8 USEFUL BIRDS. 
All animals and plants are sustained and nourished by: 
air, water, and food. Food supplies the material for growth 
and development, Its abundance increases the energy and 
fertility of a species, —its ability to produce young abun- 
dantly. The study of the food and food habits of birds and 
other animals is of the utmost importance, for by this study 
alone we are enabled to trace their life relations to each 
other, to plants, and to man. Some progress has already 
been made in this study. We know in a general way the 
character of the. food of some of the common birds of the 
United States; but we know so little as yet of the food of 
the smaller mammals, the reptiles, batrachians, many insects 
and other lower animals, that it is impossible to tell what 
may be the ultimate effect of the destruction of any one of 
these animals by birds. 
On the other hand, no one can tell what grave and far- 
reaching results might follow the extermination of a single 
species of bird; for it is probable that the food preferences 
of each species are so distinctive that no other could fill its 
place. 
Birds are guided by their natural tastes in selecting their 
food, unless driven by necessity. Of the food which suits 
their tastes, that which is most easily taken is usually first 
selected. In the main, species’ of similar structure and 
habits often choose similar food, but each species usually 
differs from its allies in the selection of some certain favorite 
insects. Were a species exterminated, however, its place 
might be taken eventually by the combined action of many 
species, for nature always operates to restore her disturbed 
balances. 
The complexity of the food relations existing between 
birds and other organisms may be indicated hypothetically 
by a brief illustration. The Eagles, larger Hawks, and Owls 
feed to some extent on Crows, and probably the nocturnal, 
tree-climbing, nest-haunting raccoon also robs them of eggs 
and young; otherwise, they seem to have very few natural 
enemies to check their increase. Crows feed on so many 
different forms of animal and vegetable life that they are 
nearly always able to find suitable food; therefore they 
are common and widely distributed. 
