6 USEFUL BIRDS. 
tions that, when shot, their distended skins burst open 
when their bodies strike the ground. This accumulation of 
fatty tissue may aid to tide the birds over a season of 
scarcity, but the moment they need food they must seek 
it far and wide, if need be, as they cannot live long with- 
outit. Birds are not always the ethereal, care-free creatures 
of the poet’s dream. In time of plenty, the joys of flight, 
of sunshine, of singing, of riding swinging boughs, or toss- 
ing to and fro on flashing waves, are theirs to the full; 
but in times of scarcity, or when rearing their helpless 
young, their daily lives are often one continued strenuous 
hunt for food. Food, therefore, is the mainspring of the 
bird’s existence. Love and fear alone are at times stronger 
than the food craving. The amount of food that birds are 
capable of consuming renders them doubly useful in case of 
an emergency. 
The utility of birds in suppressing outbreaks of other an- 
imals by massing at threatened points is of no greater value 
in the plan of nature than is the perennial regulative influ- 
ence exerted by them individually everywhere as a check on 
the undue increase of other forms of life. 
He who studies living birds, other animals, or plants, and 
the relations which these living organisms bear to one 
another, will soon learn that the main effort of each plant 
or animal is to preserve its own life and produce seed or 
young, and so multiply its kind. He will see, also, that the 
similar efforts of other organisms by which it is surrounded 
tend to hold its increase in check. 
The oak produces many hundreds of acorns; and were 
each acorn to develop into a tree, the earth eventually would 
be full of oaks, for all other trees would be crowded out. 
But many animals feed on the acorns or the young seedlings ; 
other trees crowd out the young oaks; caterpillars feed on 
the foliage ; other insects feed on the wood and bark, de- 
stroying many trees; so, on the average, each oak barely 
succeeds in producing another to occupy its place. 
Certain moths deposit hundreds of eggs in a season; and 
were each egg to hatch and each insect to come to maturity 
and go on producing young at the same rate, the entire earth 
