WHY BIRDS TRAVEL 



cut by glaciers in the rocks of Central Park show 

 us that a great ice sheet once spread southward 

 as far as New York City. 



If there were magnolia trees in Greenland, 

 it is more than probable that there were 

 also various kinds of birds that we associate 

 with these trees. And if New York was cov- 

 ered with ice, it must have been the home of 

 birds which are now found only in the Far 

 North. 



Geologists tell us that in the later history of 

 the earth there have been not one, but several, 

 climatic changes. That is, the climate at one 

 place might be warm, then cold, then warm, and 

 then cold again. When, therefore, we try to 

 explain how these variations in climate acted on 

 the birds which may have lived in a certain place 

 when first it was warm, we set ourselves no easy 

 task. 



It is a good rule not to try to answer the whole 

 of a very difficult question at once but to take 

 some little corner of what seems to be the easiest 

 part of it. So I will not now try to tell why 



