340 ELEMENTARY ZOOLOGY 



females) the males take no part in the incubation and little 

 or none in the care of the hatched young ; among most 

 monogamous birds, however, the male helps to build the 

 nest, takes his turn at sitting on the eggs, and is active in 

 bringing food for the young, and in defending them from 

 enemies. The young, when ready to hatch, break the 

 egg-shell with the " egg-tooth," a horny pointed projec- 

 tion on the upper mandible, and emerge either blind and 



FIG. 134. The nest and eggs of the black phcebe, Sayornis nigricans. 

 (Photograph by J. O. Snyder.) 



almost naked, dependent upon the parents for food until 

 able to fly (altricial young), or with eyes open and with 

 body covered with down, and able in a few hours to feed 

 themselves (precocial young). 



More details regarding the eggs, nest, and young of 

 birds will be given later in this chapter. 



Classification. The class Aves is usually divided into 

 numerous orders, the number and limits of these as pub- 

 lished in zoological manuals varying according to the 



