. Bulletin of the Department of Agriculture. Agriculture; Agriculture. BULLETIN OF THE M No. 185. Contribution from the Bureau of Biological Survey, Henry W. Henshaw, Chief. § April 17, 1915. BIRD MIGRATION. By \Yells W. Cooke, Assistant Biologist. CONTENTS. Introductiou 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 bii'ds And their way 27 Migration and molting
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. BULLETIN OF THE M No. 185. Contribution from the Bureau of Biological Survey, Henry W. Henshaw, Chief. § April 17, 1915. BIRD MIGRATION. By \Yells W. Cooke, Assistant Biologist. CONTENTS. Introductiou 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 bii'ds And their way 27 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 jiosition dm'ing migration 40 Relation between migration and temperature 41 Variations in speed of migration 43 The unknown 47 INTRODUCTION. Tiio 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 intervenmg 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 i\w. Mediterranean to winter m its depths. Witli later yeai's, h()W(n^<T, lias come a fuller knowledge of migration, csp(!cially of the ])articular region in which each species passes the cold season, and more definite information in regard to the routes followed in the sjH'iiig and fall j(jnrn(5ys. But fuller knowledge lias served t(; increase rather than to lessen interest in the SLd)ject. More persons to-day arc watching birds and noting their times of arrival and departure lliari ev(^r before. Indeed, the Biological Survey has received migration notes from more than 2,00(J diirerent observers, Note.
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