11:31        5         10:30
+        (2 birds at once)       |              5:30      10:30
+  11:03        6         11     | 11:32        6         11:30
+  11:04        3         12     | 11:33        7:30       9:30
+               5         12     |              4         10:30
+  11:05        6         10     |              6         11:30
+               5         11     |              8          9:30
+  11:06        6         10:30  | 11:35        7         10
+  11:07        3         10     |              4:30       1
+  11:08        6         11     | 11:38        6:30      11
+  11:10        7          9:30  | 11:40        5:30      12
+  11:11        5          9:15  | 11:42        4          2
+  11:13        5         12     |              5         12
+  11:14        6:30      10     |              6         10
+               5:30       1     |              4          2
+               4         12     |              5         12
+  ------------------------------+------------------------------
+
+               Table 1.--_Concluded_
+  ==============================+==============================
+  Time          In       Out    |  Time          In       Out
+  ------------------------------+------------------------------
+  11:44        8          9:30  |               8         10:15
+               7         11     |  12:16        3:30       1:30
+               6         10     |               8         11
+  11:45        5         12     |  12:23        7          1:30
+               6         10:30  |               6         12:30
+               5:45      11     |  12:36        8         11
+               4         12     |  12:37        7:30       1
+  11:46        7         11     |  12:38        7         12:30
+               6         12     |  12:40        8          1
+  11:47        8         10     |  12:45        7:30       1
+  11:48        6         10     |  12:47        5:30       1
+  11:49        6:30      10:30  |  12:48        7          1
+  11:51        8         10     |  12:52        5:30       1:30
+               8         10     |  12:54-12:55  Time out
+               8         10     |  12:56        8         10:45
+               8         10     |  12:58        5:30       1:30
+               6         10     |               7          1:30
+               8         10     |               7          2
+               6         11     |  12:59        5          3
+               7         12     |   1:00-1:30   Time out
+  11:52        5          1     |   1:37        8         12
+  11:54        7         11     |   1:38        8         12
+               6         12:30  |   1:48        7          1
+  11:55        5         12     |               7          1
+  11:56        7         10     |   1:51        5:30      11
+               5         12     |   1:57        8          1
+  11:58        8         11     |   2:07        7          2
+  11:59        5:30      12     |   2:09        9         12
+  12:00-12:03  Time out         |   2:10        8          1
+  12:03        5:30      11:30  |   2:17        9         12
+  12:04        8         11     |   2:21        6          2
+  12:07        6         12:30  |   2:30        5:30       3:15
+               7:30       1     |   2:32        8          2
+  12:08        5         10:30  |   2:46        7          1
+  12:09        5:30       1     |   3:36        9          2
+               7:30       2     |   3:39        8:30       2
+  12:10        6:30      12:45  |   3:45        6          4
+  12:13        8         11     |   3:55        9          2
+  12:14        7          1     |   4:00        8          3
+  12:15        7         12:30  |   4:03        9          2
+               7:15       1:30  |   4:30        Closed station
+  ------------------------------+------------------------------
+
+We now have a concise picture of the apparent pathways of all the
+birds recorded in each hour of observation. But the cooerdinates do not
+have the same meaning as readings of a horizontal clock on the earth's
+surface, placed in relation to the points of the compass. They are
+merely projections of the birds' courses. An equation is available for
+reversing the effect of projection and discovering the true directions
+of flight. This formula, requiring thirty-five separate computations
+for the pathways reproduced in Figure 12 alone, is far too-consuming
+for the handling of large quantities of data. A simpler procedure is
+to divide the compass into sectors and, with the aid of a reverse
+equation, to draw in the projected boundaries of these divisions on
+the circular diagrams of the moon. A standardized set of sectors, each
+22-1/2 deg. wide and bounded by points of the compass, has been evolved
+for this purpose. They are identified as shown in Figure 16. The zones
+north of the east-west line are known as the North, or N, Sectors, as
+N_{1}, N_{2}, N_{3}, etc. Each zone south of the east-west line bears
+the same number as the sector opposite, but is distinguished by the
+designation S.
+
+    [Illustration: FIG. 16. Standard sectors for designating
+       flight trends. Each zone covers a span of 22-1/2 deg.. The N_{6}
+       and N_{8}, the N_{5} and N_{7}, and their south complements,
+       where usually few birds are represented, can be combined and
+       identified as N_{6-8} and N_{5-7}, etc.]
+
+Several methods may be used to find the projection of the sector
+boundaries on the plot diagrams of Figure 15. Time may be saved by
+reference to graphic tables, too lengthy for reproduction here,
+showing the projected reading in degrees for every boundary, at every
+position of the moon; and a mechanical device, designed by C. M.
+Arney, duplicating the conditions of the original projection, speeds
+up the work even further. Both methods are based on the principle of
+the following formula:
+
+    tan [theta] = tan ([eta] - [psi]) / cos Z_{0}    (1)
+
+    [Illustration: FIG. 17. The meaning of symbols used in the
+       direction formula.]
+
+The symbols have these meanings:
+
+[theta] is the position angle of the sector boundary on the lunar
+clock, with positive values measured counterclockwise from 12 o'clock,
+negative angles clockwise (Figure 17A).
+
+[eta] is the compass direction of the sector boundary expressed in
+degrees reckoned west from the south point (Figure 17B).
+
+Z_{0} is the zenith distance of the moon's center midway through the
+hour of observation, that is, at the half hour. It represents the
+number of degrees of arc between the center of the moon and a
+point directly over the observer's head (Figure 17C).
+
+[psi] is the azimuth of the moon midway through the hour of
+observation, measured from the south point, positive values to the
+west, negative values to the east (Figure 17D).
+
+    [Illustration: FIG. 18. Form used in the computation of the
+       zenith distance and azimuth of the moon.]
+
+The angle [eta] for any sector boundary can be found immediately by
+measuring its position in the diagram (Figure 16). The form (Figure 18)
+for the "Computation of Zenith Distance and Azimuth of the Moon"
+illustrates the steps in calculating the values of Z_{0} and [psi]_{0}.
+From the American Air Almanac (Anonymous, 1945-1948), issued annually
+by the U. S. Naval Observatory in three volumes, each covering four
+months of the year, the Greenwich Hour Angle (GHA) and the declination
+of the moon may be obtained for any ten-minute interval of the date in
+question. The Local Hour Angle (LHA) of the observation station is
+determined by subtracting the longitude of the station from the GHA.
+Reference is then made to the "Tables of Computed Altitude and Azimuth,"
+published by the U. S. Navy Department, Hydrographic Office (Anonymous,
+1936-1941), and better known as the "H.O. 214," to locate the altitude
+and azimuth of the moon at the particular station for the middle of the
+hour during which the observations were made. The tables employ three
+variables--the latitude of the locality measured to the nearest degree,
+the LHA as determined above, and the declination of the moon measured
+to the nearest 30 minutes of arc. Interpolations can be made, but this
+exactness is not required. When the latitude of the observation
+station is in the northern hemisphere, the H.O. 214 tables entitled
+"Declinations Contrary Name to Latitude" are used with south
+declinations of the moon, and the tables "Declinations Same Name as
+Latitude," with north declinations. In the sample shown in Figure 15,
+the declination of the moon at 11:30 P. M., midway through the 11 to
+12 o'clock interval, was S 20 deg. 22'. Since the latitude of Progreso,
+Yucatan is N 21 deg. 17', the "Contrary Name" tables apply to this hour.
+
+Because the H.O. 214 expresses the vertical position of the moon in
+terms of its altitude, instead of its zenith distance, a conversion is
+required. The former is the number of arc degrees from the horizon to
+the moon's center; therefore Z_{0} is readily obtained by subtracting
+the altitude from 90 deg.. Moreover, the azimuth given in the H.O. 214 is
+measured on a 360 deg. scale from the north point, whereas the azimuth
+used here ([psi]_{0}) is measured 180 deg. in either direction from the south
+point, negative values to the east, positive values to the west. I
+have designated the azimuth of the tables as Az_{n} and obtained the
+desired azimuth ([psi]_{0}) by subtracting 180 deg. from Az_{n}. The sign
+of [psi]_{0} may be either positive or negative, depending on whether
+or not the moon has reached its zenith and hence the meridian of the
+observer. When the GHA is greater than the local longitude (that is,
+the longitude of the observation station), the azimuth is positive.
+When the GHA is less than the local longitude, the azimuth is
+negative.
+
+Locating the position of a particular sector boundary now becomes a
+mere matter of substituting the values in the equation (1) and
+reducing. The computation of the north point for 11 to 12 P. M. in
+the sample set of data will serve as an example. Since the north point
+reckoned west from the south point is 180 deg., its [eta] has a value of
+180 deg..
+
+    [Illustration: FIG. 19. Method of plotting sector
+       boundaries on the diagrammatic plots. The example employed is
+       the 11:00 to 12:00 P. M. diagram of Figure 15.]
+
+
+    tan [theta]_{Npt.} = tan (180 deg. - [psi]_{0}) / cos Z_{0}
+
+Substituting values of [psi]_{0} found on the form (Figure 18):
+
+    tan [theta]_{Npt.} = tan [180 deg. - (-35 deg.)] / cos 50 deg.
+                       = tan 215 deg. / cos 50 deg. = .700 / .643 = 1.09
+
+    [theta]_{Npt.} = 47 deg.28'
+
+
+    [Illustration: FIG. 20. Form for computing sector
+       densities.]
+
+Four angles, one in each quadrant, have the same tangent value.
+Since, in processing spring data, we are dealing mainly with north
+sectors, it is convenient to choose the acute angle, in this instance
+47 deg. 28'. In doubtful cases, the value of the numerator of the equation
+(here 215 deg.) applied as an angular measure from 6 o'clock will tell in
+which quadrant the projected boundary must fall. The fact that
+projection always draws the boundary closer to the 3-9 line serves as
+a further check on the computation.
+
+    [Illustration: FIG. 21. Determinationn of the angle [alpha]]
+
+In the same manner, the projected position angles of all the pertinent
+sector boundaries for a given hour may be calculated and plotted in
+red pencil with a protractor on the circular diagrams of Figure 15. To
+avoid confusion in lines, the zones are not portrayed in the black and
+white reproduction of the sample plot form. They are shown, however,
+in the shaded enlargement (Figure 19) of the 11 to 12 P. M. diagram.
+The number of birds recorded for each sector may be ascertained by
+counting the number of tally marks between each pair of boundary lines
+and the information may be entered in the columns provided in the plot
+form (Figure 15). We are now prepared to turn to the form for
+"Computations of Sector Densities" (Figure 20), which systematizes the
+solution of the following equation:
+
+         (220) 60/T (No. of Birds) (cos^2 Z_{0})
+    D =  ---------------------------------------                   (2)
+         (1 - sin^2 Z_{0} cos^2 [alpha])^0.5
+
+
+    [Illustration: FIG. 22. Facsimile of form summarizing
+       sector densities. The totals at the bottom of each column
+       give the station densities.]
+
+
+    [Illustration: FIG. 23. Determination of Net Trend Density.]
+
+
+Some of the symbols and factors, appearing here for the first time,
+require brief explanation. D stands for Sector Density. The constant,
+220, is the reciprocal of the quotient of the angular diameter of the
+moon divided by 2. T is Time In, arrived at by subtracting the total
+number of minutes of time out, as noted for each hour on the original
+data sheets, from 60. "No. of Birds" is the number for the sector and
+hour in question as just determined on the plot form. The symbol
+[alpha] represents the angle between the mid-line of the sector and
+the azimuth line of the moon. The quantity is found by the equation:
+
+    [alpha] = 180 deg. - [eta] + [psi]_{0}                             (3)
+
+The symbol [eta] here represents the position of the mid-line of the
+sector expressed in terms of its 360 deg. compass reading. This equation
+is illustrated in Figure 21. The values of [eta] for various zones are
+given in the upper right-hand corner of the form (Figure 20). The
+subsequent reductions of the equations, as they appear in the figure
+for four zones, are self-explanatory. The end result, representing the
+sector density, is entered in the rectangular box provided.
+
+After all the sector densities have been computed, they are tabulated
+on a form for the "Summary of Sector Densities" (Figure 22). By
+totaling each vertical column, sums are obtained, expressing the
+Station Density or Station Magnitude for each hour.
+
+An informative way of depicting the densities in each zone is to plot
+them as lines of thrust, as in Figure 23. Each sector is represented
+by the directional slant of its mid-line drawn to a length expressing
+the flight density per zone on some chosen scale, such as 100 birds
+per millimeter. Standard methods of vector analysis are then applied
+to find the vector resultant. This is done by considering the first
+two thrust lines as two sides of an imaginary parallelogram and using
+a drawing compass to draw intersecting arcs locating the position of
+the missing corner. In the same way, the third vector is combined
+with the invisible resultant whose distal end is represented by the
+intersection of the first two arcs. The process is repeated
+successively with each vector until all have been taken into
+consideration. The final intersection of arcs defines the length and
+slant of the Vector Resultant, whose magnitude expresses the Net Trend
+Density in terms of the original scale.
+
+The final step in the processing of a set of observations is to plot
+on graph paper the nightly station density curve as illustrated by
+Figure 24.
+
+    [Illustration: FIG. 24. Nightly station density curve at
+       Progreso, Yucatan, on April 24-25, 1948.]
+
+
+
+
+PART II. THE NATURE OF NOCTURNAL MIGRATION
+
+
+Present day concepts of the whole broad problem of bird migration are
+made up of a few facts and many guesses. The evolutionary origin of
+migration, the modern necessities that preserve its biologic utility,
+the physiological processes associated with it, the sensory mechanisms
+that make it possible, the speed at which it is achieved, and the
+routes followed, all have been the subject of some investigation and
+much conjecture. All, to a greater or less extent, remain matters of
+current controversy. All must be considered unknowns in every logical
+equation into which they enter. Since all aspects of the subject are
+intimately interrelated, since all have a bearing on the probabilities
+relating to any one, and since new conjectures must be judged largely
+in the light of old conjectures rather than against a background of
+ample facts, the whole field is one in which many alternative
+explanations of the established phenomena remain equally tenable.
+Projected into this uncertain atmosphere, any statistical approach
+such as determinations of flight density will require the accumulation
+of great masses of data before it is capable of yielding truly
+definitive answers to those questions that it is suited to solve. Yet,
+even in their initial applications, density analyses can do much to
+bring old hypotheses regarding nocturnal migration into sharper
+definition and to suggest new ones.
+
+The number of birds recorded through the telescope at a particular
+station at a particular time is the product of many potential
+variables. Some of these--like the changing size of the field of
+observation and the elevation of flight--pertain solely to the
+capacity of the observer to see what is taking place. It is the
+function of the density and direction formulae to eliminate the
+influences of these two variables insofar as is possible, so that the
+realities of the situation take shape in a nearly statistically true
+form. There remain to be considered those influences potentially
+responsible for variations in the real volume of migration at
+different times and places--things like the advance of season,
+geographic location, disposition of terrain features, hourly activity
+rhythm, wind currents, and other climatological causes. The situation
+represented by any set of observations probably is the end result of
+the interaction of several such factors. It is the task of the
+discussions that follow to analyze flight densities in the light of
+the circumstances surrounding them and by statistical insight to
+isolate the effects of single factors. When this has been done, we
+shall be brought closer to an understanding of these influences
+themselves as they apply to the seasonal movements of birds. Out of
+data that is essentially quantitative, conclusions of a qualitative
+nature will begin to take form. It should be constantly borne in mind,
+however, that such conclusions relate to the movement of birds _en
+masse+ and that caution must be used in applying these conclusions to
+any one species.
