Horizontal Distribution of Birds on Narrow Fronts 409
Density as a Function of the Hour of the Night 413
Migration in Relation to Topography 424
Geographical Factors and the Continental Density Pattern 432
Migration and Meteorological Conditions 453
CONCLUSIONS 469
LITERATURE CITED 470
LIST OF FIGURES
Figure Page
1. The field of observation as it appears to the observer 374
2. Determination of diameter of cone at any point 375
3. Temporal change in size of the field of observation 376
4. Migration at Ottumwa, Iowa 377
5. Geographic variation in size of cone of observation 378
6. The problem of sampling migrating birds 380
7. The sampling effect of a square 381
8. Rectangular samples of square areas 382
9. The effect of vertical components in bird flight 383
10. The interceptory potential of slanting lines 384
11. Theoretical possibilities of vertical distribution 388
12. Facsimile of form used to record data in the field 391
13. The identification of co-ordinates 392
14. The apparent pathways of birds seen in one hour 393
15. Standard form for plotting the apparent paths of flight 395
16. Standard sectors for designating flight trends 398
17. The meaning of symbols used in the direction formula 399
18. Form used to compute zenith distance and azimuth of the moon 400
19. Plotting sector boundaries on diagrammatic plots 402
20. Form to compute sector densities 403
21. Determination of the angle [alpha] 404
22. Facsimile of form summarizing sector densities 405
23. Determination of net trend density 406
24. Nightly station density curve at Progreso, Yucatan 407
25. Positions of the cone of observation at Tampico, Tamps 411
26. Average hourly station densities in spring of 1948 414
27. Hourly station densities plotted as a percentage of peak 415
28. Incidence of maximum peak at the various hours of the
night in 1948 416
29. Various types of density-time curves 418
30. Density-time curves on various nights at Baton Rouge 422
31. Directional components in the flight at Tampico, Tamps 428
32. Hourly station density curve at Tampico, Tamps 429
33. The nightly net trend of migrations at three stations in 1948 431
34. Stations at which telescopic observations were made in 1948 437
35. Positions of the cone of observation at Progreso, Yucatan 443
36. Hourly station density curve at Progreso, Yucatan 444
37. Sector density representation on two nights at
Rosedale, Miss. 451
38. Over-all sector vectors at major stations in spring of 1948 455
39. Over-all net trend of flight directions shown in Figure 38 456
40. Comparison of flight trends and surface weather conditions
on April 22-23, 1948 460
41. Winds aloft at 10:00 P. M. on April 22 (CST) 461
42. Comparison of flight trends and surface weather conditions
on April 23-24, 1948 462
43. Winds aloft at 10:00 P. M. on April 23 (CST) 463
44. Comparison of flight trends and surface weather conditions
on April 24-25, 1948 464
45. Winds aloft at 10:00 P. M. on April 24 (CST) 465
46. Comparison of flight trends and surface weather conditions
on May 21-22, 1948 466
47. Winds aloft at 10:00 P. M. on May 21 (CST) 467
INTRODUCTION
The nocturnal migration of birds is a phenomenon that long has
intrigued zoologists the world over. Yet, despite this universal
interest, most of the fundamental aspects of the problem remain
shrouded in uncertainty and conjecture.
Bird migration for the most part, whether it be by day or by night, is
an unseen movement. That night migrations occur at all is a conclusion
derived from evidence that is more often circumstantial than it is
direct. During one day in the field we may discover hundreds of
transients, whereas, on the succeeding day, in the same situation, we
may find few or none of the same species present. On cloudy nights we
hear the call notes of birds, presumably passing overhead in the
seasonal direction of migration. And on stormy nights birds strike
lighthouses, towers, and other tall obstructions. Facts such as these
are indisputable evidences that migration is taking place, but they
provide little basis for evaluating the flights in terms of magnitude
or direction.
