OUR GREATEST TRAVELERS 



Birds that Fly from Pole to Pole and Shun the 



Darkness; Birds that Make 2,500 



Miles in a Single Flight 



By WELLS W. COOKE 



Of the Biological Survey, U. S. Department of Agriculture 



THE migration of birds has long 

 been considered an unfathomable 

 mystery, but recent investigations 

 have furnished abundant data on the 

 when and where of migration and solved 

 many of its puzzles. The Bureau of Bio- 

 logical Survey of the United States De- 

 partment of Agriculture has collected 

 much information on the migration of 

 North American birds, and this article is 

 an attempt to put in popular form some 

 of the data that have already appeared 

 in the more technical bulletins and re- 

 ports. No correct understanding of bird 

 migration is possible until it is consid- 

 ered as a voluntary evolution. All mi- 

 gratory movements must have begun with 

 changes of location that were only very 

 slight. 



From this short migration, benefit ac- 

 crued to individuals or to their posterity. 

 Migration became a fixed habit, and the 

 distance covered gradually — very gradu- 

 ally — increased as each succeeding exten- 

 sion proved advantageous. It is not to 

 be supposed that every attempted exten- 

 sion was a success ; in fact, it is more 

 probable that only a small part of the 

 experimental pioneering routes were per- 

 manently adopted. 



INIoreover, it must be borne in mind 

 that the time occupied in the establish- 

 ment of present migration habits and 

 routes was measured in geologic ages, 

 and there is no reason to suppose that 

 changes took place during these ages any 

 faster than they do now. 



It is about a hundred years since the 

 first reliable notes on migration in the 

 United States were recorded, and this 

 period has proven too short to show any 



perceptible difference in its time, direc- 

 tion, or speed. It can be affirmed, then, 

 that the migration routes of today are 

 the results of innumerable experiments 

 as to the best way to travel from the 

 winter to the summer home and return. 

 It can also be said that food supplies 

 en route have been the determining fac- 

 tor in the choice of one course in prefer- 

 ence to another, and not the distance 

 from one food base to the next. The 

 location of plenty of suitable provender 

 having been ascertained, the birds pay no 

 attention to the length of the single flight 

 required to reach it. 



principal migration routes of north 



AMERICA 



The shape of the land areas in the 

 northern half of the Western Hemisphere 

 has tended to great variations in migra- 

 tory movements. If the whole area from 

 Brazil to Canada were a plain with the 

 general characteristics of the middle sec- 

 tion of the Mississippi \^alley, the study 

 of bird migration would lose much of its 

 fascination. There would be a simple 

 rhythmical swinging of the migration 

 pendulum back and forth spring and fall. 

 lUit a large part of the space between 

 I>razil and Canada is occupied by the 

 Ciulf of Mexico, the Caribbean Sea, and 

 parts of the Atlantic Ocean, all devoid 

 of sustenance for land birds. The two 

 areas of abundant food supplies are 

 North America and northern South 

 .America, separated by the comparatively 

 small land areas of Alexico and Central 

 .\merica. the islands of the West Indies, 

 and the great stretches of foodless 

 waters. 



