\fIORA 1 1"\ OF BIRD 



and in the ostrich and other Ratita the labor is wholl} 

 formed by the mal 



There are probably from 7000 to 8000 5 of In 



birds. Of the whole numbi well- 



marked geographical races inhabit North A merit th of 



Mexico and including Greenland, 'i graphical 



tribution of birds is somi plicated by their m \ 



tions. While tin* larger number of species are tropical, 

 arctic birds are abundant, though n them are aquatic, 



[n the United £ there arc three centres of distribution: 



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



ky Mountain plateau, and (3) the Pa 



Nearly all the birds which breed in tic initial and north- 

 era portionsof the Unit uthward in the 



autumn, and spend the winter in the wanner Southern 

 or in Central America or the Wesl Indies. The 

 causes of this regular annual migration are pr bablydue to 

 the ohai m, 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 nol migrati 

 climes; bul some Brazilians migrate southwards into 



Buenos Ayr* s; it is those birds which live far north which 

 have what i> called the " migratory instinct." B Is mi- 

 grate by lii-iii 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 Rockj .Mountains as well as of the Pacifi 

 sedentary, or migrate bul a shorl distance, h is the 1 

 era birds which migrate regularly. These pass 

 into Mexico and Guatemala, and reach South America. 

 Thus the extent of the migration var 1 

 cies only going a few degrees north and south, while others 

 migrate annually from arctic America to the tropics, and 

 every gradation occurs between I ernes. An 



those which migrate farthest are tie -f war" 



* See the writings of Boird, Www, and Wall;.. 



