BIRD TRAVELERS 11 



theories ' which have been put forward at various 

 times to explain the cause of bird migration. 



Food and temperature. It is very commonly 

 stated that lack of food and low temperatures cause 

 birds to migrate. But even a very hasty examination 

 of the facts shows that these do not explain migra- 

 tion. The fall migration begins during the late sum- 

 mer, when the temperature is still high, and at a 

 time when insect life is abundant. Furthermore, 

 during the spring migration, birds are traveling into 

 regions where the temperature is lower and insect 

 life is less abundant than in the regions which they 

 are leaving. And again, some tropical sea-birds mi- 

 grate from one section to another where the condi- 

 tions of temperature and food-supply are practically 

 the same. 



Glacial theory. One theory relates the origin of 

 bird migration closely with the glacial age. Fossils 

 which have been found show that before this age 

 North America had a warm climate, even in its 

 northern portions. This climate must have been 

 well adapted for bird life during all parts of the year. 

 As the ice-sheet began to extend south, the birds were 

 driven before it, and as it melted and receded north, 

 the birds followed it back. In accordance with this 

 theory, the habit which the birds thus acquired of 

 moving back and forth, following the oscillations 

 of the ice-sheet, was inherited eventually by the birds 

 as an instinct and still exists to this day. 



