THE TRAVELS OF BIRDS 



cause of any change in the climate there? Because 

 of any failure in the supply of food? If you have 

 ever hunted hens' eggs, have you noticed that the 

 birds often hide their nests in places where they 

 never go except to lay? Mention several places 

 of this kind. Why do you suppose the hens select 

 them? Why do many kinds of birds visit remote 

 and inaccessible islands on which to nest? Does not 

 the difference between the habits of hens and sea 

 birds in nesting in out-of-the-way places consist 

 largely in the difference of the length of their jour- 

 neys? What prompts the hen to return to her nest? 

 What prompts the sea birds to go back to their 

 islets? Compare a year of a bird's life with that 

 of a tree. What evidence have we for the belief 

 that a much warmer climate once existed in the. 

 Arctic region? How do we know that the greater 

 part of North America was once covered with ice? 

 How far south did the ice sheet come? Describe 

 the change in climate due to the gradual freezing of 

 the North, and the invasion and retreat of the ice 

 sheet. How is it believed to have affected the mi- 

 gration of birds? 



