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344 THE MIGRATION OF BIRDS. 
pelled to go by absolute necessity. In wet, springy meadows. 
in grass fields, in the eastern states, the meadow lark finds a 
cient supply of food to subsist upon, throughout the winter; wh 
on the western prairies, under a less degree of cold he migrates 
regularly as the kingbird, being unable to procure the req 
supply of food in his summer habitat. ‘The snipe and the wood 
cock linger as long as they can find unfrozen marshes and swa 
and in mild seasons, may be found about warm, springy meadows 
and coppices, sheltered from the frost, even in mid-winter. But 
were it possible for them to find a sufficiency of worms, and could 
the warblers obtain such insects as they habitually feed upon, the 
woods and marshes, instead of being nearly deserted for seve gs 
months, would remain tenanted throughout the year. Just as 
soon as the rigor of winter has passed away, and the ch 
temperature calls forth myriads of gnats and flies to swarm 
the woods, and the frozen earth thaws, and permits the worms to 
approach the surface again, they come back to their native reg 
from which they had been temporarily driven by stern necessity. — 
The supply of food, however, is -so closely governed by the 
seasons, that the migrations may be said to depend upon them, 
although, absolutely speaking, the paramount necessity of sub- 
sistence, and not the mere effect of heat and cold upon the b : 
themselves, is the main cause. Hence, we find, as a rule, that ¢ 
migrations of those birds, whose food is most affected by a change 
of temperature, are more regular and extended than those of 
species, whose subsistence is more independent of the seas 
The Colopteride, Sylvicolide, and all insectivorous birds 
capture their prey upon the wing, belong to the former class; * 
range extends, in most cases, many hundred miles north and uit 
and the migration is complete, few or no individuals lingeries 
behind the rest in their summer abodes. The granivorous 
cies on the other hand, living principally upon seeds, are m 
tionary, some of them finding a sufficient supply of food in 
native haunts, throughout the winter, while the rest Mis 
southward, though they seldom go as far as the insect 
birds. The omnivorous species are still more independent 
of them, as the raven, are strictly non-migratory. , 
- Nevertheless, in some cases, mere temperature, UNCON” 
with the question of subsistence, seems to be the motive I” 
from one region to another... There is no apparent reason W 
