Chap. 36 



BIRDS — CONQUEST OF THE AIR 



731 



Fig. 36.2. King penguins. A penguin is a bird that swims with great speed 

 usually below the surface of the sea, and dives often and swiftly. A land animal, 

 it is also superbly aquatic. It is the result of an evolution of animals that swam 

 the sea with fins, that clambered onto the land and lived there for long ages, 

 climbed trees, and eventually could fly. Sometime in the succeeding millions of 

 years they returned to the water and now their wings work only like flippers. 

 Penguins cannot fly. (Photograph courtesy. New York Zoological Society.) 



development of a feather is described with other outgrowths of the skin in 

 Chapter 8. Pinfeathers, the common name for developing ones, are a source 

 of vitamin requirement. They are enclosed in a horny, pointed sheath, the 

 "pin" which breaks as the feather grows. The sheath and other castoff bits of 

 feathers are eaten during the bird's frequent oiling and cleaning of plumage. 

 These oiled fragments have usually been exposed to sunshine. Thus, while 

 preening their feathers birds treat themselves to irradiated oil containing the 

 fat soluble vitamin D. 



Feathers do not develop equally on all parts of the body. Except in a few 

 primitive species including the penguins and ostriches they grow in tracts 

 separated by bare skin (Fig. 36.3). It is likely that in the early ancestors of 

 birds the feathers were small and covered the whole body like those of penguins. 



Types and Functions. The contour feathers are the larger ones that con- 

 tribute most to the form of the bird. The outer ones covering the body and 

 limbs are the flight feathers of wings and tail (Fig. 36.4). The bases of the 

 contour feathers of wings and tail are usually protected by smaller covert 

 feathers. The tail is primarily a rudder for steering, but it has many forms and 



