CHAPTER XXXIV 

 THE STRUCTURE AND ACTIVITIES OF BIRDS 



Birds are to the majority of people the most interesting of 

 all vertebrates. They are easily distinguished from any other 

 animals by the presence of feathers. Besides this, most birds 

 possess wings and are able to fly. Their excellent locomotor 

 powers have enabled birds to become distributed all over the 

 world and to establish themselves in every habitable region. 



The Body Built for Flight. — The body of a bird is in general 

 built for flight (Fig. 211). Its shape is such as to offer the least 

 resistance to movement through the air, and its bones are closely 

 united, giving the rigidity required by a body supported only by 

 air (Fig. 212). In the first place, although the vertebras in the 

 neck move freely upon one another, those of the back are closely 

 united, forming a firm axis (Fig. 213). The thorax is strongly 

 supported by ribs which are entirely of bone and are held to- 

 gether by projections called uncinate processes. The long wing 

 bone, although able to move freely, is more firmly connected 

 with the shoulder girdle than is the bone of the fore limb in 

 other vertebrates. Finally, the breastbone or sternum bears a 

 projection or keel to which the enormous wing muscles are at- 

 tached, and many of the bones are hollow and therefore decrease 

 the specific gravity of the body. These features are all corre- 

 lated with the flying habits of the birds. 



The Wings as Organs of Flight. — The principal organs of 

 flight are, of course, the wings. When devoid of their feathers, the 

 wings of a bird seem quite ineffective, but the bones within them 

 are very strong and closely knit together. These bones are 

 similar to those in the fore limbs of other vertebrates except the 



335 



