626 COLLEGE ZOOLOGY 



twenty. For example, the murre lays one; the mourning dove, 

 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, six 

 to eight; the ruff ed grouse, eight to fourteen ; the bob-white, ten 

 to eighteen. 



The average period of incubation for passerine birds is about 

 twelve days. The eggs of the ostrich hatch in about forty-five 

 days. In some cases the female alone incubates ; in other cases 

 both male and female assist in incubation; and in a few birds, 

 such as the ostrich, the male performs practically all of this duty. 



Two general classes of young are recognized: (i) those that 

 are able to run about, like young chickens, soon after hatching, 

 known as precocious birds; and (2) those that remain in the nest 

 for a greater or less period before they are able to take care of 

 themselves. The latter are known as altricial birds. 



g. The Economic Importance of Birds 



Commercial Value. - - Without taking into consideration the 

 more than three million dollars annually derived from poultry 

 products in this country, we may say that the principal sources 

 of revenue derived from birds are the flesh of game birds, the 

 eggs of certain colonial sea-birds, the feathers of many species 

 of use for millinery purposes, and the excreta and ejecta of certain 

 species, which have accumulated on tropical islands and are 

 known as guano. 



Guano contains two important elements of use in fertilizing 

 the soil, phosphoric acid and nitrogen. The Chincha Islands off 

 the coast of Peru have been for centuries the habitation of large 

 numbers of sea-birds, whose excreta and remains have dried and 

 formed a deposit in some places a hundred feet thick. The sup- 

 ply on these islands is now almost exhausted, though in 1853 

 the Peruvian government estimated the amount at that time at 

 12,376,100 tons. There are many other deposits in the rainless 

 latitudes of the Pacific, but none as rich as were those of the 

 Chinchas. 



