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

Figurepage
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 α]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