+
+Since the dispersal of migrants in the night sky has a fundamental
+bearing on the sampling procedure itself, and therefore on the
+reliability of figures on flight density, consideration can well be
+given first to the horizontal distribution of birds on narrow fronts.
+
+
+A. HORIZONTAL DISTRIBUTION OF BIRDS ON NARROW FRONTS
+
+Bird migration, as we know it in daytime, is characterized by spurts
+and uneven spatial patterns. Widely separated V's of geese go honking
+by. Blackbirds pass in dense recurrent clouds, now on one side of the
+observer, now on the other. Hawks ride along in narrow file down the
+thermal currents of the ridges. Herons, in companies of five to fifty,
+beat their way slowly along the line of the surf. And an unending
+stream of swallows courses low along the levees. Everywhere the
+impression is one of birds in bunches, with vast spaces of empty sky
+between.
+
+Such a situation is ill-suited to the sort of sampling procedure on
+which flight density computations are based. If birds always traveled
+in widely separated flocks, many such flocks might pass near the cone
+of observation and still, by simple chance, fail to enter the sliver
+of space where they could be seen. Chance would be the dominating
+factor in the number of birds recorded, obscuring the effects of other
+influences. Birds would seldom be seen, but, when they did appear, a
+great many would be observed simultaneously or in rapid succession.
+
+When these telescopic studies were first undertaken at Baton Rouge in
+1945, some assurance already existed, however, that night migrants might
+be so generally dispersed horizontally in the darkness above that the
+number passing through the small segment of sky where they could be
+counted would furnish a nearly proportionate sample of the total number
+passing in the neighborhood of the observation station. This assurance
+was provided by the very interesting account of Stone (1906: 249-252),
+who enjoyed the unique experience of viewing a nocturnal flight as a
+whole. On the night of March 27, 1906, a great conflagration occurred in
+Philadelphia, illuminating the sky for a great distance and causing the
+birds overhead to stand out clearly as their bodies reflected the light.
+Early in the night few birds were seen in the sky, but thereafter they
+began to come in numbers, passing steadily from the southwest to the
+northeast. At ten o'clock the flight was at its height. The observer
+stated that two hundred birds were in sight at any given moment as he
+faced the direction from which they came. This unparalleled observation
+is of such great importance that I quote it in part, as follows: "They
+[the birds] flew in a great scattered, wide-spread host, never in
+clusters, each bird advancing in a somewhat zigzag manner.... Far off in
+front of me I could see them coming as mere specks...gradually growing
+larger as they approached.... Over the illuminated area, and doubtless
+for great distances beyond, they seemed about evenly distributed.... I
+am inclined to think that the migrants were not influenced by the fire,
+so far as their flight was concerned, as those far to the right were not
+coming toward the blaze but keeping steadily on their way.... Up to
+eleven o'clock, when my observations ceased, it [the flight] continued
+apparently without abatement, and I am informed that it was still in
+progress at midnight."
+
+Similarly, in rather rare instances in the course of the present
+study, the combination of special cloud formations and certain
+atmospheric conditions has made it possible to see birds across the
+entire field of the telescope, whether they actually passed before the
+moon or not. In such cases the area of the sky under observation is
+greatly increased, and a large segment of the migratory movement can
+be studied. In my own experience of this sort, I have been forcibly
+impressed by the apparent uniformity and evenness of the procession of
+migrants passing in review and the infrequence with which birds
+appeared in close proximity.
+
+As striking as these broader optical views of nocturnal migration are,
+they have been too few to provide an incontestable basis for
+generalizations. A better test of the prevailing horizontal
+distribution of night migrants lies in the analysis of the telescopic
+data themselves.
+
+    [Illustration: FIG. 25. Positions of the cone of observation
+       at Tampico, Tamps., on April 21-22, 1948. Essential features
+       of this diagrammatic map are drawn to scale, the triangular
+       white lines representing the projections of the cone of
+       observation on the actual terrain at the mid-point of each
+       hour of observation. If the distal ends of the position lines
+       were connected, the portion of the map encompassed would
+       represent the area over which all the birds seen between
+       8:30 P. M. and 3:30 A. M. must have flown.]
+
+The distribution in time of birds seen by a single observer may be
+studied profitably in this connection. Since the cone of observation
+is in constant motion, swinging across the front of birds migrating
+from south to north, each interval of time actually represents a
+different position in space. This is evident from the map of the
+progress of the field of observation across the terrain at Tampico,
+Tamaulipas, on April 21-22, 1948 (Figure 25). At this station on this
+night, a total of 259 birds were counted between 7:45 P. M. and
+3:45 A. M. The number seen in a single hour ranged from three to
+seventy-three, as the density overhead mounted to a peak and then
+declined. The number of birds seen per minute was not kept with stop
+watch accuracy; consequently, analysis of the number of birds that
+passed before the moon in short intervals of time is not justified. It
+appears significant, however, that in the ninety minutes of heaviest
+flight, birds were counted at a remarkably uniform rate per fifteen
+minute interval, notwithstanding the fact that early in the period the
+flight rate overhead had reached a peak and had begun to decline. The
+number of birds seen in successive fifteen-minute periods was
+twenty-six, twenty-five, nineteen, eighteen, fifteen, and fifteen.
+
+Also, despite the heavy volume of migration at this station on this
+particular night, the flight was sufficiently dispersed horizontally
+so that only twice in the course of eight hours of continuous
+observation did more than one bird simultaneously appear before the
+moon. These were "a flock of six birds in formation" seen at 12:09 A. M.
+and "a flock of seven, medium-sized and distant," seen at 2:07 A. M.
+In the latter instance, as generally is the case when more than one
+bird is seen at a time, the moon had reached a rather low altitude,
+and consequently the cone of observation was approaching its maximum
+dimensions.
+
+The comparative frequency with which two or more birds simultaneously
+cross before the moon would appear to indicate whether or not there is
+a tendency for migrants to fly in flocks. It is significant, therefore,
+that in the spring of 1948, when no less than 7,432 observations were
+made of birds passing before the moon, in only seventy-nine instances,
+or 1.1 percent of the cases, was more than one seen at a time. In
+sixty percent of these instances, only two birds were involved. In one
+instance, however, again when the moon was low and the cone of
+observation near its maximum size, a flock estimated at twenty-five
+was recorded.
+
+The soundest approach of all to the study of horizontal distribution at
+night, and one which may be employed any month, anywhere, permitting the
+accumulation of statistically significant quantities of data, is to set
+up two telescopes in close proximity. Provided the flight overhead is
+evenly dispersed, each observer should count approximately the same
+number of birds in a given interval of time. Some data of this type are
+already available. On May 19-20, at Urbana, Illinois, while stationed
+twenty feet apart making parallax studies with two telescopes to
+determine the height above the earth of the migratory birds, Carpenter
+and Stebbins (_loci cit._) saw seventy-eight birds in two and one-half
+hours. Eleven were seen by both observers, thirty-three by Stebbins
+only, and thirty-four by Carpenter only. On October 10, 1905, at the
+same place, in two hours, fifty-seven birds were counted, eleven being
+visible through both telescopes. Of the remainder, Stebbins saw
+seventeen and Carpenter, twenty-nine. On September 12, 1945, at Baton
+Rouge, Louisiana, in an interval of one hour and forty minutes, two
+independent observers each counted six birds. Again, on October 17,
+1945, two observers each saw eleven birds in twenty-two minutes. On
+April 10, 1946, in one hour and five minutes, twenty-four birds were
+seen through one scope and twenty-six through the other. Likewise on May
+12, 1946, in a single hour, seventy-three birds were counted by each of
+two observers. The Baton Rouge observations were made with telescopes
+six to twelve feet apart. These results show a remarkable conformity,
+though the exceptional October observation of Carpenter and Stebbins
+indicates the desirability of continuing these studies, particularly in
+the fall.
+
+On the whole, the available evidence points to the conclusion that night
+migration differs materially from the kind of daytime migration with
+which we are generally familiar. Birds are apparently evenly spread
+throughout the sky, with little tendency to fly in flocks. It must be
+remembered, however, that only in the case of night migration have
+objective and truly quantitative studies been made of horizontal
+distribution. There is a possibility that our impressions of diurnal
+migration are unduly influenced by the fact that the species accustomed
+to flying in flocks are the ones that attract the most attention.
+
+These conclusions relate to the uniformity of migration in terms of
+short distances only, in the immediate vicinity of an observation
+station. The extent to which they may be applied to broader fronts is a
+question that may be more appropriately considered later, in connection
+with continental aspects of the problem.
+
+
+B. DENSITY AS FUNCTION OF THE HOUR OF THE NIGHT
+
+There are few aspects of nocturnal migration about which there is less
+understanding than the matter of when the night flight begins, at what
+rate it progresses, and for what duration it continues. One would think,
+however, that this aspect of the problem, above most others, would have
+been thoroughly explored by some means of objective study. Yet, this is
+not the case. Indeed, I find not a single paper in the American
+literature wherein the subject is discussed, although some attention has
+been given the matter by European ornithologists. Siivonen (1936)
+recorded in Finland the frequency of call notes of night migrating
+species of _Turdus_ and from these data plotted a time curve showing a
+peak near midnight. Bergman (1941) and Putkonen (1942), also in Finland,
+studied the night flights of certain ducks (_Clangula hyemalis_ and
+_Oidemia fusca_ and _O. nigra_) and a goose (_Branta bernicla_) and
+likewise demonstrated a peak near midnight. However, these studies were
+made at northern latitudes and in seasons characterized by evenings of
+long twilight, with complete darkness limited to a period of short
+duration around midnight. Van Oordt (1943: 34) states that in many cases
+migration lasts all night; yet, according to him, most European
+investigators are of the opinion that, in general, only a part of the
+night is used, that is, the evening and early morning hours. The
+consensus of American ornithologists seems to be that migratory birds
+begin their flights in twilight or soon thereafter and that they remain
+on the wing until dawn. Where this idea has been challenged at all, the
+implication seems to have been that the flights are sustained even
+longer, often being a continuation far into the night of movements begun
+in the daytime. The telescopic method fails to support either of these
+latter concepts.
+
+    [Illustration: FIG. 26. Average hourly station densities in
+       spring of 1948. This curve represents the arithmetic mean
+       obtained by adding all the station densities for each hour,
+       regardless of date, and dividing the sum by the number of
+       sets of observations at that hour (CST).]
+
+
+_The Time Pattern_
+
+When the nightly curves of density at the various stations are plotted
+as a function of time, a salient fact emerges--that the flow of birds is
+in no instance sustained throughout the night. The majority of the
+curves rise smoothly from near zero at the time of twilight to a single
+peak and then decline more or less symmetrically to near the base line
+before dawn. The high point is reached in or around the eleven to twelve
+o'clock interval more often than at any other time.
+
+    [Illustration: FIG. 27. Hourly station densities plotted as
+       a percentage of peak. The curve is based only on those sets
+       of data where observations were continued long enough to
+       include the nightly peak. In each set of data the station
+       density for each hour has been expressed as a percentage of
+       the peak for the night at the station in question. All
+       percentages for the same hour on all dates have been averaged
+       to obtain the percentile value of the combined station
+       density at each hour (CST).]
+
+Figure 26, representing the average hourly densities for all stations on
+all nights of observation, demonstrates the over-all effect of these
+tendencies. Here the highest density is reached in the hour before
+midnight with indications of flights of great magnitude also in the hour
+preceding and the hour following the peak interval. The curve ascends
+somewhat more rapidly than it declines, which fact may or may not be
+significant. Since there is a great disproportion in the total volume of
+migration at different localities, the thought might be entertained that
+a few high magnitude stations, such as Tampico and Progreso, have
+imposed their own characteristics on the final graph. Fortunately, this
+idea may be tested by subjecting the data to a second treatment. If
+hourly densities are expressed as a percentage of the nightly peak, each
+set of observations, regardless of the number of birds involved, carries
+an equal weight in determining the character of the over-all curve.
+Figure 27 shows that percentage analysis produces a curve almost
+identical with the preceding one. To be sure, all of the individual
+curves do not conform with the composite, either in shape or incidence
+of peak. The extent of this departure in the latter respect is evident
+from Figure 28, showing the number of individual nightly station curves
+reaching a maximum peak in each hour interval. Even this graph
+demonstrates that maximum densities near midnight represent the typical
+condition.
+
+    [Illustration: FIG. 28. Incidence of maximum peak at the
+       various hours of the night in 1948. "Number of stations"
+       represents the total for all nights of the numbers of station
+       peaks falling within a given hour.]
+
+The remarkable smoothness and consistency of the curves shown in Figures
+26 and 27 seem to lead directly to the conclusion that the volume of
+night migration varies as a function of time. Admittedly other factors
+are potentially capable of influencing the number of birds passing a
+given station in a given hour. Among these are weather conditions,
+ecological patterns, and specific topographical features that might
+conceivably serve as preferred avenues of flight. However, if any of
+these considerations were alone responsible for changes in the numbers
+of birds seen in successive intervals, the distribution of the peak in
+time could be expected to be haphazard. For example, there is no reason
+to suppose that the cone of observation would come to lie over favored
+terrain at precisely the hour between eleven and twelve o'clock at so
+many widely separated stations. Neither could the topographical
+hypothesis explain the consistently ascending and descending pattern of
+the ordinates in Figure 28. This is not to say that other factors are
+without effect; they no doubt explain the divergencies in the time
+pattern exhibited by Figure 28. Nevertheless, the underlying
+circumstances are such that when many sets of data are merged these
+other influences are subordinated to the rise and fall of an evident
+time pattern.
+
+Stated in concrete terms, the time frequencies shown in the graphs
+suggest the following conclusions: first, nocturnal migrations are not a
+continuation of daytime flights; second, nearly all night migrants come
+to earth well before dawn; and, third, in each hour of the night up
+until eleven or twelve o'clock there is typically a progressive increase
+in the number of birds that have taken wing and in each of the hours
+thereafter there is a gradual decrease. Taken at its face value, the
+evidence seems to indicate that birds do not begin their night
+migrations _en masse_ and remain on the wing until dawn and that in all
+probability most of them utilize less than half of the night.
+
+Interestingly enough, the fact that the plot points in Figure 26 lie
+nearly in line tempts one to a further conclusion. The curve behaves as
+an arithmetic progression, indicating that approximately the same number
+of birds are leaving the ground in each hour interval up to a point and
+that afterwards approximately the same number are descending within
+each hour. However, some of the components making up this curve, as
+later shown, are so aberrant in this regard that serious doubt is cast
+on the validity of this generalization.
+
+Because the results of these time studies are unexpected and
+startling, I have sought to explore other alternative explanations and
+none appears to be tenable. For example, the notion that the varying
+flight speeds of birds might operate in some way to produce a
+cumulative effect as the night progresses must be rejected on close
+analysis. If birds of varying flight speeds are continuously and
+evenly distributed in space, a continuous and even flow would result
+all along their line of flight. If they are haphazardly distributed in
+space, a correspondingly haphazard density pattern would be expected.
+
+Another explanation might be sought in the purely mathematical effects
+of the method itself. The computational procedure assumes that the
+effective area of the sample is extremely large when the moon is low,
+a condition that usually obtains in the early hours of the evening in
+the days surrounding the full moon. Actually no tests have yet been
+conducted to ascertain how far away a silhouette of a small bird can
+be seen as it passes before the moon. Consequently, it is possible
+that some birds are missed under these conditions and that the
+effective field of visibility is considerably smaller than the
+computed field of visibility. The tendency, therefore, may be to
+minimize the densities in such situations more than is justified.
