2 USEFUL BIRDS. 
they may bear to the unnatural and semi-artificial conditions 
produced by the agriculturist may then be better under- 
stood. The position occupied by birds among the forces of 
nature is unique in one respect at least; their structure fits 
them to perform the office of a swiftly moving force of 
police, large bodies of which can be assembled at once to 
correct disturbances caused by abnormal outbreaks of plant 
or animal life. This function is well performed. A swarm 
of locusts appears, and birds of many species congregate to 
feed upon locusts. An irruption of field mice, lemmings, or 
gophers occurs, and birds of prey gather to the feast from 
far and near. 
This habit of birds is also serviceable in clearing the earth 
of decaying materials, which otherwise might pollute both 
air and water. A great slaughter of animals takes place, 
and Eagles, Vultures, Crows, and other scavengers hasten to 
tear the flesh from the carcasses. A dead sea monster is 
cast upon the shore, and sea birds promptly assemble to 
devour its wasting tissues. The gathering of birds to feed 
is commonly observed in the flocking of Crows in meadows 
where. grasshoppers or grubs abound, the assembling of 
Crows and Blackbirds in cornfields, and in the massing of 
shore birds on flats or marshes where the receding tide 
exposes their food. 
A study of the structure and habits of birds shows how 
well fitted they are to check excessive multiplication of 
injurious creatures or to remove offensive material. Birds 
are distinguished from all other animals by their complex, 
feathered wings, —the organs of perfect flight. 
The tremendous muscular power exhibited by birds is only 
such as might be expected in creatures provided with such 
perfect respiratory, circulatory, and assimilative organs. The 
strength of birds as compared with that of man is enormously 
out of proportion to their size ; but it is largely concentrated 
in the muscles that move the wings, for it is by flight that 
the bird is enabled to live. No other animals have such 
sustained power of flight or such perfect command over 
themselves while in the air. Even the bat, which is a most 
skillful flyer, being remarkably quick in aerial evolutions, 
