CLASS AVES 



411 



The tip of the wing traces a path in the air which is characterized by 

 long downward and forward strokes, alternating with shorter upward 

 and backward strokes. This action is modified in other modes of flight. 

 After making several strokes, some birds hold their wings motionless 

 and glide for a considerable distance before again making several more. 

 Thus gliding is a second form of flight. In some cases before a high 

 wind a bird will partly flex the wings and permit itself to be carried by 

 the wind. This is a form of flight known as flex gliding. Another 

 modification of flight is known as soaring, characterized by the bird, 

 usually at a high elevation, describing great circles without any move- 



FiG. 299. — Restoration of a bipedal dinosaur, Ornithomimus. 



"Reptiles and Amphibians.") 



{Redrawn from Barbour, 



ments of the wings whatever. As it describes these circles it gradually 

 works along with the wind. There is no doubt that soaring is usually 

 due to the bird taking advantage of the upward rush of currents of 

 air, though it may be that the bird can soar by taking advantage of a 

 wind blowing horizontally. Still another form of flight is known as 

 hovering, in which the bird remains poised in the air before a flower or 

 above an object upon the ground, the tip of its wings apparently describ- 

 ing a figure eight. 



430. The Bird as a Flying Animal. — A bird flies on the principle 

 of an airplane, or heavier-than-air machine, rather than on that of 

 a balloon, or lighter-than-air machine. Such a machine requires light- 

 ness and rigidity, which are secured by the character of the bird's skeleton. 



