470 VERTEBRATE LIFE AND ORGANIZATION 



Figure 24.2. Birds in flight. A, A photograph showing the flapping flight of pigeons; 

 B, an osprev soaring. (A, U. S. Army photograph; B, photograph by Allan D. Cruick- 

 shank, National Audubon Society.) 



ascending thermal currents and differences in wind velocity to maintain 

 its forward movement. In each type of flight, the tail helps to support 

 and balance the body and is used as a rudder. 



Wing shape and area vary considerably with the species, depending 

 upon the size of the bird and the speed and type of flight. Since weight 

 increases as the cube of the linear dimension and surface as the square, 

 large birds need relatively larger wing areas than small birds. However, 

 the body and wings cannot increase in size indefinitely, for there is a 

 limit to the strength of the flight muscles. Birds that fly fast can and do 

 have a smaller wing area than others, for increased speed of the air- 



