. Bird migration . BULLETIN or THE No. 185 Contribution from the Bureau of Biological Survey, Henry W. HenshawApril 17, 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 2? Migration and molting 31 Casualties during migration 31 Are bii-ds exhausted by long fl
. Bird migration . BULLETIN or THE No. 185 Contribution from the Bureau of Biological Survey, Henry W. HenshawApril 17, 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 2? Migration and molting 31 Casualties during migration 31 Are bii-ds 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 41 Variations in speed of migration 43 The unknown 47 INTRODUCTION. The mystery of bird migration has proved a fascinating subject forspeculation and study from earliest times. Long ago it was noticedthat birds disappeared in fall and reappeared in spring, but, not know-ing where they spent the intervening period, many fanciful theorieswere advanced to account for the
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