358 PRACTICAL ZOOLOGY 



young. The young of precocial birds are able to run about like 

 young chickens soon after they are hatched ' (Fig. 232). The 

 eggs of these birds must be correspondingly large in order to 

 contain food material (yolk and white) enough to enable the 

 young to reach such an advanced stage in development. The 

 killdeer, nighthawks, bobwhites, and ducks are common pre- 

 cocial birds. 



The young of altricial birds, on the other hand, hatch in a 

 very immature condition (Fig. 233) and must remain in the nest 

 a long time until their feathers are grown and they become strong 

 enough to walk or fly. Most of our common birds are of this sort. 



Birds' Eggs. — The eggs of birds are covered by a hard shell 

 of calcium carbonate (Fig. 234). Eggs are single cells, their 

 enormous size being due to the accumulation of food material 

 within them. The shell is either pure white, as in many of the 

 birds like woodpeckers and kingfishers that lay eggs in dark 

 places, or variously colored and covered with specks, spots, and 

 lines of different hues. Colored eggs are adapted to their sur- 

 roundings, since they are less conspicuous amidst the green vege- 

 tation than white eggs would be. 



Number of Eggs. — The number of eggs laid in a single 

 nest, called a " clutch " or " set," is usually the same for in- 

 dividuals of one species, but differs in different species. The 

 passenger pigeon lays one egg; the mourning dove lays two; 

 the red-tailed hawk two or three; the robin three or four; the 

 blue jay four or five ; the bank swallow six; the flicker and king- 

 fisher six or eight ; and the game birds, like the bobwhite, from 

 a dozen to twenty. 



Incubation. — The eggs must be kept warm or incubated in 

 order to develop; this is accomplished by the bird sitting on 

 them. Sometimes the female alone performs this duty; some- 

 times both birds take turns; and in a few instances, like that of 

 the ostrich, the male alone incubates the eggs. The period of 

 incubation lasts from ten or twelve days among the smaller 

 birds to over a month in the case of the largest species. 



