206 ZOOLOGY. 



{Dendrceca), and oUior fly-catching warblers (MniofiUidcB), 

 many of wliicli bix'cd on the shores of Hndsou's Bay, and 

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

 cies of birds breed in Caniida 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 otlier West Indian islands, 

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

 ern States and passes northward, till in June it reaches 

 Canada and extends west to the Saskatchewan Kiver in 54° 

 north latitude. 



Says Baird: "While birds proceed generally in the spring 

 to the very spot of birth, and by a definite route, their re- 

 turn in autumn is not necessarily in the same line. Many 

 birds are familiar visitors in abundance in certain locali- 

 ties in either spring or autumn, and are not known there 

 in the other season." He thinks that in very many in- 

 stances birds proceed northward along the valley of the 

 Mississippi, to i-eturn along the coast of the Atlantic. In 

 general, also, the northward vernal movement is performed 

 much more rapidly, and with fewer stops by the way, than 

 the autumnal. " Birds generally make tlieir apjiearance in 

 given localities with wonderful regularity in the spring — the 

 SylvicoUdcB especially; a difference of a few days in succes- 

 sive years attracting the notice of the careful observer: this 

 difference is generally influenced by the season. The time 

 of autumnal return is, perluips, less definite." (Baird.) 



Wh.ile there are a number of veiy strange exiinct birds, 

 one of which called the Archceopferyx is the connecting 

 link between reptiles and birds, and there are fossil birds 

 with teeth, all the living species lielong to two single sub- 

 classes. 



Sub Classes of Existing Birds. 



1. Sternum smootli-, wings rudimentary Hntilce (Ostricli). 



2. Sternum Iceeled ; wings well developed. . . . Caiiiialw (Robin, etc.). 