+However, in many, if not most, cases, the plotting of the actual
+number of birds seen, devoid of any mathematical procedures, results
+in an ascending and descending curve.
+
+    [Illustration: FIG. 29. Various types of density-time curves.
+       (A) Near typical, Ottumwa, April 22-23; (B) random fluctuation,
+       Stillwater, April 23-24; (C) bimodal, Knoxville, April 22-23;
+       (D) sustained peak, Ottumwa, April 21-22; (E) early peak, Oak
+       Grove, May 21-22; (F) late peak, Memphis, April 23-24.]
+
+A third hypothesis proposes that all birds take wing at nearly the
+same time, gradually increase altitude until they reach the mid-point
+of their night's journey, and then begin a similarly slow descent.
+Since the field of observation of the telescope is conical, it is
+assumed that the higher the birds arise into the sky the more they
+increase their chances of being seen. According to this view, the
+changes in the density curve represent changes in the opportunity to
+see birds rather than an increase or decrease in the actual number of
+migrants in the air. Although measurements of flight altitude at
+various hours of the night have not been made in sufficient number to
+subject this idea to direct test, it is hardly worthy of serious
+consideration. The fallacy in the hypothesis is that the cone of
+observation itself would be rising with the rising birds so that
+actually the greatest proportion of birds flying would still be seen
+when the field of observation is in the supine position of early
+evening.
+
+It cannot be too strongly emphasized that the over-all time curves
+just discussed have been derived from a series of individual curves,
+some of which differ radically from the composite pattern. In Figure
+29, six dissimilar types are shown. This variation is not surprising
+in view of the fact that many other causative factors aside from time
+operate on the flow of birds from hour to hour. Figure 29A illustrates
+how closely some individual patterns conform with the average. Figure
+29B is an example of a random type of fluctuation with no pronounced
+time character. It is an effect rarely observed, occurring only in the
+cases where the number of birds observed is so small that pure chance
+has a pronounced effect on the computed densities; its vacillations
+are explicable on that account alone. Errors of sampling may similarly
+account for some, though not all, of the curves of the bimodal type
+shown in Figure 29C. Some variation in the curves might be ascribed to
+the variations in kinds of species comprising the individual flights
+at different times at different places, provided that it could be
+demonstrated that different species of birds show dissimilar temporal
+patterns. The other atypical patterns are not so easily dismissed and
+will be the subject of inquiry in the discussions that follow. It is
+significant that in spite of the variety of the curves depicted, which
+represent every condition encountered, in not a single instance is the
+density sustained at a high level throughout the night. Moreover,
+these dissident patterns merge into a remarkably harmonious, almost
+normal, average curve.
+
+When, at some future date, suitable data are available, it would be
+highly desirable to study the average monthly time patterns to
+ascertain to what extent they may deviate from the over-all average.
+At present this is not justifiable because there are not yet enough
+sets of data in any two months representing the same selection of
+stations.
+
+_Correlations with Other Data_
+
+It is especially interesting to note that the data pertaining to this
+problem derived from other methods of inquiry fit the conclusions
+adduced by the telescopic method. Overing (1938), who for several
+years kept records of birds striking the Washington Monument, stated
+that the record number of 576 individuals killed on the night of
+September 12, 1937, all came down between 10:30 P. M. and midnight.
+His report of the mortality on other nights fails to mention the time
+factor, but I am recently informed by Frederick C. Lincoln (_in
+litt._) that it is typical for birds to strike the monument in
+greatest numbers between ten and twelve o'clock at night. At the
+latter time the lights illuminating the shaft are extinguished, thus
+resulting in few or no casualties after midnight. The recent report by
+Spofford (1949) of over 300 birds killed or incapacitated at the
+Nashville airport on the night of September 9-10, 1948, after flying
+into the light beam from a ceilometer, is of interest in this
+connection even though the cause of the fatality is shrouded in
+mystery. It may be noted, however, that "most of the birds fell in the
+first hour," which, according to the account, was between 12:30 A. M.
+and 1:30 A. M. Furthermore, birds killed at the Empire State
+Building in New York on the night of September 10-11, 1948, began to
+strike the tower "shortly after midnight" (Pough, 1948). Also it will
+be recalled that the observations of Stone (_loc. cit._), already
+referred to in this paper (page 410), show a situation where the
+flight in the early part of the night was negligible but mounted to a
+peak between ten and eleven o'clock, with continuing activity at least
+until midnight.
+
+All of these observations are of significance in connection with the
+conclusions herein advanced, but by far the most striking correlation
+between these present results and other evidences is found in the
+highly important work of various European investigators studying the
+activity of caged migratory birds. This work was recently reviewed and
+extended by Palmgren (1944) in the most comprehensive treatise on the
+subject yet published. Palmgren recorded, by an electrically operated
+apparatus, the seasonal, daily, and hourly activity patterns in caged
+examples of two typical European migrants, _Turdus ericetorum
+philomelas_ Brehm and _Erithacus rubecula_ (Linnaeus). Four rather
+distinct seasonal phases in activity of the birds were discerned:
+_winter non-migratory_, _spring migratory_, _summer non-migratory_,
+and _autumn migratory_. The first of these is distinguished by morning
+and evening maxima of activity, the latter being better developed but
+the former being more prolonged. Toward the beginning of migration,
+these two periods of activity decline somewhat. The second, or spring
+migratory phase, which is of special interest in connection with the
+present problem, is characterized by what Palmgren describes as
+nightly migratory restlessness (_Zugunruhe_). The morning maximum,
+when present, is weaker and the evening maximum often disappears
+altogether. Although variations are described, the migratory
+restlessness begins ordinarily after a period of sleep ("sleeping
+pause") in the evening and reaches a maximum and declines before
+midnight.
+
+This pattern agrees closely with the rhythm of activity indicated by
+the time curves emerging from the present research. Combining the two
+studies, we may postulate that most migrants go to sleep for a period
+following twilight, thereby accounting for the low densities in the
+early part of the night. On awakening later, they begin to exhibit
+migratory restlessness. The first hour finds a certain number of birds
+sufficiently stimulated so that they rise forthwith into the air. In
+the next hour still others respond to this urge and they too mount
+into the air. This continues until the "restlessness" begins to abate,
+after which fewer and fewer birds take wing. By this time, the birds
+that began to fly early are commencing to descend, and since their
+place is not being filled by others leaving the ground, the density
+curve starts its decline. Farner (1947) has called attention to the
+basic importance of the work by Palmgren and the many experimental
+problems it suggests. Of particular interest would be studies
+comparing the activity of caged American migrant species and the
+nightly variations in the flight rates.
+
+_The Baton Rouge Drop-off_
+
+As already stated, the present study was initiated at Baton Rouge,
+Louisiana, in 1945, and from the outset a very peculiar density time
+pattern was manifest. I soon found that birds virtually disappeared
+from the sky after midnight. Within an hour after the termination of
+twilight, the density would start to ascend toward a peak which was
+usually reached before ten o'clock, and then would begin, surprisingly
+enough, a rapid decline, reaching a point where the migratory flow was
+negligible. In Figure 30 the density curves are shown for five nights
+that demonstrate this characteristically early decline in the volume
+of migration at this station. Since, in the early stages of the work,
+cooerdinates of apparent pathways of all the birds seen were not
+recorded, I am unable now to ascertain the direction of flight and
+thereby arrive at a density figure based on the dimension of the cone
+and the length of the front presented to birds flying in certain
+directions. It is feasible, nevertheless, to compute what I have
+termed a "plus or minus" flight density figure stating the rate of
+passage of birds in terms of the maximum and minimum corrections which
+all possible directions of flight would impose. In other words,
+density is here computed, first, as if all the birds were flying
+perpendicular to the long axis of the ellipse, and, secondly, as if
+all the birds were flying across the short axis of the ellipse. Since
+the actual directions of flight were somewhere between these two
+extremes, the "plus or minus" density figure is highly useful.
+
+    [Illustration: FIG. 30. Density-time curves on various nights
+       at Baton Rouge. (A) April 25, 1945; (B) April 15, 1946;
+       (C) May 10, 1946; (D) May 15, 1946; (E) April 22-23, 1948.
+       These curves are plotted on a "plus or minus" basis as
+       described in the text, with the bottom of the curve
+       representing the minimum density and the top of the curve
+       the maximum.]
+
+The well-marked decline before midnight in the migration rates at
+Baton Rouge may be regarded as one of the outstanding results emerging
+from this study. Many years of ornithological investigation in this
+general region failed to suggest even remotely that a situation of
+this sort obtained. Now, in the light of this new fact, it is possible
+for the first time to rationalize certain previously incongruous data.
+Ornithologists in this area long have noted that local storms and
+cold-front phenomena at night in spring sometimes precipitate great
+numbers of birds, whereupon the woods are filled the following day
+with migrants. On other occasions, sudden storms at night have
+produced no visible results in terms of bird densities the following
+day. For every situation such as described by Gates (1933) in which
+hordes of birds were forced down at night by inclement weather, there
+are just as many instances, even at the height of spring migration,
+when similar weather conditions yielded no birds on the ground.
+However, the explanation of these facts is simple; for we discover
+that storms that produced birds occurred before midnight and those
+that failed to produce birds occurred after that time (the storm
+described by Gates occurred between 8:30 and 9:00 P. M.).
+
+The early hour decline in density at Baton Rouge at first did not seem
+surprising in view of the small amount of land area between this
+station and the Gulf of Mexico. Since the majority of the birds
+destined to pass Baton Rouge on a certain night come in general from
+the area to the south of that place, and since the distances to
+various points on the coast are slight, we inferred that a three-hour
+flight from even the more remote points would probably take the bulk
+of the birds northward past Baton Rouge. In short, the coastal plain
+would be emptied well before midnight of its migrant bird life, or at
+least that part of the population destined to migrate on any
+particular night in question. Although data in quantity are not
+available from stations on the coastal plain other than Baton Rouge,
+it may be pointed out that such observations as we do have, from
+Lafayette and New Orleans, Louisiana, and from Thomasville, Georgia,
+are in agreement with this hypothesis.
+
+A hundred and seventy miles northward in the Mississippi Valley, at
+Oak Grove, Louisiana, a somewhat more normal density pattern is
+manifested. There, in four nights of careful observation, a pronounced
+early peak resulted on the night of May 21-22 (Figure 29E), but on the
+other three nights significant densities held up until near twelve
+o'clock, thereby demonstrating the probable effect of the increased
+amount of land to the south of the station.
+
+Subsequent studies, revealing the evident existence of an underlying
+density time pattern, cast serious doubt on the explanations just
+advanced of the early decline in the volume of migration at Baton
+Rouge. It has as yet been impossible to reconcile the early drop-off
+at this station with the idea that birds are still mounting into the
+air at eleven o'clock, as is implied by the ideal time curves.
+
+
+C. MIGRATION IN RELATION TO TOPOGRAPHY
+
+To this point we have considered the horizontal distribution of birds
+in the sky only on a very narrow scale and mainly in terms of the
+chance element in observations. Various considerations have supported
+the premise that the spread of nocturnal migration is rather even, at
+least within restricted spacial limits and short intervals of time.
+This means that in general the flow of birds from hour to hour at a
+single station exhibits a smooth continuity. It does not mean that it
+is a uniform flow in the sense that approximately the same numbers of
+birds are passing at all hours, or at all localities, or even on all
+one-mile fronts in the same locality. On the contrary, there is
+evidence of a pronounced but orderly change through the night in the
+intensity of the flight, corresponding to a basic and definitely timed
+cycle of activity. Other influences may interfere with the direct
+expression of this temporal rhythm as it is exhibited by observations
+at a particular geographical location. Among these, as we have just
+seen, is the disposition of the areas that offer suitable resting
+places for transient birds and hence contribute directly and
+immediately to the flight overhead. A second possible geographical
+effect is linked with the question of the tendency of night migrants
+to follow topographical features.
+
+_General Aspects of the Topographical Problem_
+
+That many diurnal migrants tend to fly along shorelines, rivers, and
+mountain ridges is well known, but this fact provides no assurance
+that night migrants do the same thing. Many of the obvious advantages
+of specialized routes in daylight, such as feeding opportunities, the
+lift provided by thermal updrafts, and the possible aid of certain
+landmarks in navigation, assume less importance after night falls.
+Therefore, it would not be safe to conclude that _all_ nocturnal
+migrants operate as do _some_ diurnal migrants. For instance, the
+passage of great numbers of certain species of birds along the Texas
+coast in daylight hours cannot be regarded as certain proof that the
+larger part of the nocturnal flight uses the same route. Neither can
+we assume that birds follow the Mississippi River at night simply
+because we frequently find migrants concentrated along its course in
+the day. Fortunately we shall not need to speculate indefinitely on
+this problem; for the telescopic method offers a means of study based
+on what night migrants are doing _at night_. Two lines of attack may
+be pursued. First we may compare flight densities obtained when the
+field of the telescope lies over some outstanding topographical
+feature, such as a river, with the recorded volume of flight when the
+cone of observation is directed away from that feature. Secondly, we
+may inquire how the major flight directions at a certain station are
+oriented with respect to the terrain. If the flight is concentrated
+along a river, for instance, the flight density curve should climb
+upward as the cone of observation swings over the river, _regardless
+of the hour at which it does so_. The effect should be most pronounced
+if the observer were situated on the river bank, so that the cone
+would eventually come to a position directly along the watercourse.
+Though in that event birds coming up the river route would be flying
+across the short axis of an elliptical section of the cone, the fact
+that the whole field of observation would be in their path should
+insure their being seen in maximum proportions. If, on the other hand,
+the telescope were set up some distance away from the river so that
+the cone merely moved _across_ its course, only a section of the
+observation field would be interposed on the main flight lane.
+
+The interaction of these possibilities with the activity rhythm should
+have a variety of effects on the flight density curves. If the cone
+comes to lie over the favored topographical feature in the hour of
+greatest migrational activity, the results would be a simple sharp
+peak of doubtful meaning. However, since the moon rises at a different
+time each evening, the cone likewise would reach the immediate
+vicinity of the terrain feature at a different time each night. As a
+result, the terrain peak would move away from its position of
+coincidence with the time peak on successive dates, producing first,
+perhaps, a sustention of peak and later a definitely bimodal curve.
+Since other hypotheses explain double peaks equally well, their mere
+existence does not necessarily imply that migrants actually do travel
+along narrow topographical lanes. Real proof requires that we
+demonstrate a moving peak, based on properly corrected density
+computations, corresponding always with the position of the cone over
+the most favored terrain, and that the flight vectors be consistent
+with the picture thus engendered.
+
+_The Work of Winkenwerder_
+
+To date, none of the evidence in favor of the topographical hypothesis
+completely fills these requirements. Winkenwerder (_loc. cit._), in
+analyzing the results of telescopic counts of birds at Madison and
+Beloit, Wisconsin, Detroit and Ann Arbor, Michigan, and at Lake
+Forest, Illinois, between 1898 and 1900, plotted the number of birds
+seen at fifteen-minute intervals as a function of the time of the
+night. He believed that the high points in the resulting frequency
+histograms represented intervals when the field of the telescope was
+moving over certain topographically determined flight lanes, though he
+did not specify in all cases just what he assumed the critical
+physiographic features to be. Especially convincing to him were
+results obtained at Beloit, where the telescope was situated on the
+east bank of the Rock River, on the south side of the city.
