AMERIOAN ORNITHOLOGY. 



85 



the case of the warbler, they are annually affected by an impulse to hasten 

 to a certain place. The impulse is periodic, and in a sense, common to all 

 birds. There is a nesting season in the tropics, just as there is a regular 

 nesting season in the Arctic region. 



Why is it, then, since there is a nesting season in the tropics, that some of 

 the species which migrate to the tropics do not remain there, and when the 

 period of reproduction approaches, seek some retired spot where they can 

 nest in safety.^ Because as the summer months advance, the heat of the 

 tropical region becomes unbearable to those species which are used to a tem- 

 perate clime. 



Photo from life by J. 

 Catbird on Nest. 



H. Miller. 



