536 ZOOLOGY. 
of the discovery of food to their young or their mates. They 
have an ear for music; some speciés, 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 Naturalist (Vol. 13, p. 21) the songs of a number 
of our birds. The singular antics, dances, mid-air evolu- 
tions, struts, and posturings of different 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, 
while the woodpecker excavates holes in dead trees. Asa 
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 Ratite the labor is mostly per- 
formed by the males. 
There are probably from 7000 to 8000 species of living 
birds; Gray’s ‘‘ Handlist’’ enumerates 11,162, but many 
of these are not good species. Of the whole number, about 
00 distinct species or well-marked geographical races in- 
habit North America north of Mexico. The geographical 
distribution of birds is somewhat complicated by their mi- 
grations. While 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, 