+Immediately below Beloit the river turns southwestward and continues
+in this direction about five miles before turning again to flow in a
+southeastward course for approximately another five miles. In this
+setting, on two consecutive nights of observation in May, the number
+of birds observed increased tremendously in the 2 to 3 A. M. interval,
+when, according to Winkenwerder's interpretation of the data (he did
+not make the original observations at Beloit himself), the telescope
+was pointing directly down the course of the river. This conclusion is
+weakened, however, by notable inconsistencies. Since the moon rises
+later each evening, it could not have reached the same position over
+the Rock River at the same time on both May 12-13 and May 13-14, and
+therefore, if the peaks in the graph were really due to a greater
+volume of migration along the watercourse, they should not have so
+nearly coincided. As a matter of fact the incidence of the peak on
+May 12-13 should have preceded that of the peak on May 13-14; whereas
+his figure shows the reverse to have been true. Singularly enough,
+Winkenwerder recognized this difficulty in his treatment of the data
+from Madison, Wisconsin. Unable to correlate the peak period with the
+Madison terrain by the approach used for Beloit, he plotted the
+observations in terms of hours after moonrise instead of standard
+time. This procedure was entirely correct; the moon does reach
+approximately the same position at each hour after its rise on
+successive nights. The surprising thing is that Winkenwerder did not
+seem to realize the incompatibility of his two approaches or to
+realize that he was simply choosing the method to suit the desired
+results.
+
+Furthermore, as shown in Part I of this paper, the number of birds
+seen through the telescope often has only an indirect connection with
+the actual number of birds passing over. My computations reveal that
+the highest counts of birds at Beloit on May 12-13 were recorded when
+the moon was at an altitude of only 8 deg. to 15 deg. and, that when
+appropriate allowance is made for the immense size of the field of
+observation at this time, the partially corrected flight density for
+the period is not materially greater than at some other intervals in
+the night when the telescope was not directed over the course of the
+Rock River. These allowances do not take the direction factor into
+consideration. Had the birds been flying at right angles to the short
+axis of an elliptical section of the cone throughout the night, the
+flight density in the period Winkenwerder considered the peak would
+have been about twice as high as in any previous interval. On the
+other hand, if they had been flying across the long axis at all times,
+the supposed peak would be decidedly inferior to the flight density at
+10 to 11:00 P. M., before the cone came near the river.
+
+Admittedly, these considerations contain a tremendous element of
+uncertainty. They are of value only because they expose the equal
+uncertainty in Winkenwerder's basic evidence. Since the cooerdinates of
+the birds' apparent pathways at Beloit were given, I at first
+entertained the hope of computing the flight densities rigorously, by
+the method herein employed. Unfortunately, Winkenwerder was apparently
+dealing with telescopes that gave inverted images, and he used a
+system for recording cooerdinates so ambiguously described that I am
+not certain I have deciphered its true meaning. When, however, his
+birds are plotted according to the instructions as he stated them, the
+prevailing direction of flight indicated by the projection formula
+falls close to west-northwest, not along the course of the Rock River,
+but _at direct right angles to it_.
+
+    [Illustration: FIG. 31. Directional components in the flight
+       at Tampico on three nights in 1948. The lengths of the
+       sector vectors are determined by their respective densities
+       expressed as a percentage of the station density for that
+       night; the vector resultants are plotted from them by
+       standard procedure. Thus, the nightly diagrams are not on the
+       same scale with respect to the actual number of birds involved.]
+
+
+    [Illustration: FIG. 32. Hourly station density curve at
+       Tampico, Tamaulipas, on the night of April 21-22, 1948 (CST).]
+
+_Interpretation of Recent Data_
+
+I am in a position to establish more exact correlations between flight
+density and terrain features in the case of current sets of
+observations. Some of these data seem at first glance to fit the idea
+of narrow topographically-oriented flight lanes rather nicely. At
+Tampico, where six excellent sets of observations were made in March
+and April, 1948, the telescope was set up on the beach within a few
+yards of the Gulf of Mexico. As can be seen from Figure 25 (_ante_),
+the slant of the coastline at this point is definitely west of north,
+as is also the general trend of the entire coast from southern
+Veracruz to southern Tamaulipas (see Figure 34, beyond). The over-all
+vector resultant of all bird flights at this station was N 11 deg. W, and,
+as will be seen from Figure 31, none of the nightly vector resultants
+in April deviates more than one degree from this average. Thus the
+prevailing direction of flight, as computed, agrees with the trend of
+the coast at the precise point of the observations, at least to the
+extent that both are west of north. To be sure, the individual sector
+vectors indicate that not all birds were following this course;
+indeed, some appear to have been flying east of north, heading for a
+landfall in the region of Brownsville, Texas, and a very few to have
+been traveling northeastward toward the central Gulf coast. But it
+must be remembered that a certain amount of computational deviation
+and of localized zigzagging in flight must be anticipated. Perhaps
+none of these eastward vectors represents an actual extended flight
+path. The nightly vector resultants, on the other hand, are so
+consistent that they have the appearance of remarkable accuracy and
+tempt one to draw close correlations with the terrain. When this is
+done, it is found that, while the prevailing flight direction is 11 deg.
+west of north, the exact slant of the coastline at the location of the
+station is about 30 deg. west of north, a difference of around 19 deg.. It
+appears, therefore, that the birds were not following the shoreline
+precisely but cutting a chord about ten miles long across an
+indentation of the coast. If it be argued that the method of
+calculation is not accurate enough to make a 19 deg. difference
+significant, and that most of the birds might have been traveling
+along the beach after all, it can be pointed out with equal
+justification that, if this be so, the 11 deg. divergence from north does
+not mean anything either and that perhaps the majority of the birds
+were going due north. We are obliged to conclude either that the main
+avenue of flight paralleled the disposition of the major topographical
+features only in a general way or that the angle between the line of
+the coast and true north is not great enough to warrant any inference
+at all.
+
+Consideration of the Tampico density curves leads to similarly
+ambiguous results. On the night of April 21-22, as is evident from a
+comparison of Figures 25 and 32, the highest flight density occurred
+when the projection of the cone on the terrain was wholly included
+within the beach. This is very nearly the case on the night of April
+23-24 also, the positions of the cone during the peak period of
+density being only about 16 deg. apart. (On the intervening date, clouds
+prevented continuous observation during the critical part of the
+night.) These correlations would seem to be good evidence that most of
+these night migrants were following the coastline of the Gulf of
+Mexico. However, the problem is much more complicated. The estimated
+point of maximum flight density fell at 10:45 P. M. on April 21-22
+and 11:00 P. M. on April 23-24, both less than an hour from the peak
+in the ideal time curve (Figure 26, _ante_). We cannot be sure,
+therefore, that the increase in density coinciding with the position
+of the moon over the beach is not an increase which would have
+occurred anyway. Observations conducted several nights before or after
+the second quarter, when the moon is not on or near its zenith at the
+time of the predictable peak in the density curve, would be of
+considerable value in the study of this particular problem.
+
+The situation at Tampico has been dealt with at length because, among
+all the locations for which data are available, it is the one that
+most strongly supports the topographical hypothesis. In none of the
+other cases have I been able to find a definite relation between the
+direction of migration and the features of the terrain. Studies of
+data from some of these stations disclose directional patterns that
+vary from night to night only slightly more than does the flight at
+Tampico. In three nights of observation at Lawrence, Kansas, marked by
+very high densities, the directional trend was north by
+north-northeast with a variation of less than 8 deg., yet Lawrence is so
+situated that there seems to be no feature of the landscape locally or
+in the whole of eastern Kansas or of western Missouri that coincides
+with this heading. At Mansfield, Louisiana, in twelve nights of
+observation, the strong east by northeast trend varied less than 15 deg.,
+but again there appears to be no correlation over a wide area between
+this direction and any landmarks. And, at Progreso, Yucatan, where the
+vector resultants were 21 deg. and 27 deg. on successive nights, most of the
+birds seen had left the land and were beginning their flight northward
+over the trackless waters of the Gulf of Mexico. Furthermore, as I
+have elsewhere pointed out (1946: 205), the whole northern part of the
+Yucatan Peninsula itself is a flat terrain, unmarked by rivers,
+mountains, or any other strong physiographic features that conceivably
+might be followed by birds.
+
+    [Illustration: FIG. 33. The nightly net trend of migrations
+       at three stations in 1948. Each arrow is the vector resultant
+       for a particular night, its length expressing the nightly
+       density as a percentage of the total station density for the
+       nights represented. Thus, the various station diagrams are
+       not to the same scale.]
+
+In Figure 33 I have shown the directional patterns at certain stations
+where, unlike the cases noted above, there is considerable change on
+successive nights. Each vector shown is the vector resultant for one
+particular night. The lengths of the vectors have been determined by
+their respective percentages of the total computed density, or total
+station magnitude, for all the nights in question. In other words, the
+lengths of the individual vectors denote the percentile role that each
+night played in the total density. From the directional spread at
+these stations it becomes apparent that if most of the birds were
+traveling along a certain topographic feature on one night, they
+could not have been traveling along the same feature on other nights.
+
+The possibility should be borne in mind, however, that there may be
+more than one potential topographic feature for birds to follow at
+some stations. Moreover, it is conceivable that certain species might
+follow one feature that would lead them in the direction of their
+ultimate goal, whereas other species, wishing to go in an entirely
+different direction, might follow another feature that would lead them
+toward their respective destination. It would seem unlikely, however,
+that the species composition of the nocturnal flights would change
+materially from night to night, although there is a strong likelihood
+that it might do so from week to week and certainly from month to
+month.
+
+By amassing such data as records of flight direction along the same
+coast from points where the local slant of the shoreline is materially
+different, and comparisons of the volume of migration at night along
+specialized routes favored during the day with the flight densities at
+progressive distances from the critical terrain feature involved, we
+shall eventually be able to decide definitely the role topography
+plays in bird migration. We cannot say on the basis of the present
+ambiguous evidence that it is not a factor in determining which way
+birds fly, but, if I had to hazard a guess one way or the other, I
+would be inclined to discount the likelihood of its proving a major
+factor.
+
+
+D. GEOGRAPHICAL FACTORS AND THE CONTINENTAL DENSITY PATTERN
+
+A study of the total nightly or seasonal densities at the various
+stations brings forth some extremely interesting factors, many of
+which, however, cannot be fully interpreted at this time. A complete
+picture of the magnitude of migration at a given station cannot be
+obtained from the number of birds that pass the station on only a few
+nights in one spring. Many years of study may be required before hard
+and fast principles are justifiable. Nevertheless, certain salient
+features stand out in the continental density pattern in the spring of
+1948. (The general results are summarized in Tables 2-5; the location
+of the stations is shown in Figure 34.) These features will be
+discussed now on a geographical basis.
+
+    TABLE 2.--Extent of Observations and Seasonal Station
+                Densities at Major Stations in 1948
+
+  ==========================================================================
+                       |Nights of observation| Hours of observation|
+  OBSERVATION STATION  |---------------------+---------------------|Season
+                       |March|April|May|Total|March|April|May|Total|density
+  ---------------------+-----+-----+---+-----+-----+-----+---+-----+--------
+  CANADA               |     |     |   |     |     |     |   |     |
+    Pt. Pelee          |     |     | 1 |  1  |     |     | 6 |   6 |   2,500
+                       |     |     |   |     |     |     |   |     |
+  MEXICO               |     |     |   |     |     |     |   |     |
+    S. L. P.: Ebano    |  1  |     |   |  1  |  3  |     |   |   3 |   1,300
+    Tamps.: Tampico    |  3  |  3  |   |  6  | 20  | 20  |   |  40 | 140,300
+    Yuc.: Progreso     |     |  3  |   |  3  |     | 18  |   |  18 |  60,500
+                       |     |     |   |     |     |     |   |     |
+  UNITED STATES        |     |     |   |     |     |     |   |     |
+    Fla.: Pensacola    |     |  2  | 2 |  4  |     |  8  | 7 |  15 |   1,500
+      Winter Park      |     |  5  | 6 | 11  |     | 39  |38 |  77 |  21,700
+    Ga.: Athens        |     |  2  |   |  2  |     | 10  |   |  10 |   4,000
+      Thomasville      |     |  1  | 1 |  2  |     |  8  | 8 |  16 |   4,700
+    Iowa: Ottumwa      |     |  5  | 5 | 10  |     | 16  |28 |  44 | 134,400
+    Kans.: Lawrence    |  2  |  1  |   |  3  | 16  |  4  |   |  20 |  68,700
+    Ky.: Louisville    |     |  3  | 2 |  5  |     | 20  |14 |  34 |  49,300
+      Murray           |     |  2  |   |  2  |     | 13  |   |  13 |  26,200
+    La.: Baton Rouge   |     |  3  |   |  3  |     | 15  |   |  15 |  11,000
+      Lafayette        |     |  1  |   |  1  |     |  5  |   |   5 |   2,800
+      Mansfield        |  1  |  5  | 4 | 10  |  2  | 16  |22 |  40 |  22,400
+      New Orleans      |  1  |  1  |   |  2  |  5  |  2  |   |   7 |   1,900
+      Oak Grove        |     |  2  | 2 |  4  |     | 16  |15 |  31 |  33,900
+    Mich.: Albion      |     |  1  |   |  1  |     |  3  |   |   3 |   1,100
+    Minn.: Hopkins     |     |     | 1 |  1  |     |     | 4 |   4 |   2,000
+    Miss.: Rosedale    |     |  1  | 1 |  2  |     |  6  | 8 |  14 |  12,600
+    Mo.: Columbia      |     |  2  | 1 |  3  |     |  8  | 6 |  14 |  13,100
+    Liberty            |     |  1  | 1 |  2  |     |  7  | 7 |  14 |   4,800
+    Okla.: Stillwater  |  1  |  2  | 1 |  4  |  5  | 11  | 3 |  19 |   8,400
+    S. Car.: Charleston|  1  |  1  | 1 |  3  |  5  |  8  | 9 |  22 |   3,000
+    Tenn.: Knoxville   |     |  2  | 2 |  4  |     | 18  |14 |  32 |  35,400
+      Memphis          |  2  |  3  | 2 |  7  | 13  | 20  |12 |  45 |  29,700
+    Tex.: College      |     |  3  | 1 |  4  |     | 19  | 8 |  27 |  32,200
+      Station Rockport |     |  1  |   |  1  |     |  4  |   |   4 |   6,200
+  --------------------------------------------------------------------------
+
+    TABLE 3.--Average Hourly Station Densities in 1948
+
+  ========================================================
+  OBSERVATION STATION     | March | April |  May  | Season
+  ------------------------+-------+-------+-------+-------
+  CANADA                  |       |       |       |
+    Pt. Pelee             |       |       |   400 |   400
+                          |       |       |       |
+  MEXICO                  |       |       |       |
+    S. L. P.: Ebano       |   400 |       |       |   400
+    Tamps.: Tampico       |   700 | 6,300 |       | 3,500
+    Yuc.: Progreso        |       | 2,800 |       | 2,800
+                          |       |       |       |
+  UNITED STATES           |       |       |       |
+    Fla.: Pensacola       |       |     0+|   200 |   100
+      Winter Park         |       |   300 |   200 |   300
+    Ga.: Athens           |       |   400 |       |   400
+      Thomasville         |       |   500 |   100 |   300
+    Iowa: Ottumwa         |       | 1,700 | 3,800 | 3,100
+    Kans.: Lawrence       | 4,000 | 1,400 |       | 3,400
+    Ky.: Louisville       |       | 2,000 |   700 | 1,500
+      Murray              |       | 2,000 |       | 2,000
+    La.: Baton Rouge      |       |   700 |       |   700
+      Lafayette           |       |   600 |       |   600
+      Mansfield           |     0 |   700 |   800 |   600
+      New Orleans         |    60 |   800 |       |   300
+      Oak Grove           |       | 1,400 |   800 | 1,100
+    Mich.: Albion         |       |   400 |       |   400
+    Minn.: Hopkins        |       |       |   500 |   500
+    Miss.: Rosedale       |       | 1,100 |   700 |   900
+    Mo.: Columbia         |       |   400 | 1,700 |   900
+      Liberty             |       |   500 |   200 |   300
+    Okla.: Stillwater     |   500 |   200 | 1,000 |   400
+    S. Car.: Charleston   |   200 |   200 |     0+|   100
+    Tenn.: Knoxville      |       | 1,300 |   800 | 1,100
+      Memphis             |   300 |   800 |   900 |   700
+    Tex.: College Station |       | 1,100 | 1,500 | 1,200
+      Rockport            |       | 1,600 |       | 1,600
+  --------------------------------------------------------
+
+    TABLE 4.--Maximum Hourly Station Densities in 1948
+
+  ======================================================
+  OBSERVATION STATION     |  March  |  April  |   May
+  ------------------------+---------+---------+---------
+  CANADA                  |         |         |
+    Pt. Pelee             |         |         |  1,400
+                          |         |         |
+  MEXICO                  |         |         |
+    S. L. P.: Ebano       |    600  |         |
+    Tamps.: Tampico       |  3,100  | 21,200  |
+    Yuc.: Progreso        |         | 11,900  |
+                          |         |         |
+  UNITED STATES           |         |         |
+    Fla.: Pensacola       |         |    100  |    700
+      Winter Park         |         |  2,300  |  1,000
+    Ga.: Athens           |         |    900  |
+      Thomasville         |         |  1,500  |    200
+    Iowa: Ottumwa         |         |  3,800  | 12,500
+    Kans.: Lawrence       | 14,500  |  2,200  |
+    Ky.: Louisville       |         |  5,000  |  1,400
+      Murray              |         |  3,700  |
+    La.: Baton Rouge      |         |  3,400  |
+      Lafayette           |         |  1,800  |
+      Mansfield           |         |  2,100  |  1,600
+      New Orleans         |    200  |  1,100  |
+      Oak Grove           |         |  2,700  |  2,500
+    Mich.: Albion         |         |    700  |
+    Minn.: Hopkins        |         |         |  1,100
+    Miss.: Rosedale       |         |  2,200  |  1,400
+    Mo.: Columbia         |         |    800  |  3,400
+      Liberty             |         |    800  |    800
+    Okla.: Stillwater     |    900  |    700  |  1,400
+    S. Car.: Charleston   |    400  |    600  |    200
+    Tenn.: Knoxville      |         |  5,800  |  1,900
+      Memphis             |  1,200  |  3,400  |  2,100
+    Tex.: College Station |         |  3,400  |  3,100
+      Rockport            |         |  2,400  |
+  ------------------------------------------------------
+
+    TABLE 5.--Maximum Nightly Densities at Stations with More
+                Than One Night of Observation
+
+  ======================================================
+  OBSERVATION STATION     |  March  |  April  |   May
+  ------------------------+---------+---------+---------
+                          |         |         |
+  MEXICO                  |         |         |
+    Tamps.: Tampico       |  5,500  | 63,600  |
+    Yuc.: Progreso        |         | 31,600  |
+                          |         |         |
+  UNITED STATES           |         |         |
+    Fla.: Winter Park     |         |  6,200  |
+    Ga.: Athens           |         |  2,600  |
+      Thomasville         |         |  3,900  |
+    Iowa: Ottumwa         |         | 15,300  | 54,600
+    Kans.: Lawrence       | 51,600  |  5,400  |
+    Ky.: Louisville       |         | 17,000  |  8,400
+      Murray              |         | 16,400  |
+    La.: Baton Rouge      |         |  6,200  |
+      Mansfield           |         |  4,900  |  5,200
+      Oak Grove           |         | 13,600  |  5,800
+    Miss.: Rosedale       |         |  6,800  |  5,800
+    Mo.: Columbia         |         |  1,400  | 10,300
+    Okla.: Stillwater     |  2,700  |  1,900  |  3,000
+    Tenn.: Knoxville      |         | 15,200  |  9,000
+      Memphis             |  3,600  |  7,900  |  7,000
+    Tex.: College Station |         |  6,200  | 13,200
+  ------------------------------------------------------
+
+    [Illustration: FIG. 34. Stations at which telescopic
+       observations were made in 1948.]
