BULLETIN OF THE 



No. 185 



Contribution from the Bureau of Biological Survey, Henry W. Henshaw 

 April 17, 1915. 



BIRD MIGRATION. 



By Wells W. Cooke, Assistant Biologist. 



CONTENTS. 



Page. 



Introduction 1 



Causes of migration 2 



Relation of migration to -weather 4 



Day and night migrants 5 



Distance of migration 7 



Routes of migration 11 



Direct and circuitous migration routes 19 



Eccentric migration routes 21 



Wide and narrow migration routes 23 



Slow and rapid migration 25 



Page. 



How birds find their way 27 



Migration and molting 31 



Casualties during migration 31 



Are birds exhausted by long flight? 33 



Evolution of migration routes 35 



Normal and abnormal migration 37 



Relative position during migration 40 



Relation between migration and temperature 11 



Variations in speed of migration 43 



The unknown 47 



INTRODUCTION. 



The mystery of bird migration has proved a fascinating subject for 

 speculation and study from earliest times. Long ago it was noticed 

 that birds disappeared in fall and reappeared in spring, but, not know- 

 ing where they spent the intervening period, many fanciful theories 

 were advanced to account for their disappearance, as hibernation in 

 hollow trees or in the mud of streams or ponds. Within the century 

 stories were current of whole flocks that were seen to disappear 

 beneath the waves of the Mediterranean to winter in its depths. 

 With later years, however, has come a fuller knowledge of migration, 

 especially of the particular region in which each species passes the 

 cold season, and more definite information in regard to the routes 

 followed in the spring and fall journeys. But fuller knowledge has 

 served to increase rather than to lessen interest in the subject. More 

 persons to-day are watching buds and noting their times of arrival 

 and departure than ever before. Indeed, the Biological Survey has 

 received migration notes from more than 2,000 different observers, 



Note.— This bulletin discusses the subject of bird migration. Of interest to nature, students and to 

 investigators of the economic relation of birds to agriculture. 

 76048°— Bull. 185—15 1 



