UNITED STATES DEPARTMENT OF AGRICULTURE 

 BULLETIN No. 185 



Contribution from the Bureau of Biological Survey 

 HENRY W. HENSHAW, Chief 



Washington, D. C. 



April 17, 1915 



BIRD MIGRATION 



By 

 WELLS W. COOKE, Assistant Biologist 



CONTENTS 



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 



How Birds Find Their Way ...... 27 



Page 



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 Tem- 

 perature 41 



Variations in Speed of Migration ... 43 



The Unknown 47 



WASHINGTON 



GOVERNMENT PRINTING OFFICE 



1915 