+
+_Gulf Migration: A Review of the Problem_
+
+In view of the controversy in recent years pertaining to migration
+routes in the region of the Gulf of Mexico (Williams, 1945 and 1947;
+Lowery, 1945 and 1946), the bearing of the new data on the problem is
+of especial interest. While recent investigations have lent further
+support to many of the ideas expressed in my previous papers on the
+subject, they have suggested alternative explanations in the case of
+others. In the three years that have elapsed since my last paper
+dealing with Gulf migration, some confusion seems to have arisen
+regarding the concepts therein set forth. Therefore, I shall briefly
+re-state them.
+
+It was my opinion that evidence then available proved conclusively
+that birds traverse the Gulf frequently and intentionally; that the
+same evidence suggested trans-Gulf flights of sufficient magnitude to
+come within the meaning of migration; that great numbers of birds move
+overland around the eastern and western edges of the Gulf; that it was
+too early to say whether the coastal or trans-Gulf route was the more
+important, but that enough birds cross the water from Yucatan to
+account for transient migration in the extreme lower Mississippi
+Valley; and, that, in fair weather, most trans-Gulf migrants continue
+on inland for some distance before coming to land, creating an area of
+"hiatus" that is usually devoid of transient species. I tried to make
+it emphatically clear that I realized that many birds come into Texas
+from Mexico overland, that I did not think the hordes of migrants
+normally seen on the Texas coast in spring were by any means all
+trans-Gulf migrants. I stated (1946: 206): "Proving that birds migrate
+in numbers across the Gulf does not prove that others do not make the
+journey by the coastal routes. But that is exactly the point. No one
+has ever pretended that it does." Although some ornithologists seem to
+have gained the impression that I endorse only the trans-Gulf route,
+this is far from the truth. I have long held that the migrations
+overland through eastern Mexico and southern Texas on one hand, and
+the over-water flights on the other, are each part of the broad
+movement of transients northward into the United States. There are
+three avenues of approach by which birds making up the tremendous
+concentrations on the Texas coast may have reached there: by a
+continental pathway from a wintering ground in eastern and southern
+Mexico; by the over-water route from Yucatan and points to the
+southward; and, finally, by an overland route from Central America via
+the western edge of the Gulf. As a result of Louisiana State
+University's four-year study of the avifauna in eastern Mexico, I
+know that migrants reach Texas from the first source. As a consequence
+of my studies in Yucatan of nocturnal flight densities and their
+directional trends, I strongly believe that migrants reach Texas from
+this second source. As for the third source, I have never expressed an
+opinion. I am not prepared to do so now, for the reason that today, as
+three years ago, there is no dependable evidence on which to base a
+judgment one way or another.
+
+    TABLE 6.--Computed Hourly Densities at Tampico, Tamps.,
+                in Spring of 1948
+
+  =========================================================================
+             |                    Average hour of observation
+    DATE     |-----+------+-------+-------+------+------+------+------+----
+             | 8:30| 9:30 | 10:30 | 11:30 |12:30 | 1:30 | 2:30 | 3:30 |4:30
+  -----------|-----+------+-------+-------+------+------+------+------+----
+  22-23 March|  600|  700 | 1,000 |   800 |  100 |  100 |    0 |  100 | ..
+  23-24 March|    0|  400 | 1,200 | 3,100 |  800 |  ..  |  ..  |  ..  | ..
+  24-25 March|  300|  700 |   800 | 1,600 |1,100 |  ..  |  ..  |  ..  | ..
+  21-22 April|1,100|7,000 |14,900 |12,900 |8,100 |3,800 |3,500 |  200 | ..
+  22-23 April|  700|2,900 | 7,500 |   ..  |  ..  |  ..  |  ..  |  ..  | ..
+  23-24 April|  600|4,700 |19,100 |21,200 |5,500 |5,900 |4,000 |2,000 |200
+  -------------------------------------------------------------------------
+
+
+_Western Gulf Area_
+
+Among the present flight density data bearing on the above issues, are
+the six sets of observations from the vicinity of Tampico, Tamaulipas,
+already referred to. These were secured in the spring of 1948 by a
+telescope set up on the Gulf beach just north of the Miramar pavilion
+and only a hundred feet from the surf (see Figure 25, _ante_). The
+beach here is approximately 400 feet wide and is backed by
+scrub-covered dunes, which rapidly give way toward the west to a
+rather dense growth of low shrubs and trees. One might have expected
+that station densities at Tampico in March would be rather high.
+Actually, though they are the second highest recorded for the month,
+they are not impressive and afford a striking contrast with the record
+flights there in April (Table 6). Unfortunately, only a few stations
+were operating in March and thus adequate comparisons are impossible;
+but the indications are that, in March, migration activity on the
+western edges of the Gulf is slight. It fails even to approach the
+volume that may be observed elsewhere at the same time, as for
+example, in eastern Kansas where, however, the migration is not
+necessarily correlated with the migration in the lower Gulf area.
+Strangely enough, on the night of March 22-23, at Tampico,
+approximately 85 per cent of the birds were flying from north of an
+east-west line to south of it, opposite to the normal trend of spring
+migration. This phenomenon, inexplicable in the present instance, will
+be discussed below. On the other two nights in March, the directional
+trend at Tampico was northward with few or no aberrant components.
+Observations made approximately thirty-five miles inland from the
+Gulf, at Ebano, San Luis Potosi, on the night of March 25-26, show
+lower station densities than the poorest night at Tampico, but since
+they cover only a three-hour watch, they reveal little or nothing
+concerning the breadth of the so-called coastal flyway.
+
+April flight densities at Tampico are the highest recorded in the
+course of this study. The maximum hourly density of 21,200 birds is 46
+per cent higher than the maximum hourly density anywhere else. The
+average hourly density of 6,300 in April is more than twice as great
+as the next highest average for that month. These figures would seem
+to satisfy certain hypotheses regarding a coastwise flight of birds
+around the western edge of the Gulf. Other aspects of the observations
+made at that time do not satisfy these hypotheses. Texas
+ornithologists have found that in periods of heavy spring migration,
+great numbers of birds are invariably precipitated by rainy weather.
+On April 23, in the midst of the record-breaking telescopic studies at
+Tampico, Mr. Robert J. Newman made a daytime census immediately
+following four hours of rain. He made an intensive search of a small
+area of brush and low growth back of the beach for traces of North
+American migrants. In his best hour, only thirteen individual birds
+out of seventy-five seen were of species that do not breed there. The
+transient species were the Ruby-throated Hummingbird (1),
+Scissor-tailed Flycatcher (1), Western Wood Pewee (1), Black-throated
+Green Warbler (2) Orchard Oriole (7), and Baltimore Oriole (1), all of
+which winter extensively in southern Mexico. Perhaps, however, the
+apparent scarcity of transients on this occasion is not surprising in
+the light of the analysis of flight density in terms of bird density
+on the ground which I shall develop beyond. My only point here is to
+demonstrate that rain along the coast does not always produce birds.
+
+As large as the nocturnal flights at Tampico have so far proved to be,
+they are not commensurate with the idea that nearly all birds follow a
+narrow coastwise route around the Gulf. To establish the latter idea,
+one must be prepared to show that the migrant species returning to the
+United States pass along two flyways a few miles wide in the immense
+volume necessary to account for their later abundance on a 1500-mile
+front extending across eastern North America. One might expect at
+least ten to twenty fold the number observable at any point in the
+interior of the United States. In actuality, the highest nightly
+density of 63,600 birds at Tampico is barely sufficient to account for
+the highest nightly density of 54,600 at Ottumwa, Iowa, alone.
+
+Of course, there is no way of knowing how closely a ratio of anywhere
+from ten to one through twenty to one, employed in this comparison,
+expresses the true situation. It may be too high. It could be too low,
+particularly considering that preliminary studies of flight density in
+Florida indicate that the western shores of the Gulf of Mexico must
+carry the major part of the traffic if migratory flights back to the
+United States in spring take place only along coastwise routes.
+Consideration of the data obtained in Florida in 1948 will serve to
+emphasize the point.
+
+_Eastern Gulf Area_
+
+At Winter Park, Florida, seventy-seven hours were spent at the
+telescope in April and May. This was 71 per cent more hours of actual
+observation than at the next highest station. Nevertheless, the total
+seasonal density amounted to only 21,700 birds. The average hourly
+density was only 300 birds, with the maximum for any one hour being
+2,300 birds. In contrast, forty-five hours of observation at Tampico,
+Tamaulipas, in March and April, yielded a total station density of
+140,300 birds. At the latter place, on the night of April 23-24,
+almost as many birds passed _in a single hour_ as passed Winter Park
+in all of its seventy-seven hours of observation.
+
+Should future telescopic studies at Florida stations fail to produce
+densities appreciably higher than did Winter Park in 1948, the
+currently-held ideas that the Florida Peninsula is a major flyway will
+be seriously shaken. But one consideration must be kept in mind
+regarding the present picture. No observations were made at Winter
+Park in March, when it is conceivable that densities may have been
+materially higher. We know, for instance, that many of the early
+migrants to the southern United States are species whose winter homes
+are in the West Indies. Numbers of Vireonidae and Parulidae (notably
+the genera _Vireo_, _Parula_, _Protonotaria_, _Mniotilta_, _Seiurus_,
+_Geothlypis_, _Setophaga_, and certain _Dendroica_ and _Vermivora_)
+winter extensively in this region and are among the first birds to
+return to the southern states in the spring. Many of them often reach
+Louisiana and other states on the Gulf coastal plain by mid-March. In
+the same connection, it may be mentioned that many of the outstanding
+instances of birds striking lighthouses in southern Florida occurred
+in March and early April (Howell, 1932).
+
+_Yucatan Area_
+
+I have long felt that the answers to many of the questions which beset
+us in our study of Gulf migration are to be found on the open waters
+of the Gulf of Mexico itself or on the northern tip of the Yucatan
+Peninsula. Accordingly, in the spring of 1945 I crossed the Gulf by
+slow freighter for the purpose of determining how many and what kinds
+of birds might be seen between the mouth of the Mississippi River and
+the Yucatan Peninsula in fair weather, when it could not be argued
+that the birds had been blown there by inclement weather. To my own
+observations I was able to add those of other ornithologists who
+likewise had been aboard ship in the Gulf.
+
+The summary of results proved that birds of many species cross the
+Gulf and do so frequently. It failed to demonstrate beyond all doubt
+that they do so in large numbers. Nor had I expected it to do so. The
+consensus of Gulf coast ornithologists seemed to be that transient
+migration in their respective regions is often performed at too high
+an elevation to be detected unless the birds are forced to earth by
+bad weather. I saw no reason to anticipate that the results would be
+otherwise over the waters of the Gulf of Mexico.
+
+The application of the telescopic method held promise of supplying
+definite data on the numbers of trans-Gulf migrants, however high
+their flight levels. The roll and vibration of the ship had prevented
+me in 1945 from making telescopic observations at sea. Since no
+immediate solution to the technical difficulties involved presented
+itself, I undertook to reach one of the small cays in Alacran Reef,
+lying seventy-five miles north of Yucatan and in line with the coast
+of southern Louisiana. Because of transportation difficulties, my
+plans to place a telescopic station in this strategic location failed.
