206 ZOOLOGY. 



(Dendrceca), and other fly-catching warblers (MniotiUidcB), 

 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- 

 ern States and passes northward, till in June it reaches 

 Canada and extends west to the Saskatchewan River 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 return 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 their appearance in 

 given localities with wonderful regularity in the spring — the 

 Sylvicolidce 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, perhaps, less definite." (Baird.) 



While there are a number of very strange extinct birds, 

 one of which called the Arclmopteryx is the connecting 

 link between reptiles and birds, and there are fossil birds 

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

 classes. 



Sub-Classes op Existing Birds. 



1. Sternum smooth; wings rudimentary Ratitce (Ostrich). 



2. Sternum keeled; wings well developed. . . .Carinatm (Robin, etc.). 



