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 160; 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 Dendraca, and other Ameri- 
can flycatching warblers (Mnzotiltidw), 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 eastern species; though a few 
Rocky 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 lunifrons) 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 (TZvroglodytes ludovicianus) 
is another bird which has spread considerably northwards since 