+Consequently, I returned in 1948 by freighter to Progreso, Yucatan,
+where telescopic counts were made for three nights, one of which was
+rendered almost valueless by the cloud cover.
+
+    [Illustration: FIG. 35. Positions of the cone of
+       observation at Progreso, Yucatan, on the night of April
+       23-24, 1948, from 8:53 P. M. to 3:53 A. M. Essential
+       features of this map are drawn to scale. The telescope was
+       set up on the end of a one-mile long wharf that extends
+       northward from the shore over the waters of the Gulf of
+       Mexico. The triangular (white) lines represent the
+       projections of the cone of visibility on the earth at the
+       mid-point of each hour of observation. Only briefly, in the
+       first two hours, did the cone lie even in part over the
+       adjacent mainland. Hence, nearly all of the birds seen in the
+       course of the night had actually left the land behind.]
+
+The observation station at Progreso was situated on the northern
+end of the new wharf which projects northward from the beach to
+a point one mile over the Gulf. As will be seen from Figure 35, the
+entire cone of observation lay at nearly all times over the intervening
+water between the telescope on the end of the wharf and the
+beach. Therefore, nearly all of the birds seen were actually observed
+leaving the coast and passing out over the open waters of the
+Gulf. The hourly station densities are shown in Table 7 and Figures
+24 and 36. In the seventeen hours of observation on the nights of
+April 23-24 and April 24-25, a total computed density of 59,200 birds
+passed within one-half mile of each side of Progreso. This is the
+third highest density recorded in the course of this study. The
+maximum for one hour was a computed density of 11,900 birds. This
+is the fourth highest hourly density recorded in 1948.
+
+    [Illustration: FIG. 36. Hourly station density curve for
+       night of April 23-24, 1948, at Progreso, Yucatan.]
+
+    TABLE 7.--Computed Hourly Densities at Progreso, Yuc.,
+                in Spring of 1948
+
+  ===========+============================================================
+             |               Average hour of observation
+     DATE    +-----+------+------+-------+------+------+------+-----+-----
+             |8:30 | 9:30 |10:30 | 11:30 |12:30 | 1:30 | 2:30 |3:30 |4:30
+  -----------+-----+------+------+-------+------+------+------+-----+-----
+  23-24 April| 400 |3,000 |5,100 |10,000 |9,000 |2,800 |  900 | 400 |....
+  24-25 April|   0 |  500 |3,700 |11,900 |7,900 |1,900 |1,100 | 400 | 200
+  -----------+-----+------+------+-------+------+------+------+-----+-----
+
+
+It is not my contention that this many birds leave the northern coast
+of Yucatan every night in spring. Indeed, further studies may show
+negligible flight densities on some nights and even greater densities
+on others. As a matter of fact several hours of observation on the
+night of April 25-26, at Merida, Yucatan, approximately twenty-five
+miles inland from Progreso, indicated that on this night the density
+overhead was notably low, a condition possibly accounted for by a
+north wind of 10 mph blowing at 2,000 feet. I merely submit that on
+the nights of April 23-24 and 24-25, birds were leaving the coast of
+Yucatan _at Progreso_ at the rate indicated. But, as I have emphasized
+in this paper and elsewhere (1946: 205-206), the northern part of the
+Yucatan Peninsula is notably unmarked by streams or any other
+physiographic features which birds might follow. The uniformity of the
+topography for many miles on either side of Progreso, if not indeed
+for the entire breadth of the Peninsula, makes it probable that
+Progreso is not a particularly favored spot for observing migration,
+and that it is not the only point along the northern coast of Yucatan
+where high flight densities can be recorded. This probability must be
+considered when comparisons are made between Progreso densities and
+those at Tampico. The argument could be advanced that the present
+densities from Tampico do not sufficiently exceed those at Progreso to
+establish the coastal route as the main avenue of traffic in spring,
+since there is every reason to suspect topography of exerting some
+influence to produce a channeling effect in eastern Mexico. Here the
+coast parallels the directional trend of the migratory movement for
+more than 600 miles. Likewise the Sierra Madre Oriental of eastern
+Mexico, situated approximately 100 miles inland (sometimes less), lies
+roughly parallel to the coast. Because of the slant of the Mexican
+land mass, many winter residents in southern Mexico, by short
+northward movements, would sooner or later filter into the coastal
+plain. Once birds are shunted into this lowland area, it would seem
+unlikely that they would again ascend to the top of the Sierra Madre
+to the west. In this way the great north-south cordillera of mountains
+may act as a western barrier to the horizontal dispersion of
+transients bound for eastern North America. Similarly, the Gulf itself
+may serve as an eastern barrier; for, as long as migrants may progress
+northward in the seasonal direction of migration and still remain over
+land, I believe they would do so.
+
+To put the matter in a slightly different way, the idea of a very
+narrow flight lane is inherent in the idea of coastwise migration.
+For, as soon as we begin to visualize flights of great volume over
+fronts extending back more than fifty miles from the shore line, we
+are approaching, if indeed we have not already passed, the point where
+the phenomenon is no longer coastwise in essence, but merely overland
+(as indeed my own unprocessed, telescopic data for 1949 indicate may
+be the case). In actuality, those who have reported on the migration
+along the western edge of the Gulf of Mexico have never estimated the
+width of the main flight at more than fifty miles and have intimated
+that under some circumstances it may be as narrow as two miles. No
+evidence of such restrictions can be discerned in the case of the
+trans-Gulf flights. If it cannot be said that they may be assumed to
+be as wide as the Gulf itself, they at least have the potential
+breadth of the whole 260-mile northern coast of the Yucatan Peninsula.
+On these premises, to be merely equal in total magnitude, the
+coastwise flights must exhibit, depending on the particular situation,
+from five to 130 times the concentrations observable among trans-Gulf
+migrants. This point seems almost too elementary to mention, but I
+have yet to find anyone who, in comparing the two situations, takes it
+into consideration.
+
+Judged in this light, the average hourly density of 2,800 birds at
+Progreso in April would appear to be indicative of many more migrants
+on the entire potential front than the 6,300 birds representing the
+average hourly density for the same month at Tampico.
+
+That the Progreso birds were actually beginning a trans-Gulf flight
+seems inevitable. The Yucatan Peninsula projects 200 miles or more
+northward into the vast open expanses of the Gulf of Mexico and the
+Caribbean Sea, with wide stretches of water on either side. The great
+majority of the birds were observed _after_ they had proceeded beyond
+the northern edge of this land mass. Had they later veered either to
+the east or the west, they would have been obliged to travel several
+hundred miles before again reaching land, almost as far as the
+distance straight across the Gulf. Had they turned southward, some
+individuals should have been detected flying in that direction. As can
+be seen from Figures 23, 42, and 44, not one bird observed was heading
+south of east or south of west on either night. No other single piece
+of evidence so conclusively demonstrates that birds cross the Gulf of
+Mexico in spring in considerable numbers as do flight density data
+recorded from Progreso in 1948.
+
+_Northern Gulf Area_
+
+Unfortunately only a few data on flight density are available from
+critical localities on the northern shores of the Gulf in spring. As
+the density curves in Figure 30 demonstrate, several sets of
+observation, including some phenomenal flights, have been recorded at
+Baton Rouge. This locality, however, lies sixty-four miles from the
+closest point on the Gulf coast, and the point due southward on the
+coast is eighty-four miles distant. Since all of the birds seen at
+Baton Rouge on any one night may have come from the heavily forested
+area between Baton Rouge and the coast of the Gulf, we cannot use data
+from Baton Rouge as certainly representative of incoming trans-Gulf
+flights. Data from repeated observations at stations on the coast
+itself are needed to judge the degree of trans-Gulf migration
+northward. On the few nights of observation at such localities
+(Cameron and Grand Isle, Louisiana, and Pensacola, Florida), flight
+densities have been zero or negligible. To be sure, negative results
+have been obtained at stations in the interior of the United States,
+and flights of low density have been recorded on occasion at stations
+where the flight densities are otherwise high. Nevertheless, in view
+of the volume of migration departing from Progreso, Yucatan, it would
+appear, upon first consideration, that we should at times record on
+the coast of Louisiana enough birds arriving in a night of continuous
+observation to yield a high density figure.
+
+Upon further consideration, however, there are factors mitigating
+against heavy densities of birds in northern flight on the northern
+coast of the Gulf. In the first place, presuming the main trans-Gulf
+flight to originate from northern Yucatan, and that there is a
+directional fanning to the northward, the birds leave on a 260-mile
+front, and arrive on a front 400 miles or more wide. Consequently,
+other factors remaining the same, there would be only approximately
+half the number of birds on the coast of arrival, at a given time and
+place, as there was on the coast of departure. Secondly, we may now
+presume on the basis of the telescopic studies at Progreso, that most
+migrants leaving northern Yucatan do so in the few hours centering
+about midnight. The varying speeds of the birds making the 580-mile
+flight across the Gulf distribute them still more sparsely on the
+north coast of the Gulf both in time and in space. Also we can see
+only that segment of the flight, which arrives in that part of a
+twenty-four hour period when the moon is up. This circumstance further
+reduces the interceptive potential because the hours after dark, to
+which the present telescopic studies have been restricted, comprise
+the period in which the fewest migrants arrive from over the water. To
+illustrate: it is a mathematical certainty that _none_ of the birds
+leaving Yucatan in the hours of heaviest flight, before 12 P. M.,
+and flying on a straight course at a speed of approximately 33 mph
+will reach the northern Gulf coast after nightfall; they arrive in the
+daytime. It will be useful to devise a technique for employing the sun
+as a background for telescopic observation of birds, thereby making
+observations possible on a twenty-four hour basis, so as to test these
+inferences by objective data.
+
+When a whole night's observation (1949 data not yet processed) at Port
+Aransas, on the southern coast of Texas, on the great overland route
+from eastern Mexico, yields in one night in April only seven birds,
+the recording of no birds at a station near the mouth of the
+Mississippi River becomes less significant.
+
+As I have previously remarked in this paper, the new data obtained
+since 1946, when I last wrote on the subject of migration in the
+region of Gulf of Mexico, requires that I alter materially some of my
+previously held views. As more and more facts come to light, I may be
+compelled to alter them still further. For one thing, I have come to
+doubt seriously the rigidity of the coastal hiatus as I envisioned it
+in 1945. I believe instead that the scarcity of records of transient
+migrants on the Gulf coastal plain in fair weather is to a very large
+extent the result of a wide dispersion of birds in the dense cover
+that characterizes this general region. I now question if appreciable
+bird densities on the ground ever materialize anywhere except when the
+sparseness of suitable habitat for resting or feeding tends to
+concentrate birds in one place, or when certain meteorological
+conditions erect a barrier in the path of an oncoming migratory
+flight, precipitating many birds in one place.
+
+This retrenchment of ideas is a direct consequence of the present
+study, for time and again, as discussed in the case of Tampico
+densities, maximal nightly flights have failed to produce a visible
+abundance of transients on land the following day. A simple example
+may serve to illustrate why. The highest one-hour density recorded in
+the course of this study is 21,200 birds. That means that this many
+birds crossed a line one mile long on the earth's surface and at right
+angles to the direction of flight. Let us further assume that the
+average flight speed of all birds comprising this flight was 30 mph.
+Had the entire flight descended simultaneously, it would have been
+dispersed over an area one mile wide and thirty miles long, and the
+precipitated density on the ground would have been only 1.1 birds per
+acre. Moreover, if as many as ten species had been involved in the
+flight, this would have meant an average per species of less than one
+bird per nine acres. This would have failed, of course, to show
+appreciable concentrations to the observer in the field the following
+day. If, however, on the other hand, the same flight of 21,200 birds
+had encountered at one point a weather barrier, such as a cold-front
+storm, all 21,200 birds might have been precipitated in one place and
+the field observer would have recorded an "inundation of migrants."
+This would be especially true if the locality were one with a high
+percentage of open fields or prairies and if the flight were mainly of
+woodland dwelling species, or conversely, if the locality were densely
+forested with few open situations and the flight consisted mainly of
+open-country birds. As explained on page 389, the density formula may
+be too conservative in its expression of actual bird densities. Even
+if the densities computed for birds in the air are only half as high
+as the actual densities in the air, the corresponding ground density
+of 2.2 birds per acre that results if all the birds descended
+simultaneously would hardly be any more impressive than the 1.1 bird
+per acre.
+
+This consideration is doubtless highly modified by local
+circumstances, but, in general, it seems to suggest a working
+hypothesis that provides an explanation for many of the facts that we
+now have. For example, on the coast of Texas there are great expanses
+of terrain unattractive to such birds as warblers, vireos, tanagers,
+and thrushes. The precipitation there by bad weather of even a
+mediocre nightly flight composed of birds of the kinds mentioned would
+surely produce an overwhelming concentration of birds in the scattered
+woods and shrubs.
+
+In spite of all that has been written about the great concentrations
+of transient migrants on the coast of Texas in spring, I am not convinced
+that they are of a different order of magnitude than those concentrations
+that sometimes occur along the cheniers and coastal islands
+of Louisiana and Mississippi. I have read over and over the
+highly informative accounts of Professor Williams (_loci cit._) and the
+seasonal summaries by Davis (1936-1940) and Williams (1941-1945).
+I have conversed at length with Mrs. Jack Hagar, whom I
+regard as one of the leading authorities on the bird life of the
+Texas coast, and she has even permitted me access to her voluminous
+records covering a period of fifteen years residence at Rockport.
+Finally, I have spent a limited amount of time myself on the Texas
+coast studying first-hand the situation that obtains there in order
+that I might be in a position to compare it with what I have learned
+from observations elsewhere in the region of the Gulf of Mexico,
+Louisiana, Florida, Yucatan, and eastern Mexico.
+
+Although the concentrations of birds on some days near the mouth of
+the Mississippi River are almost incalculable, the fact remains that
+in Texas the densities of transient species on the ground are more
+consistently high from day to day. The reason for this may be simple.
+As birds move up daily from Mexico overland, a certain percentage
+would be destined to come down at all points along the route but so
+dispersed in the inland forest that they might pass unnoticed.
+However, that part of the same flight settling down in coastal areas,
+where trees are scarce, would produce visible concentrations of
+woodland species. With the advent of a cold-front storm, two
+diametrically opposite effects of the same meteorological phenomenon
+would tend to pile up great concentrations of migrants of two
+classes--the overland and the trans-Gulf flights. During the
+prepolar-front weather the strong southerly (from the south) and
+southeasterly winds would tend to displace much of the trans-Gulf
+segment to the western part of the Gulf. With the shift of the winds
+to the north and northwest, which always occurs as the front passes,
+the overland flight still in the air would tend to be banked up
+against the coast, and the incoming trans-Gulf flight would be
+confronted with a barrier, resulting in the precipitation of birds on
+the first available land.
+
+These postulated conditions are duplicated in part in autumn along the
+Atlantic coast of the eastern United States. There, as a result of the
+excellent work of Allen and Peterson (1936) and Stone (1937), a
+similar effect has been demonstrated when northwest winds shove the
+south-bound flights up against the coast of New Jersey and concentrate
+large aggregations of migrants there.
+
+_Interior of the United States_
+
+Attention has been drawn already to the nature of the nightly flights
+at stations immediately inland from the Gulf coast, where densities
+decline abruptly well before midnight. I have suggested that this
+early drop-off is mainly a result of the small amount of terrain south
+of these stations from which birds may be contributed to a night's
+flight. At Oak Grove, Louisiana, the flight exhibited a strong
+directional trend with no significant aberrant components. Therefore,
+one may infer that a considerable part of the flight was derived from
+regions to the south of the station.
