Chapter I: Part II: The Nature of Nocturnal Migration 408

Horizontal Distribution of Birds on Narrow Fronts 409

Density as a Function of the Hour of the Night 413

Migration in Relation to Topography 424

Geographical Factors and the Continental Density Pattern 432

Migration and Meteorological Conditions 453

CONCLUSIONS 469

LITERATURE CITED 470

LIST OF FIGURES

Figure Page

1. The field of observation as it appears to the observer 374

2. Determination of diameter of cone at any point 375

3. Temporal change in size of the field of observation 376

4. Migration at Ottumwa, Iowa 377

5. Geographic variation in size of cone of observation 378

6. The problem of sampling migrating birds 380

7. The sampling effect of a square 381

8. Rectangular samples of square areas 382

9. The effect of vertical components in bird flight 383

10. The interceptory potential of slanting lines 384

11. Theoretical possibilities of vertical distribution 388

12. Facsimile of form used to record data in the field 391

13. The identification of co-ordinates 392

14. The apparent pathways of birds seen in one hour 393

15. Standard form for plotting the apparent paths of flight 395

16. Standard sectors for designating flight trends 398

17. The meaning of symbols used in the direction formula 399

18. Form used to compute zenith distance and azimuth of the moon 400

19. Plotting sector boundaries on diagrammatic plots 402

20. Form to compute sector densities 403

21. Determination of the angle [alpha] 404

22. Facsimile of form summarizing sector densities 405

23. Determination of net trend density 406

24. Nightly station density curve at Progreso, Yucatan 407

25. Positions of the cone of observation at Tampico, Tamps 411

26. Average hourly station densities in spring of 1948 414

27. Hourly station densities plotted as a percentage of peak 415

28. Incidence of maximum peak at the various hours of the
night in 1948 416

29. Various types of density-time curves 418

30. Density-time curves on various nights at Baton Rouge 422

31. Directional components in the flight at Tampico, Tamps 428

32. Hourly station density curve at Tampico, Tamps 429

33. The nightly net trend of migrations at three stations in 1948 431

34. Stations at which telescopic observations were made in 1948 437

35. Positions of the cone of observation at Progreso, Yucatan 443

36. Hourly station density curve at Progreso, Yucatan 444

37. Sector density representation on two nights at
Rosedale, Miss. 451

38. Over-all sector vectors at major stations in spring of 1948 455

39. Over-all net trend of flight directions shown in Figure 38 456

40. Comparison of flight trends and surface weather conditions
on April 22-23, 1948 460

41. Winds aloft at 10:00 P. M. on April 22 (CST) 461

42. Comparison of flight trends and surface weather conditions
on April 23-24, 1948 462

43. Winds aloft at 10:00 P. M. on April 23 (CST) 463

44. Comparison of flight trends and surface weather conditions
on April 24-25, 1948 464

45. Winds aloft at 10:00 P. M. on April 24 (CST) 465

46. Comparison of flight trends and surface weather conditions
on May 21-22, 1948 466

47. Winds aloft at 10:00 P. M. on May 21 (CST) 467