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diff --git a/37894-8.txt b/37894-8.txt new file mode 100644 index 0000000..47ac191 --- /dev/null +++ b/37894-8.txt @@ -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: 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. 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