416 



ZOOLOCiY 



describes a oo ; and since the bones of the wing — its most 

 rigid part — are at its front edge, the hinder edge tends to 

 lag Ix'iiind the front edge, so that the wing as a whole hits the 

 air in a plane which is not perpendicular but obliciue to the 

 direction of impact. Consequently on both the up and the 

 down stroke one component pushes backward and tends to 

 drive the bird forward (Fig. 387). In gliding, the wings are 

 spread, but are not flapped; progression depends ujjon an 



acquired velocit}', as in 

 *■ ^ aeroplanes, or upon the 



wind. In soaring, the 

 ^nngs remain motion- 

 less, and the bird does 

 not lose its velocity nor 

 tend to fall. The way 

 in which the bird is 

 supported and carried 

 along is uncertain. It 

 seems to depend upon certain favorable currents in the air. 

 Birds, like insects, have the closest economic relations with 

 man. A few of them, chieflj- Ijelonging to the orders of swim- 

 mers and grouse, are very important as human food ; but 

 most of them concern man on account of their feeding habits, 

 which are either favoraljle to man, as when noxious insects are 

 destroyed, or injurious, as when grain-fields are ravaged or 

 other Ijirds are preyed ujion. Unquestionably the vast ma- 

 joritj' of Ijirds are commercially ad\'antageous to man. The 

 Raptores are only partially so, for they feed entirely upon 

 animal food, chiefly liirds and small (usually destructi^'e) 

 mammals. The bobolink and the American crow, to be sure, 

 together annually destroy millions of dollars' worth of grain, 



Fig. .3<S". — Diagram showing resolution of 

 forces (.1) on down and on up strokes of the 

 wing ; tlie component B, in both cases exerts 

 a Ijackward pressure that drives the bird 

 forward ; while the component C keeps the 

 bird up. 



