THE STRUCTURE AND ACTIVITIES OF BIRDS 339 



swimmer keeps at the surface by the movements of his arms. 

 These same strokes of the bird are made at such an angle that 

 the wings form an inclined plane, thereby propelling the body 

 forward (see insect flight, p. 10). When brought forward, the 

 wings are bent at the wrist joint, with the narrow edge in front, 

 so that very little resistance is encountered. 



Steering the Body during 

 Flight. — The wings also in 

 part steer the bird through 

 the air, since a more powerful 

 stroke on one side swerves 

 the body toward the other 

 side, just as does a stronger 

 pull on one oar of a boat. 

 Steering is, however, largely 

 done with the tail, which 

 serves as a rudder, directing 

 the body upward or down- 

 ward and from side to side 

 according to the position in 

 which it is held. 



How the Feet are Used. — 

 While on the ground birds 

 walk, run, or hop, and when 

 in trees, they cling to the (b) 



Fig. 216. — The structure of feathers 

 I. Contour feather. a, quill; 



vane ; 

 II. 



Contour feather. 



, shaft. 



Part of shaft (a) with two barbs 



III. Two barbules bearing hooklets 



(c). (From Coleman.) 



twigs with their claws. Their 



hind limbs must therefore be 



adapted for these various purposes, as well as for obtaining 



food, for building nests, and for fighting with other animals. 



Feet are Adapted to Various Purposes (Fig. 214). — In 

 birds that ordinarily perch on limbs the feet are strong and fitted 

 for grasping. Swimming birds have their toes entirely or par- 

 tially connected by webs. Wading birds have long legs and long, 

 slender toes which prevent them from sinking into the mud. 

 The toes of the birds of prey are very strong and bear sharp, 



