12 BIRD FRIENDS 



Physiological explanation. None of these theories 

 is generally accepted by bird students as giving a 

 satisfactory explanation of migration. It is probable 

 that birds have a physiological instinct which prompts 

 them to migrate in order to rear their young, just as 

 their instinct leads to other actions, such as singing, 

 mating, nest-building, egg-laying, and incubating. 

 But this statement, of course, gives no explanation 

 as to how and why this instinct originated. 



