MIGRATION OF BIRDS. 
206 
and in the ostrich and other Eatitce the labor is wholly per- 
formed by the males. 
There are probably from 7000 to 8000 species of living 
birds. Of the whole number, 878 distinct species or well- 
marked geographical races inhabit North America north of 
Mexico and including Greenland. The geographical dis- 
tribution of birds is somewhat complicated by their migra- 
tions. 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. 
Nearly all the birds which breed in the central and north- 
ern portions of the United States migrate southward in the 
autumn, and spend the winter in the warmer Southern 
States or in Central America or the West Indies. The 
causes of this regular annual migration are probably due to 
the changes of the season, and to the want of food in the 
winter time, and also to the breeding habits of birds. Trop- 
ical birds which breed at home do not migrate to other 
climes; but some Brazilian species migrate southwards into 
Buenos Ayres; it is those birds which live far north which 
have what is called the migratory instinct." Birds mi- 
grate by night as well as by day; and the young return the 
following spring to their birthplace. 
In North America* the birds of the Western plains and 
of the Eocky Mountains as well as of the Pacific coast are 
sedentary, or migrate but a short distance. It is the east- 
ern birds which migrate regularly. These pass southwards 
into Mexico and Guatemala, and reach South America, 
Thus the extent of the migration varies greatly, some spe- 
cies only going a few degrees north and south, while others 
migrate annually from arctic America to the tropics, and 
every gradation occurs between these extremes. Among 
those which migrate farthest are the species of warblers 
* See the writings of Baird, Allen, and Wallace. 
