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 BioLocicaL Survey, U. S. DEPARTMEN’T OF AGRICULTURE 
HE 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. 
Moreover, 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 Valley, 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. 
But a large part of the space between 
Brazil and Canada is occupied by the 
Gulf 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 Mexico and Central 
America, the islands of the West Indies, 
and the great stretches of foodless 
waters. 
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