HABITS OF BIRDS. 649 



high temperature of about 40 C. being sustained through- 

 out. 



HABITS AND FUNCTIONS OF BIRDS. 



Flight. As birds are characteristically flying animals, 

 many of their peculiarities may be interpreted in adaptation 

 to this mode of motion. 



(a) Shape and general structure of the &?d?y.--The 

 resistance offered by the air to the passage of a body through 

 it depends in part on the shape of the body, and the boat- 

 like shape of the bird is such that it offers relatively little 

 resistance. The attachment of the wings high up on the 

 thorax, the high position of such light organs as lungs and 

 air-sacs, the low position of the heavy muscles, the sternum, 

 and the digestive organs, the consequently low centre of 

 gravity, are also structural facts of importance. But it must 

 be remembered that the frictional resistance of the air is slight. 



(V) The muscles of flight '.--The pectoralis major brings 

 the wing downward, forward, and backward, keeping the 

 bird up and carrying it onward. As it has most work to do, 

 it is by far the largest. The pectoralis minor raises the 

 wing for the next stroke. Besides these two main muscles, 

 there are others of minor importance, the deltoides externus 

 and three coraco-brachials, which help to raise the wing. 

 On an average these muscles weigh about one-sixth of the 

 whole bird, but the proportion is often much greater, 

 amounting to nearly one-half in some pigeons. B.uffon 

 noted that eagles disappeared from sight in about three 

 minutes, and a common rate of flight is about fifty feet per 

 second. In migration many birds fly at a rate of from 

 100 to 200 miles an hour. 



(c] The skeleton.- -The. rigidity of the dorsal part of 

 the backbone, due to fusion of vertebrae, is of advantage in 

 affording a firm fulcrum for the wing - strokes, while the 

 arched clavicles (meeting in an interclavicle and often fused 

 in front to the sternum) and the strong coracoids (which 

 articulate with the sternum) are adapted to resist the in- 

 ward pressure of the down-stroke. As the keel of the breast- 

 bone serves in part for the insertion of the two chief muscles, 

 its size bears some proportion to the strength of flight. It 

 is absent in the running birds, such as the ostriches, and 



