TIJH ENGLISH SPARROW 



415 



the centre of gravity is quickly shifted. In starting to fly, tlie 

 bird gains an initial velocitj', if on the ground, bj' springing into 

 the air, or if on a tree, by combining the velocity due to gravity 

 with a pushing from the limb ; but acjuatic birds strike the 



Fig. .38G. — Wood-thrush {Turdus nuistclinus). 



surface of the water with their -nings. The best fliers have 

 relatively large, j^ointed wings. Three methods of flight are 

 employed by birds when once in the air : (a) stroking the air 

 with the wing ; (b) gliding or skimming ; and (c) sailing or soar- 

 ing. Some birds can u.se 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 



