THE MIGRATION OF BIRDS. 345 
* 
pine grosbeak should not find as abundant a supply of food in the 
northern forests during cold winters as in mild; yet it is only 
during the former that it descends to the latitude of New York; 
while in the latter it does not migrate as far south by one hundred 
and fifty or two hundred miles. The pine finch, and the crossbills 
are similar instances. On the other hand, a long, hot summer is 
apt to entice some southern birds farther north than usual. 
Man exercises a very considerable influence upon the migration 
of some birds. The clearing away of forests, and the planting of 
trees.upon the prairies, attract species that formerly could not 
find the means of support in those regions; and compel others 
to shun localities which they were wont to frequent. Fifty or 
sixty years ago, according to Audubon, the mallard and the wild 
goose, as well as some other species of water-fowl, bred in consid- 
erable numbers in the Mississippi valley ; but as the settlement of 
the country progressed, they retreated farther and farther north, 
until at the present day, very few raise their young east of the 
Missouri and Red rivers, or south of the British boundary, al- 
though some still nest in central and western Minnesota, and 
northern Iowa. In this case the necessity of reaching a secure, 
safe retreat, remote from the settlements, has caused them to ex- 
tend their migration far beyond its former limit. Doubtless a 
Similar motive has acted in other instances with similar results. 
The desire to rear their young in quiet and seclusion, is a very 
strong one in many birds; and if disturbed or annoyed in an 
` Way, they will soon abandon the region, and seek another where 
they can pass the breeding season unmolested. 
Violent storms, and sudden changes in the weather, are often 
preceded by, or accompanied with, extraordinary migration among 
irds. The immense flocks of pigeons and blackbirds that occa- 
sionally pass through the country are familiar to every one. Sev- 
` eral years ago I witnessed an unusual migration of the latter bird, 
which I have never seen equalled, either before or since not even 
in the coast marshes where they sometimes congregate in enor- 
mous flocks. The latter part of February, and first week of 
March had been very mild and warm, and great numbers of crows, 
redwinged and cow blackbirds had gone north. There came a 
sudden, violent storm from the north one night, accompanied by 
showers of hail, snow and sleet, continuing all next day, and 
driving before it, immense multitudes of blackbirds. Vast flocks, 
