THE BIRD STUDY BOOK 

 for a short time to fly; but being at home in the water, 

 where they secure theirfood, they are not left in the 

 helpless, even desperate, condition in which a land 

 bird would find itself if unable to fly. In a few cases 

 birds begin to migrate before this moulting takes 

 place, but with the great majority the moult is com- 

 plete before they leave their summer homes. 



Why Birds Migrate. — Why birds migrate we can 

 only conjecture. Without doubt the growing scar- 

 city of food in autumn is the controlling factor with 

 many of them ; and this would seem to be an excellent 

 reason for leaving the region of their summer sojourn. 

 Cold weather alone would not drive all of them south- 

 ward, else why do many small birds pass the winter 

 in northern latitudes where severe climatic conditions 

 prevail? Should we assume the failing food supply 

 to be the sole cause of migration, we would find 

 ourselves at fault when we came to consider that 

 birds leave the tropic regions in spring, when food is 

 still exceedingly abundant, and journey northward 

 thousands of miles to their former summer haunts. 

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