12 USEFUL BIRDS. 
the smaller birds, the survivors of which they have unwit- 
tingly furnished with more insect food, thus making con- 
ditions favorable for the increase of the smaller birds. 
These oscillations or alternate expansions and contractions 
in the numbers of birds or insects are usually so slight as 
to escape common observation. It is only in those cases 
where they are carried to extremes that they result disas- 
trously. Under nature the checks on the increase of birds 
are essential, else they would increase in numbers until 
their food supply had become exhausted, when they would 
starve, and other consequences even more grave and much 
more complex would then follow. 
While these examples of the way in which the balance of 
nature is preserved may be regarded as somewhat hypothet- 
ical, they probably approximate what actually takes place, 
although the feeding habits of birds undoubtedly produce 
far more complicated results than are here outlined. 
It is a law of nature that the destroyer is also the protector. 
Birds of prey save the species on which they prey from 
overproduction and consequent starvation. They also serve 
such species in at least two other ways: (1) the more 
powerful bird enemies of a certain bird usually prey upon 
some of its weaker enemies; (2) these powerful birds also 
check the propagation of weakness, disease, or unfitness, by 
killing off the weaker or most unfit individuals among the 
species on which they prey, for these are most easily captured 
and killed. 
We have seen already that Jays, which are enemies of 
the smaller birds, are preyed upon by the more powerful 
Crows, Hawks, and Owls. These latter also destroy skunks, 
weasels, squirrels, mice, and snakes, all of which are also 
enemies of the smaller birds. No doubt these animals would 
be much more injurious to the smaller birds were they with- 
out these wholesome feathered checks on their increase. 
In a state of nature, albino birds or those that are rendered 
conspicuous to their enemies by any unusual mark or color 
are soon captured by some bird of prey, and seldom live to 
perpetuate their unfitness. 
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