420 



METAZOAN PHYLA 



The time of incubation varies somewhat with the size of the bird, 

 the cowbird having an incubation period of only about ten days; the 

 ordinary song bird, about two weeks; fowls, three weeks; ducks, geese, 

 and swans, from four to five weeks; the ostriches, from seven to eight 

 and a half weeks; and the Australian emu, ten or eleven weeks. Usually 

 the duty of incubation is assumed mainly by the female, and in other 

 cases the two share in it. In a few instances, however, the male does 

 all of the incubating. The last is true in the case of the American ostrich 

 and also in the case of a small sandpiper-like bird known as the phalarope. 

 The female of the African ostrich participates very little in incubation. 



Fig. 310. — A simple nest of the mourning dove, Zenaidura macroura Linnaeus, made of 

 loose straws and stems on the ground. (Photographed, copyrighted, and contributed by 

 Gayle Pickwell.) 



Two types of young birds are recognized, depending upon the degree 

 of development at the time of hatching. Some are known as 'precocial (Fig. 

 306) and form a group called nidifugae (nest-fleeing). They are covered 

 with down at the time of hatching, have their eyes open, and run or 

 swim as soon as their plumage is dried. Examples of this type of bird are 

 the fowls, ducks, geese, and water birds generally. An extreme case 

 is that of the brush turkeys of Australia and the East Indies. Neither 

 parent incubates; the eggs are hatched by the heat of the sun, and when 

 the young birds leave the eggs they are in full plumage and able to take 

 care of themselves. Others are known as altricial (Fig. 308) and form the 

 group nidicolae (nest-living). The young of these birds are naked and 

 blind when hatched but gradually acquire the down plumage, which is 

 later replaced by a mature plumage before they are able to leave the 

 nest. Our ordinary song birds all belong to this group. 