+
+At Mansfield, Louisiana, thirty-eight hours of observation in April
+and May resulted in flight densities that are surprisingly low--much
+lower, in fact, than at Oak Grove. In eleven of the hours of
+observation no birds at all were seen. A possible explanation for
+these low densities lies in the fact that eastern Texas and western
+Louisiana, where, probably, the Mansfield flights originated, is not
+an especially attractive region to migrants because of the great
+amount of deforested and second growth pine land. Oak Grove, in
+contrast, is in the great Tensas-Mississippi River flood plain,
+characterized by an almost solid stand of deciduous forest extending
+over thousands of square miles in the lower Mississippi valley.
+
+    [Illustration: FIG. 37. Sector density representation on
+       two nights at Rosedale, Mississippi, in 1948. The white lines
+       are the vector resultants.]
+
+In further contrast to the considerable flight densities and
+pronounced directional trend at Oak Grove, we have the results from
+Rosedale, Mississippi, only seventy miles to the north and slightly to
+the east. At Rosedale the densities were mediocre and the flight
+directions were extremely divergent. Many of the nights of observation
+at this locality were seriously interrupted by clouds, but such counts
+as were made on those dates indicated little migration taking place.
+On two nights, however, April 21-22 and May 20-21, visibility was
+almost continuous and densities were moderately high. In Figure 37 I
+have shown the flight directions for these two nights. The lengths of
+the individual sector vectors are plotted as a percentage of the total
+station density for each of the two nights (5,800 and 6,800 birds,
+respectively). Although the vector resultants show a net movement of
+birds to the northeast, there are important divergent components of
+the flights. This "round-the-compass" pattern is characteristic of
+stations on the edge of meteorological disturbances, as was Rosedale
+on April 21-22, but not on the night of May 20-21. If bats are
+presumed to have played a role in these latter observations, their
+random flights would tend to cancel out and the vector resultant
+would emerge as a graphic representation of the actual net trend
+density of the birds and its prevailing direction of flow. Although I
+do not believe that bats are the real reason for the diverse
+directional patterns at Rosedale, I can offer no alternative
+explanation consistent with data from other stations.
+
+Moving northward in the valley of the Mississippi and its tributaries,
+we find a number of stations that yielded significantly high densities
+on most nights when weather conditions were favorable for migration.
+Louisville and Murray, Kentucky, and Knoxville, Tennessee, each show
+several nights with many birds flying, but only Lawrence, Kansas, and
+Ottumwa, Iowa, had migrations that approach in magnitude the record
+station densities at Tampico. Indeed, these were the only two stations
+in the United States that produced flights exceeding the densities at
+Progreso, Yucatan. The densities at Lawrence are unique in one
+respect, in that they were extremely high in the month of March. Since
+there were very few stations in operation then, these high densities
+would be of little significance were it not for the fact that at no
+time in the course of this study from 1945 to the present have
+comparable densities been obtained this early in the migration period.
+Examination of the "Remarks" section of the original data sheets from
+Lawrence show frequent mention of "duck-like" birds passing before the
+moon. We may infer from these notations that a considerable part of
+the overhead flight was composed of ducks and other aquatic birds that
+normally leave the southern United States before the main body of
+transient species reach there. The heavy flight densities at Lawrence
+may likewise have contained certain Fringillidae, Motacillidae,
+Sylviidae, and other passerine birds that winter mainly in the
+southern United States and which are known to begin their return
+northward in March or even earlier. Observations in 1948 at Lawrence
+in April were hindered by clouds, and in May no studies were
+attempted. However, we do have at hand two excellent sets of data
+recorded at Lawrence on the nights of May 3-4 and May 5-6, 1947, when
+the density was also extremely high.
+
+At Ottumwa, Iowa, where a splendid cooperative effort on the part of
+the local ornithologists resulted in forty-four hours of observation
+in April and May, densities were near the maximum for all stations.
+Considering this fact along with results at Lawrence and other
+mid-western stations where cloud cover did not interfere at the
+critical periods of observation, we have here evidence supporting the
+generally held thesis that eastern Kansas, Missouri, and Iowa lie on a
+principal migratory flyway. Stations in Minnesota, Illinois,
+Michigan, Massachusetts, and Ontario were either operated for only
+parts of one or two nights, or else clouds seriously interfered with
+observations, resulting in discontinuous counts. It may be hoped that
+future studies will include an adequate representation of stations in
+these states and that observations will be extensive enough to permit
+conclusions regarding the density and direction of migration.
+
+Charleston, South Carolina, which does not conveniently fall in any of
+the geographic regions so far discussed, had, to me, a surprisingly
+low flight density; twenty-two hours of observation there in March,
+April, and May yielded a total flight density of only 3,000 birds.
+This is less, for example, than the number of birds computed to have
+passed Lawrence, Kansas, in one hour, or to have passed Progreso,
+Yucatan, in one twenty-minute interval! Possibly observations at
+Charleston merely chanced to fall on nights of inexplicably low
+densities; further observations will be required to clear up this
+uncertainty.
+
+
+E. MIGRATION AND METEOROLOGICAL CONDITIONS
+
+The belief that winds affect the migration of birds is an old one. The
+extent to which winds do so, and the precise manner in which they
+operate, have not until rather recently been the subject of real
+investigation. With modern advances in aerodynamics and the
+development of the pressure-pattern system of flying in aviation,
+attention of ornithologists has been directed anew to the part that
+air currents may play in the normal migrations of birds. In America, a
+brief article by Bagg (1948), correlating the observed abundance of
+migrants in New England with the pressure pattern obtaining at the
+time, has been supplemented by the unpublished work of Winnifred
+Smith. Also Landsberg (1948) has pointed out the close correspondence
+between the routes of certain long-distance migrants and prevailing
+wind trajectories. All of this is basis for the hypothesis that most
+birds travel along definite air currents, riding with the wind. Since
+the flow of the air moves clockwise around a high pressure area and
+counterclockwise around a low pressure area, the birds are directed
+away from the "high" and toward the center of the "low." The arrival
+of birds in a particular area can be predicted from a study of the
+surrounding meteorological conditions, and the evidence in support of
+the hypothesis rests mainly upon the success of these predictions in
+terms of observations in the field.
+
+From some points of view, this hypothesis is an attractive one. It
+explains how long distances involved in many migrations may be
+accomplished with a minimum of effort. But the ways in which winds
+affect migration need analysis on a broader scale than can be made
+from purely local vantage points. Studies of the problem must be
+implemented by data accumulated from a study of the process in action,
+not merely from evidence inferred from the visible results that follow
+it. Although several hundred stations operating simultaneously would
+surely yield more definite results, the telescopic observations in
+1948 offer a splendid opportunity to test the theory on a continental
+scale.
+
+The approach employed has been to plot on maps sector vectors and
+vector resultants that express the directional trends of migration in
+the eastern United States and the Gulf region, and to compare the data
+on these maps with data supplied by the U. S. Weather Bureau regarding
+the directions and velocities of the winds, the location of high and
+low pressure areas, the movement of cold and warm fronts, and the
+disposition of isobars or lines of equal pressure. It should be borne
+in mind when interpreting these vectors that they are intended to
+represent the directions of flight only at the proximal ends, or
+junction points, of the arrows. The tendency of the eye to follow a
+vector to its distal extremity should not be allowed to create the
+misapprehension that the actual flight is supposed to have continued
+on in a straight line to the map location occupied by the arrowhead.
+
+A fundamental difficulty in the pressure-pattern theory of migration
+has no doubt already suggested itself to the reader. The difficulty to
+which I refer is made clear by asking two questions. How can the birds
+ever get where they are going if they are dependent upon the whim of
+the winds? How can pressure-pattern flying be reconciled with the
+precision birds are supposed to show in returning year after year to
+the same nesting area? The answer is, in part, that, if the wind is a
+major controlling influence on the routes birds follow, there must be
+a rather stable pattern of air currents prevailing from year to year.
+Such a situation does in fact exist. There are maps showing wind roses
+at 750 and 1,500 meters above mean sea level during April and May
+(Stevens, 1933, figs. 13-14, 17-18). Similarly, the "Airway
+Meteorological Atlas for the United States" (Anonymous, 1941) gives
+surface wind roses for April (Chart 6) and upper wind roses at 500 and
+1,000 meters above mean sea level for the combined months of March,
+April, and May (Charts 81 and 82). The same publication shows wind
+resultants at 500 and 1,000 meters above mean sea level (Charts 108
+and 109). Further information permitting a description in general
+terms of conditions prevailing in April and May is found in the
+"Monthly Weather Review" covering these months (_cf._ Anonymous,
+1948 _a_, Charts 6 and 8; 1948 _b_, Charts 6 and 8).
+
+    [Illustration: FIG. 38. Over-all sector vectors at major
+       stations in the spring 1948. See text for explanation of
+       system used in determining the length of vectors. For
+       identification of stations, see Figure 34.]
+
+    [Illustration: FIG. 39. Over-all net trend of flight
+       directions at stations shown in Figure 38. The arrows
+       indicate direction only and their slants were obtained by
+       vector analysis of the over-all sector densities.]
+
+First, however, it is helpful as a starting point to consider the
+over-all picture created by the flight trends computed from this
+study. In Figure 38, the individual sector vectors are mapped for the
+season for all stations with sufficient data. The length of each
+sector vector is determined as follows: the over-all seasonal density
+for the station is regarded as 100 percent, and the total for the
+season of the densities in each individual sector is then expressed as
+a percentage. The results show the directional spread at each station.
+In Figure 39, the direction of the over-all vector resultant, obtained
+from the sector vectors on the preceding map, is plotted to show the
+net trend at each station.
+
+As is evident from the latter figure, the direction of the net trend
+at Progreso, Yucatan, is decidedly west of north (N 26 deg. W). At Tampico
+this trend is west of north (N 11 deg. W), but not nearly so much so as at
+Progreso. In Texas, Louisiana, Georgia, Tennessee, and Kentucky, it is
+decidedly east of north. In the upper Mississippi Valley and in the
+eastern part of the Great Plains, the flow appears to be northward or
+slightly west of north. At Winter Park, Florida, migration follows in
+general the slant of the Florida Peninsula, but, the meager data from
+Thomasville, Georgia, do not indicate a continuation of this trend.
+
+It might appear, on the basis of the foregoing data, that birds
+migrate along or parallel to the southeast-northwest extension of the
+land masses of Central America and southern Mexico. This would carry
+many of them west of the meridian of their ultimate goal, obliging
+them to turn back eastward along the lines of net trend in the Gulf
+states and beyond. This curved trajectory is undoubtedly one of the
+factors--but certainly not the only factor--contributing to the effect
+known as the "coastal hiatus." The question arises as to whether this
+northwestward trend in the southern part of the hemisphere is a
+consequence of birds following the land masses or whether instead it
+is the result of some other natural cause such as a response to
+prevailing winds. I am inclined to the opinion that both factors are
+important. Facts pertinent to this opinion are given below.
+
+In April and May a high pressure area prevails over the region of the
+Gulf of Mexico. As the season progresses, fewer and fewer cold-front
+storms reach the Gulf area, and as a result the high pressure area
+over the Gulf is more stable. Since the winds move clockwise around a
+"high," this gives a general northwesterly trajectory to the air
+currents in the vicinity of the Yucatan Peninsula. In the western area
+of the Gulf, the movement of the air mass is in general only slightly
+west of north, but in the central Gulf states and lower Mississippi
+Valley the trend is on the average northeasterly. In the eastern part
+of the Great Plains, however, the average circulation veers again
+slightly west of north. The over-all vector resultants of bird
+migration at stations in 1948, as mapped in Figure 39, correspond
+closely to this general pattern.
+
+Meteorological data are available for drawing a visual comparison
+between the weather pattern and the fight pattern on individual
+nights. I have plotted the directional results of four nights of
+observation on the Daily Weather Maps for those dates, showing surface
+conditions (Figures 40, 42, 44 and 46). Each sector vector is drawn in
+proportion to its percentage of the corresponding nightly station
+density; hence the vectors at each station are on an independent
+scale. The vector resultants, distinguished by the large arrowheads,
+are all assigned the same length, but the nightly and average hourly
+station densities are tabulated in the legends under each figure. For
+each map showing the directions of flight, there is on the facing page
+another map showing the directions of winds aloft at 2,000 and 4,000
+feet above mean sea level on the same date (see Figures 41-47). The
+maps of the wind direction show also the velocities.
+
+Unfortunately, since there is no way of analyzing the sector trends in
+terms of the elevations of the birds involved, we have no certain way
+of deciding whether to compare a given trend with the winds at 2,000,
+1,000, or 0 feet. Nor do we know exactly what wind corresponds to the
+average or median flight level, which would otherwise be a good
+altitude at which to study the net trend or vector resultant.
+Furthermore, the Daily Weather Map illustrates conditions that
+obtained at 12:30 A. M. (CST); the winds aloft are based on
+observations made at 10:00 P. M. (CST); and the data on birds covers
+in most cases the better part of the whole night. Add to all this the
+fact that the flight vectors, their resultants, and the wind
+representations themselves are all approximations, and it becomes
+apparent that only the roughest sort of correlations are to be
+expected.
+
+However, as will be seen from a study of the accompanying maps
+(Figures 40-47), the shifts in wind direction from the surface up to
+4,000 feet above sea level are not pronounced in most of the
+instances at issue, and such variations as do occur are usually in a
+clockwise direction. All in all, except for regions where frontal
+activity is occurring, the weather maps give a workable approximation
+to the average meteorological conditions on a given night.
+
+The maps (Figures 40-47) permit, first, study of the number of
+instances in which the main trend of flight, as shown by the vector
+resultant, parallels the direction of wind at a reasonable potential
+mean flight elevation, and, second, comparison of the larger
+individual sector vectors and the wind currents at any elevation below
+the tenable flight ceiling--one mile.
+
+On the whole, inspection of the trend of bird-flight and wind
+direction on specific nights supports the principle that the flow of
+migration is in general coincident with the flow of air. It might be
+argued that when the flow of air is toward the north, and when birds
+in spring are proceeding normally in that direction, no significance
+can be attached to the agreement of the two trends. However, the same
+coincidence of wind directions and bird flights seems to be maintained
+when the wind currents deviate markedly from a northward trajectory.
+Figures 46 and 47, particularly in regard to the unusual slants of the
+flight vectors at Ottumwa, Knoxville, and Memphis, illustrate that
+this coincidence holds even when the wind is proceeding obliquely
+eastward or westward. On the night of May 22-23, when a high pressure
+area prevailed from southern Iowa to the Atlantic coast, and the
+trajectory of the winds was northward, migration activity at Knoxville
+and Ottumwa was greatly increased and the flow of birds was again
+northward in the normal seasonal direction of migration.
+
+Further study of the data shows fairly conclusively that maximum
+migration activity occurs in the regions of high barometric pressure
+and that the volume of migration is either low or negligible in
+regions of low pressure. The passage of a cold-front storm may almost
+halt migration in spring. This was demonstrated first to me by the
+telescopic method at Baton Rouge, on April 12, 1946, following a
+strong cold front that pushed southeastward across the Gulf coastal
+plain and over the eastern Gulf of Mexico. The winds, as usual,
+shifted and became strong northerly. On this night, following the
+shift of the wind, only three birds were seen in seven hours of
+continuous observation. Three nights later, however, on April 15, when
+the warm air of the Gulf was again flowing from the south, I saw 104
+birds through the telescope in two hours. Apropos of this
+consideration in the 1948 data are the nights of May 21-22 and 22-23.
