304 ZOOLOGY. 



and the condition of the glottis (glottides) can only In- 

 modified by those which raise or depress the trachea. 



442. Birds are oviparous, and the young require no 

 nourishment from a breast. The period of incubation of the 

 young in the egg varies with the species, but is nearly the 

 same in the individuals of a species. For the humming bird, 

 the smallest of the class, the period is twelve days ; for the 

 canary, it varies from fifteen to eighteen days ; for the domes- 

 tic fowl, twenty-one days; twenty-five for the duck; and 

 from forty to forty-five for the swan. The heat of the sun 

 suffices for the incubation of some tropical birds, but in 

 general it is quite otherwise, and the eggs require to be 

 placed in a nest, and carefully and sedulously hatched by the 

 mother. 



Fig. 254,. The Eider (Goose). 



It is in the construction of the nest that birds display that 

 wonderful hereditary instinct of which we have already 

 spoken at considerable length ; a few merely scrape a hole in 

 the soil, and deposit therein the egg or eggs to be hatched, 

 but with the greater number it is quite otherwise, and the 

 >]Hvirs may often be known by the form of the nest ( 328). 

 The warm and light substance, called edredon, usnl in 

 domestic economy, is the soft down which the bird pulls from 

 its breast to line the nest. 



The lay of eggs (ponte) takes place generally once a yrar. 

 sometimes twice; but in a domestic state, the fecundity is 



