THE VERTEBRATES: BIRDS 219 



The eggs are fertilized within the body of the female, the 

 mating time of most birds being in the spring or early summer. 

 Some kinds, the English sparrow, for example, rear numerous 

 broods each year, but most species have only one or at most 

 two. The eggs vary greatly in size and color-markings, 

 and in number from one, as with many of the Arctic ocean 



FIG. 114. The nest and eggs of the black phoebe, Sayornis tiigricans. 

 (Photograph by J. O. Snyder.) 



birds, to six or ten, as with most of the familiar song-birds, 

 or from ten to twenty, as with some of the pheasants and 

 grouse. The duration of incubation (outside the body) 

 varies from ten to thirty days among the more familiar birds, 

 to nearly fifty among the ostriches. The temperature neces- 

 sary for incubation is about 40 C. (100 F.). Among polyga- 

 mous birds (species in which a male mates with several 

 or many 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 



