24 DISPERSAL AND MIGRATION. [part I. 



northward as far as Canada, where, according to Mr. Allen, more 

 species breed than in the warm Southern States. Even in the 

 extreme north, beyond the limit of forests, there are no less than 

 60 species which breed ; in Canada about 1 60 ; while in 

 Carolina there are only 135, and in Louisiana, 130. The extent 

 of the migration varies greatly, some species only going a few 

 degrees north and south, while others migrate annually from 

 the tropics to the extreme north of the continent ; and every 

 gradation occurs between these extremes. Among those which 

 migrate furthest are the species of Dendrceca, and other Ameri- 

 can flycatching warblers (Mniotiltidce), many of which breed 

 on the shores of Hudson's Bay, and spend the winter in Mexico 

 or the West Indian islands. 



The great migratory movement of American birds is almost 

 wholly confined to the east coast ; the birds of the high central 

 plains and of California being for the most part sedentary, or 

 only migrating for short distances. All the species which reach 

 South America, and most of those which winter in Mexico 

 and Guatemala, are exclusively eas'tern species ; though a few 

 Eocky Mountain birds range southward along the plateaux of 

 Mexico and Guatemala, but probably not as regular annual 

 migrants. 



In America as in Europe birds appear in spring with great 

 regularity, while the time of the autumnal return is less con- 

 stant. More curious is the fact, also observed in both hemi- 

 spheres, that they do not all return by the same route followed in 

 going northwards, some species being constant visitors to certain 

 localities in spring but not in autumn, others in autumn but not 

 in spring. 



Some interesting cases have been observed in America of a 

 gradual alteration in the extent of the migration of certain birds. 

 A Mexican swallow (Hirundo klnifrons) first appeared in Ohio 

 in 1815. Year by year it increased the extent of its range till 

 by 1845 it had reached Maine and Canada; and it is now quoted 

 by American writers as extending its annual migrations to 

 Hudson's Bay. An American wren (Troglodytes ludovicianus) 

 is another bird which has spread considerably northwards since 



