MIGRATIONS AND NESTING HABITS. 281 



a mate, with many birds, covers a period as long as the 

 life of the birds. It is believed of the eagle that his choice 

 of a mate is a life-long choice. The EngUsh sparrow is 

 believed to be polygamous, and our domestic fowls are 

 also. The bob-whites are monogamous, a dual union 

 being an exception among them, although they belong to 

 the same order of birds as do our domestic fowls. 



The number of eggs in the full set for different birds 

 varies from one to twenty. The young of some birds 

 are hatched with a full covering of down, and can run 

 about or swim from the time of hatching; such birds are 

 called prscocial. Of other birds, the young are hatched 

 naked and helpless, and must be long fed and nourished 

 in the nest; these birds are called altricial. Such birds 

 are in a much less advanced condition than are the prae- 

 cocial birds, and the nest becomes a home where the 

 development takes place under the care of one or both 

 parents. This fact has biological significance; the longer 

 the time of development from birth to maturity, the 

 greater the degree of complexity which may be reached, 

 and the higher the position of the animal in the scale of 

 life. In accord with this fact, we have within the class 

 Aves, a possible ranking of the families of birds; and we 

 shall find that the birds of highest nervous- organization 

 are the altricial birds, while the prscocial birds plainly 

 rank lower in the bird scale. 



The period of incubation is the period intervening 

 between the laying of the eggs and the hatching of the 

 young birds. With some birds the eggs are left loosely 

 covered but exposed to the rays of the sun, the loose 

 covering serving to prevent undue radiation at night. 

 With many birds the eggs are covered by the female, 

 the male during the entire time taking upon himself 