+
+    [Illustration: FIG. 40. Comparison of flight trends and
+       surface weather conditions on April 22-23, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on April 23. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          5. Louisville: 9,100 (1,100)
+          6. Murray: 16,300 (2,700)
+          8. Stillwater: 1,900 (500)
+          9. Knoxville: 15,200 (1,700)
+         13. Oak Grove: 13,600 (1,700)
+         16. College Station: 13,300 (1,900)
+         17. Baton Rouge: 6,200 (1,000)
+         19. Lafayette: 2,800 (600)
+         21. Winter Park: 6,200 (700)
+         23. Tampico: 11,100 (3,700)]
+
+    [Illustration: FIG. 41. Winds aloft at 10:00 P. M. on
+       April 22 (CST). Winds at 2,000 feet above mean sea level are
+       shown in black; those at 4,000 feet, in white. Velocities are
+       indicated by standard Beaufort Scale of Wind Force. The
+       numbers in circles refer to the stations shown in Figure 40.]
+
+    Correction: Figures 41 and 45 were inadvertently transposed.
+
+    [Illustration: FIG. 42. Comparison of flight trends and
+       surface weather conditions on April 23-24, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on April 24. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          1. Albion: 1,100 (300)
+          2. Ottumwa: 5,500 (900)
+          4. Lawrence: 5,400 (1,400)
+          5. Louisville: 13,300 (2,700)
+          6. Murray: 9,800 (1,400)
+          8. Stillwater: 800 (100)
+          9. Knoxville: 8,000 (900)
+         10. Memphis: 7,900 (1,000)
+         14. Mansfield: 4,900 (1,200)
+         16. College Station: 700 (100)
+         17. Baton Rouge: 1,700 (400)
+         18. Pensacola: migration negligible
+         20. New Orleans: 1,600 (800)
+         21. Winter Park: 2,700 (300)
+         23. Tampico: 63,600 (6,300)
+         24. Progreso: 31,300 (3,900)]
+
+    [Illustration: FIG. 43. Winds aloft at 10:00 P. M. on
+       April 23 (CST). Winds at 2,000 feet above mean sea level are
+       shown in black; those at 4,000 feet, in white. Velocities are
+       indicated by standard Beaufort Scale of Wind Force. The
+       numbers in circles refer to the stations shown in Figure 42.]
+
+    [Illustration: FIG. 44. Comparison of flight trends and
+       surface weather conditions on April 24-25, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on April 25. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          1. Albion: migration negligible
+          2. Ottumwa: 4,600 (1,500)
+          3. Columbia: 1,400 (400)
+          5. Louisville: 1,700 (200)
+         10. Memphis: 6,600 (900)
+         12. Rosedale: 1,100 (100)
+         14. Mansfield: 1,700 (400)
+         18. Pensacola: migration negligible
+         21. Winter Park: 600 (100)
+         24. Progreso: 27,300 (3,000)]
+
+    [Illustration: FIG. 45. Winds aloft at 10:00 P. M. on
+       April 24 (CST). Winds at 2,000 feet above mean sea level are
+       shown in black; those at 4,000 feet, in white. Velocities are
+       indicated by standard Beaufort Scale of Wind Force. The
+       numbers in circles refer to the stations shown in Figure 44.]
+
+    Correction: Figures 41 and 45 were inadvertently transposed.
+
+    [Illustration: FIG. 46. Comparison of flight trends and
+       surface weather conditions on May 21-22, 1948. The
+       meteorological data were taken from the U. S. Weather Bureau
+       Daily Weather Map for 12:30 A. M. (CST) on May 22. The
+       nightly station densities and the average hourly station
+       density (shown in parentheses) are as follows:
+
+          2. Ottumwa: 6,900 (1,400)
+          5. Louisville: 1,500 (200)
+          9. Knoxville: 3,200 (500)
+         10. Memphis: 7,000 (1,200)
+         13. Oak Grove: 5,800 (800)
+         14. Mansfield: 2,500 (800)
+         18. Pensacola: migration negligible
+         21. Winter Park: 1,200 (200)]
+
+    [Illustration: FIG. 47. Winds aloft at 10:00 P. M. on May
+       21 (CST). Winds at 2,000 feet above mean sea level are shown.
+       Velocities are indicated by standard Beaufort Scale of Wind
+       Force. The numbers in circles refer to the stations shown in
+       Figure 46.]
+
+On the first night, following the passage of a cold front, migration
+at Ottumwa was comparatively low (6,900 birds in five hours). On the
+following night, when the trajectory of the winds was toward the
+north, the volume of migration was roughly twice as high (22,300 birds
+in eight hours). At Louisville, on May 21-22, the nightly station
+density was only 1,500 birds in seven hours, whereas on the following
+night, it was 8,400 birds in the same length of time, or about six
+times greater.
+
+The evidence adduced from the present study gives support to the
+hypothesis that the continental pattern of spring migration in eastern
+North America is regulated by the movement of air masses. The
+clockwise circulation of warm air around an area of high pressure
+provides, on its western edge, tail winds which are apparently
+favorable to northward migration. High pressure areas exhibit a
+centrifugal force outward from the center, which may tend to disperse
+the migratory flight originating at any given point. In contrast, the
+circulation of air in the vicinity of a low pressure area is
+counterclockwise with the force tending to be directed inward toward
+the center. Since the general movement of the air is from the high
+pressure area toward a low pressure area, birds starting their
+migrations with favorable tail winds, are often ultimately carried to
+a region where conditions are decidedly less favorable. In the
+vicinity of an area of low pressure the greater turbulence and high
+wind velocities, combined with the possibly slightly less buoyant
+property of the air, cause birds to descend. Since low pressure areas
+in spring generally precede cold fronts, with an attending shift of
+the wind to the north, an additional barrier to the northward
+migration of birds is imposed. The extreme manifestation of low
+pressure conditions and the manner in which they operate against bird
+flight, are associated with tropical hurricanes. There, the
+centripetal force of the wind is so great that it appears to draw
+birds into the "eye" of the hurricane. A classic example of this
+effect is seen in the case of the birds that came aboard the "West
+Quechee" when this vessel passed through the "eye" of a hurricane in
+the Gulf of Mexico in August, 1927. I have already discussed the
+details of this incident in a previous paper (1946:192). There is also
+the interesting observation of Mayhew (1949), in which a similar
+observation was made of large numbers of birds aboard a ship passing
+through one of these intense low-pressure areas.
+
+Although the forces associated with an ordinary low-pressure area are
+by no means as intense as those associated with a tropical hurricane,
+the forces operating are much the same. Consequently birds conceivably
+might tend to be drawn toward a focal point near the center of the
+low, where the other factors already mentioned would tend to
+precipitate the entire overhead flight. Visible evidence of migration
+would then manifest itself to the field ornithologists.
+
+
+
+
+CONCLUSIONS
+
+
+  1. Telescopic counts of birds passing before the moon may be used
+     to determine reliable statistical expressions of the volume of
+     migration in terms of direction and of definite units of time
+     and space.
+
+  2. Night migrants fly singly more often than in flocks, creating a
+     remarkably uniform dispersion on a local scale throughout the
+     sky, quite unlike the scattered distributions observable in the
+     daytime.
+
+  3. The nocturnal migration of birds is apparently preceded by a
+     resting or feeding pause during which there are few migrants in
+     the air. It is not to an important degree a non-stop continuation
+     of flights begun in the daylight.
+
+  4. Nightly migrational activity in North America varies from hour to
+     hour according to a definite temporal pattern, corresponding to
+     the _Zugunruhe_ of caged European birds, and expressed by
+     increasingly heavy flights up until the hour before midnight,
+     followed by a pronounced decline.
+
+  5. The visible effects of the time pattern are subject to
+     modification at a particular station by its location with respect
+     to the resting areas from which the night's flight originates.
+
+  6. Quantitative and directional studies have so far failed to prove
+     that nocturnal migrants favor narrow, topographically-determined
+     flight lanes to an important degree.
+
+  7. Flight densities on the east coast of Mexico, though of first
+     magnitude, have not yet been demonstrated in the volume demanded
+     by the premise that almost all migrants returning to the
+     United States from regions to the south do so by coastal routes.
+
+  8. Heavy flights have been recorded from the northern coast of
+     Yucatan under circumstances leading inevitably to the conclusion
+     that birds migrate across the Gulf of Mexico in considerable
+     numbers.
+
+  9. There is reason to believe that the importance of the Florida
+     Peninsula as an April and May flyway has been over-estimated,
+     as regards the numbers of birds using it in comparison with the
+     numbers of birds using the Mexican and Gulf routes.
+
+ 10. The amount of migration is apparently seldom sufficient to produce
+     heavy densities of transient species on the ground without
+     the operation of concentrative factors such as ecological patterns
+     and meteorological forces.
+
+ 11. The absence or scarcity of transients in some areas in fine
+     weather may be explained by this consideration.
+
+ 12. A striking correlation exists between air currents and the
+     directional flight trends of birds, suggesting that most night
+     migrants travel by a system of pressure-pattern flying.
+
+
+
+LITERATURE CITED
+
+
+  ALLEN, R. P., AND R. T. PETERSON
+
+    1936.    The hawk migrations at Cape May Point, New Jersey. Auk,
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+
+
+  ANONYMOUS
+    1936-1941. Tables of computed altitude and azimuth. U. S. Navy
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+    1941.    Airway meteorological atlas for the United States.
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+    1945-1948. The American air almanac. U. S. Naval Observatory.
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+    1948_b_. Meteorological and climatological data for May 1948.
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+
+
+  BAGG, A. M.
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+    1948.    Barometric pressure-patterns and spring migration.
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+
+
+  BERGMAN, G.
+
+    1941.    Der Fruhlingszug von _Clangula hyemalis_ (L.) und
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+  BRAY, R. A.
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+  CARPENTER, F. W.
+
+    1906.    An astronomical determination of the height of birds
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+
+  CHAPMAN, F. M.
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+  DAVIS, L. I.
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+    1936-1940. The season: lower Rio Grande Valley region. Bird-Lore
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+  F. [ARNER], D. [ONALD] S.
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+    1947.    Studies on daily rhythm of caged migrant birds (review of
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+
+
+  GATES, W. H.
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+
+  LANSBERG, H.
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+
+  LIBBY, O. G.
+
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+
+  LOWERY, G. H., JR.
+
+    1945.    Trans-Gulf spring migration of birds and the coastal
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+
+  MAYHEW, D. F.
+
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+
+  OVERING, R.
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+
+  PALMGREN, P.
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+  POUGH, R. H.
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+    1948.    Out of the night sky. Audubon Mag., 50:354-355.
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+
+  PUTKONEN, T. A.
+
+    1942.    Kevaetmuutosta Viipurinlakdella. Ornis Fennica, 19:33-44.
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+
+  RENSE, W. A.
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+    1946.    Astronomy and ornithology. Popular Astronomy, 54:55-73.
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+
+  SCOTT, W. E. D.
+
+    1881_a._ Some observations on the migration of birds. Bull. Nuttall
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+
+  SIIVONEN, L.
+
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+
+    1949.    Mortality of birds at the ceilometer of the Nashville
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+
+  STEBBINS, J.
+
+    1906.    A method of determining height of migrating birds.
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+
+  STEVENS, LLOYD A.
+
+    1933.    Upper-air wind roses and resultant winds for the eastern
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+
+
+  STONE, W.
+
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+    1937.    Bird studies at Old Cape May. Delaware Valley Orni. Club,
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+
+  THOMSON, A. L.
+
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+
+  VAN OORDT, G.
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+    1943.    Vogeltrek. E. J. Brill, Leiden, xii + 145 pp.
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+
+  VERY, F. W.
+
+    1897.    Observations of the passage of migrating birds across the
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+             Science, 6:409-411.
+
+
+  WALTERS, W.
+
+    1927.    Migration and the telescope. Emu, 26:220-222.
+
+
+  WEST, R. H.
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+    1896.    Flight of birds across the moon's disc. Nature (London),
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+  WILLIAMS, G. G.
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+    1941-1948. The season: Texas coastal region. Audubon Mag., 43-50.
+
+    1945.    Do birds cross the Gulf of Mexico in spring? Auk,
+             62:98-111.
+
+    1947.    Lowery on trans-Gulf migration. Auk, 64:217-238.
+
+
+  WINKENWERDER, H. A.
+
+    1902_a_. The migration of birds with special reference to nocturnal
+             flight. Bull. Wisconsin Nat. Hist. Soc., 2:177-263.
+
+    1902_b_. Some recent observations on the migration of birds. Bull.
+             Wisconsin Nat. Hist. Soc., 2:97-107.
+
+
+  Transmitted June 1, 1949.
+
+
+
+    []
+  23-1020
+
+
+
+
+UNIVERSITY OF KANSAS PUBLICATIONS
+
+
+The University of Kansas Publications, Museum of Natural History, are
+offered in exchange for the publications of learned societies and
+institutions, universities and libraries. For exchanges and
+information, address the Exchange Desk, University of Kansas Library,
+Lawrence, Kansas, U. S. A.
+
+MUSEUM OF NATURAL HISTORY.--E. Raymond Hall, Chairman, Editorial
+Committee.
+
+This series contains contributions from the Museum of Natural History.
+Cited as Univ. Kans. Publ., Mus. Nat. Hist.
+
+  Vol. 1.  (Complete) Nos. 1-26.  Pp. 1-638. August 15, 1946-January 20,
+           1951.
+
+  Vol. 2.  (Complete)  Mammals of Washington. By Walter W. Dalquest.
+           Pp. 1-444, 140 figures in text. April 9, 1948.
+
+  Vol. 3.  1.  The avifauna of Micronesia, its origin, evolution, and
+               distribution. By Rollin H. Baker. Pp. 1-359, 16 figures
+               in text. June 12, 1951.
+
+           2.  A quantitative study of the nocturnal migration of birds.
+               By George H. Lowery, Jr. Pp. 361-472, 47 figures in text.
+               June 29, 1951.
+
+
+
+
+
+  Transcriber's Notes
+
+  With the exception of the typographical corrections detailed below
+  and some minor corrections for missing periods or extra punctuation
+  (item 28 in List of Figures), the text presented here is that
+  contained in the original printed version. A transcription of the
+  Data presented in Figure 12 was added to illustrate the information
+  contained on that sheet. Some text was moved to rejoin paragraphs.
+  The list of UK publications was moved to the end of the document.
+
+  In writing variables for formulae, superscripted characters are
+  shown using a caret (^). So, X squared would be X^2. Subscripts are
+  shown using an underscore. Carbon dioxide is CO_2. Where several
+  superscript or subscript character(s) are required or to aid in
+  clarity, they are placed in braces (ex., H_{2}O for water and
+  [theta]_{Npt.} for theta degrees from the North point).
+
+  Emphasis Notation
+
+      _Text_  = Italics
+
+  Typographical Corrections
+
+  Page   Correction
+
+  385   flght => flight
+  394   diargrams => diagrams
+  404   Determinaton => Determination
+  411   obsever => observer
+  419   Morover => Moreover
+  425   Mississippii => Mississippi
+  425   a => as
+  430   at => and
+  431   inserted "a"
+         ("...traveling along a certain topographic feature...")
+  442   concensus => consensus
+  472   Stephens, Loyd A. => Stevens, Lloyd A.
+
+
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of A Quantitative Study of the Nocturnal
+Migration of Birds., by George H. Lowery.
+
+*** END OF THIS PROJECT GUTENBERG EBOOK A QUANTITATIVE STUDY OF THE ***
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