THE ENGLISH SPARROW AND ITS ALLIES 311 



form of the body is made conical, so as to offer little re- 

 sistance to the air, while, by varying the position of the 

 head, wings, and tail, the centre of gravity is quickly 

 shifted. In starting to fly, the bird sftiins an initial veloc- 



o / * o 



ity, if 011 the ground, by springing into the air, or if on a 

 tree, by combining the velocity due to gravity with a push- 

 ing from the limb ; but aquatic birds strike the surface of 

 the water with their wings. The best fliers have relatively 

 large, pointed wings. Three methods of flight are em- 

 ployed by birds when once in the air : () stroking the air 

 with the wing; (>) gliding or skimming ; and (c) sailing or 

 soaring. Some birds can use all three methods, and all 

 good fliers use the first two. In the stroke the wing moves 

 downward and forward, backward and upward, so that the 

 tip of the wing describes a QO; and the plane of the wing 

 constantly changes so as to push downward against the 

 air, and thus keep the bird up, and to push backward 

 against the air, and thus drive the bird forward. At the 

 same time, the wing must be brought back to its upper and 

 anterior position without offering great resistance to the 

 air. In gliding, the wings are spread, but are not flapped, 

 - progression depends upon an acquired velocity or upon 

 the wind. In soaring, the wings remain motionless, and 

 the bird does not lose its velocity nor tend to fall. Tne 

 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, chiefly belonging to the orders 

 of Natatores and Gallinacei, are very important as human 

 food; but most of them concern man on account of their 

 feeding habits, which are either favorable to man, as when 



