LOCOMOTION OF BIRDS. 115 



thereby raised in the same manner as in leaping from the ground. 

 The impulse being once given, the bird folds the wings by bending 

 the different joints, and raises them preparatory to another stroke. 



Velocity of flight depends upon the rapidity with which the 

 wing-strokes succeed each other ; and the ratio of the resist- 

 ance of the air is not as the velocity simply, but as the square 

 of the velocity. A downward stroke would only tend to raise 

 the bird in the air ; to carry it forward the wings require to be 

 moved in an oblique plane, so as to strike backward as well as 

 downward. The turning in flight to the right or to the left is 

 principally effected by an inequality in the vibrations of the wings. 

 To wheel to the rio;ht the left 

 wing must be plied with greater 

 frequency or force, and vice versa. 



The outspread tail contributes 

 to sustain the posterior part of 

 the body ; and, its plane being 

 horizontal, serves chiefly by its 

 movements to lift or lower the 

 head. If a bird, flying in the 

 direction of its axis, q, f, fio;. 36, , ^. ,,.,.«.,, ,.,.,., 



y J y J y O ^ ^ ? Action of tail in flight, cxxxi. 



brings the tail into the position 



h h, parallel to o n, the resistance of the air will depress h toward 

 k, and, causing the bird to rotate on its centre of gravity c, mil 

 raise the head from a towards Z. If the tail be moved into the 

 position h i, parallel to I k, the resistance of the air mil raise the 

 point b toward Ji and depress the head toward o. By partially 

 folding the fan, or bending the tail to one side, it may be made 

 to act like a rudder in the manifold modifications of the course of 

 flight. In Waders and Anserines the tail, represented by the 

 caudal quill feathers, is very short, and the office of the rudder is 

 transferred to the legs, which are extended backward in flight, 

 and counterbalance the long outstretched neck and head. 



In descending from a great height birds usually incline the 

 axis of the body obliquely downward, as in fig. 1, the resistance 

 of the air in a vertical direction upAvard equilibrating the force 

 of gravity acting upon the body vertically downward, so that the 

 motion of the bird becomes uniform without requiring any move- 

 ment of the wings. ' Another mode of descent is performed with 

 greater celerity by elevating the wings at an angle of nearly 45° 

 above the plane of the horizon, as in fig. 37, by which the resist- 

 ance of the air, compared with the resistance to the wing when 

 horizontal, is diminished in the ratio of the radius to the cube of 



I 2 



