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- 

 out it. 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 