Many of the resulting uncertainties surrounding the nocturnal
migration of birds have a quantitative aspect; their resolution hinges
on how many birds do one thing and how many do another. If we knew,
for instance, how many birds are usually flying between 2 and 3 A. M.
and how this number compares with other one-hour intervals in the
night, we would be in a position to judge to what extent night flight
is sustained from dusk to dawn. If we could measure the number of
birds passing selected points of observation, we could find out
whether such migration in general proceeds more or less uniformly on a
broad front or whether it follows certain favored channels or flyways.
This in turn might give us a clearer insight into the nature of the
orienting mechanism and the extent to which it depends on visual
clues. And, if we had some valid way of estimating the number of birds
on the wing under varying weather conditions, we might be able to
understand better the nature and development of migration waves so
familiar to field ornithologists. These are just random examples
suggesting some of the results that may be achieved in a broad field
of inquiry that is still virtually untouched--the quantitative study
of migratory flights.
This paper is a venture into that field. It seeks to evaluate on a
more factual basis the traditional ideas regarding these and similar
problems, that have been developed largely from circumstantial
criteria. It is primarily, therefore, a study of comparative
quantities or volumes of migration--or what may be conveniently called
flight densities, if this term be understood to mean simply the number
of birds passing through a given space in a given interval of time.
In the present study, the basic data permitting the numerical
expression of such migration rates from many localities under many
different sets of circumstances were obtained by a simple method. When
a small telescope, mounted on a tripod, is focused on the moon, the
birds that pass before the moon's disc may be seen and counted, and
their apparent pathways recorded in terms of cooerdinates. In bare
outline, this approach to the problem is by no means new.
Ornithologists and astronomers alike have recorded the numbers of
birds seen against the moon in stated periods of time (Scott, 1881a
and 1881b; Chapman, 1888; Libby, 1889; West, 1896; Very, 1897;
Winkenwerder, 1902a and 1902b; Stebbins, 1906; Carpenter, 1906).
Unfortunately, as interesting as these observations are, they furnish
almost no basis for important generalizations. Most of them lack
entirely the standardization of method and the continuity that would
make meaningful comparisons possible. Of all these men, Winkenwerder
appears to have been the only one to follow up an initial one or two
nights of observation with anything approaching an organized program,
capable of leading to broad conclusions. And even he was content
merely to reproduce most of his original data without correlation or
comment and without making clear whether he fully grasped the
technical difficulties that must be overcome in order to estimate the
important flight direction factor accurately.
The present study was begun in 1945, and early results obtained were
used briefly in a paper dealing with the trans-Gulf migration of birds
(Lowery, 1946). Since that time the volume of field data, as well as
the methods by which they can be analyzed, has been greatly expanded.
In the spring of 1948, through the cooperation and collaboration of a
large number of ornithologists and astronomers, the work was placed on
a continent-wide basis. At more than thirty stations (Figure 34, page
437) on the North American continent, from Yucatan to Ontario, and
from California to South Carolina, observers trained telescopes
simultaneously on the moon and counted the birds they saw passing
before its disc.
Most of the stations were in operation for several nights in the full
moon periods of March, April, and May, keeping the moon under constant
watch from twilight to dawn when conditions permitted. They have
provided counts representing more than one thousand hours of
observation, at many places in an area of more than a million square
miles. But, as impressive as the figures on the record sheets are,
they, like the published observations referred to above, have dubious
meaning as they stand. Were we to compare them directly, station for
station, or hour for hour, we would be almost certain to fall into
serious errors. The reasons for this are not simple, and the measures
that must be taken to obtain true comparisons are even less so. When I
first presented this problem to my colleague, Professor William A.
Rense, of the Department of Physics and Astronomy at Louisiana State
University, I was told that mathematical means exist for reducing the
data and for ascertaining the desired facts. Rense's scholarly insight
into the mathematics of the problem resulted in his derivation of
formulae that have enabled me to analyze on a comparable basis data
obtained from different stations on the same night, and from the same
station at different hours and on different nights. Astronomical and
technical aspects of the problem are covered by Rense in his paper
(1946), but the underlying principles are discussed at somewhat
greater length in this paper.