536 ZOOLOGY. 



of the discovery of food to their young or their mates. They 

 have an ear for music ; some species, as the mocking-bird, 

 will imitate the notes of other birds. The songs of birds 

 ■can be set to music. Mr. X. Clark has published in the 

 American Xatiiralist (\<A. 13, p. 21) the songs of a number 

 of our birds. The singular antics, dances, mid-air evolu- 

 tions, struts, and posturings of difEerent birds, are without 

 doubt the visible signs of emotions which in other birds find 

 vent in vocal music. 



The nesting habits of birds are varied. Many birds, as 

 the gulls, auks, etc., drop their eggs on bare ground or rocks ; 

 as extremes in the series are the elaborate nests of the 

 tailor-bird, and the hanging nest of the Baltimore oriole, 

 •svhile the ■woodpecker excavates holes in dead trees. As a 

 rule, birds build their nests concealed from sight ; in tropi- 

 cal forests they hang them, in some cases, out of reach of pred- 

 atory monkeys and reptiles. Birds may change their nesting 

 habits sufficiently to prove that they have enough reasoning 

 powers to meet the exigencies of their life. Parasitic birds, 

 like the cuckoo and cow-birds, lay their eggs by stealth in 

 the nests of other birds. 



The duties of incubation are, as a rule, performed by the 

 female, but in most Passerine birds and certain species of 

 other groups, the males divide the work with the females, 

 and in the ostrich and other Ratitm the labor is mostly per- 

 formed by the males. 



There are probably from 7000 to 8000 species of living 

 birds; Gray's "Handlist" enumerates ll,16-2, but many 

 of these are not good species. Of the whole number, about 

 700 distinct species or well-marked geographical races in- 

 habit Xorth America north of Mexico. The geographical 

 -distribution of birds is somewhat complicated by their mi- 

 grations. T\'hile the larger number of species are tropical, 

 arctic birds are abundant, though most of them are aquatic. 

 In the United States there are three centres of distribution : 

 (1) the Atlantic States and Mississippi Valley ; (2) the 

 Rocky Mountain plateau, and (3) the Pacific coast. The 

 migrations of birds will be treated of near the close of this 

 volume. 



