CHARACTERS AND HABITS OF BIRDS 13 



twenty to one hundred fifty downward strokes of its wings a 

 minute. As each downward stroke must be preceded by an up- 

 ward stroke, this means that the wings make from two hundred 

 forty to three hundred separate movements a minute. In such 

 swift-flying birds as the pigeon the movements of the wings 

 can be distinguished but cannot be counted. The fastest move- 

 ments of the wings are not made by the swiftest fliers. In order 

 to fly at all some land birds with comparatively small wings have 

 to move them so fast "that the movements make a blur and a 

 whirring noise. The partridge is an illustration of a bird of 

 this class. 



If the supporting surface of the wing of a bird were made of 

 skin, like the web of the foot of a swimming bird, it would be 

 necessary to fold the wing for each upward stroke. It is here 

 that the structure of feathers adapts itself to the rapid action re- 

 quired for movement in the air. The wing is not one surface 

 but a series of narrow surfaces lapping in such a manner that 

 they unite to form one broad surface when the downward stroke 

 is made, and with the upward stroke are separated so that the 

 air passes between them. Greater power in the downward stroke 

 and less resistance in the upward stroke are also secured by the 

 curvature of the wing. The under side is concave, the upper 

 side convex. Thus in the downward stroke the wing gathers 

 the air under it and so increases the pressure, while in the up- 

 ward stroke it scatters the air and reduces the pressure. 



If the wing were equally rigid throughout, the movement of 

 the bird would be mostly upward. The bird in flying moves 

 forward because the front of the wing is rigid and the tips of 

 the feathers, which are directed backward, are flexible. So the 

 air compressed by the wing in the downward stroke escapes 

 backward, and in doing so propels the bird forward. The 

 principle is the same that is applied in the screw propeller of 

 a boat or an airship, except that the wing vibrates while the 

 propeller revolves. 



