MIGRATION OF BIRD8. 205 



and in the ostrich and other Ratitce 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) th« Atlantic States and Mississippi Valley; (2) the 

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



Nearly all the bii-ds which breed in the central and north- 

 ern porti(jnsof 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 bi'eed 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 niglit 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 ai-e 

 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 tlie 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 



* Si;e Itie writings of Baird, Allen, and Wallace. 



