1 7 2 ANIMAL BIOLOGY 



in the warm countries in summer, the food found in north- 

 ern countries in summer would be unused, while they 

 would have to compete with the numerous tropical birds 

 for food, and they and their eggs would be in danger from 

 snakes, wild cats, and other beasts of prey so numerous in 

 warm climates. These are the best reasons so far given 

 for migration. 



The manner and methods of migration have been studied 

 more carefully in Europe than in America. Migration is 



FIG. 316. EUROPEAN SWALLOWS (Hirundo urbicd), assembling for autumn 

 flight to South. 



not a blind, infallible instinct, but the route is learned and 

 taught by the old birds to the young ones ; they go in 

 flocks to keep from losing the way (Fig. 316); the oldest 

 and strongest birds guide the flocks (Fig. 317). The birds 

 which lose their way are young ones of the last brood, or 

 mothers that turn aside to look for their strayed young. 

 The adult males seldom lose their way unless scattered 

 by a storm. Birds are sometimes caught in storms or 

 join flocks of another species and arrive in countries 

 unsuited for them, and perish. For example, a sea or 



