210 FIRST LESSONS IN ZOOLOGY. 



eal, 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 Val- 

 ley ; (3) the Eocky 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 southwards 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 shori; 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 

 (Dendrwca), and the fly-catching warblers {Mniotiltidm), 

 many of which breed on the shores of Hudson's Bay, and 

 spend the winter in Mexico or the West Indies. More spe- 

 cies of birds breed in Canada than in the warm Southern 

 States. Birds have been known to extend their range of 

 migrations; the rice-bird or bobolink continually widens 

 its range as rice and wheat are more extensively cultivated. 

 This bird winters in Cuba and other West Indian islands, 

 and probably also in Mexico. In April it enters the South- 



