BIRDS 375 



feathers are thus attached to definite regions of the body, 

 they ordinarily cover the surface, and of course prevent 

 undue loss of heat. It is evident that the warm- 

 blooded type of organization developed along with the 

 hair or feathers which helped to conserve the heat and 

 protect the body from rapid changes of temperature. 

 In the case of those mammals which have lost the hairy 

 covering, special arrangements attaining the same end 

 are found, thick layers of fat in whales and porpoises, 

 clothes and houses for man. No birds are as naked as 

 man, but some have large bare areas. The turkey 

 vulture, for example, has the head and neck bare, 

 because its habit of feeding upon carrion would make 

 it impossible to keep feathers in that region decent. 

 The colored bare areas about the heads of various birds 

 appear to serve for ornament. 



3. A typical large feather, used in flight, consists of structure 

 a main shaft, from which arises on each side an oblique ^feathers 

 series of barbs. These barbs can be seen under the 

 microscope to be compound, giving rise on each side 

 to a series of barbules. Thus the structure resembles 

 that of a bipinnate leaf. The barbules, however, are 

 provided with little hooks, the barbicels, which hold on 

 to the barbules of the adjacent barbs and thus keep the 

 surface of the feather intact, enabling it to resist the 

 pressure of the air. On closer examination, it is seen 

 that not only do the various feathers on a single bird 

 differ in structure, but different kinds of birds have 

 different feathers. If we knew nothing of birds but 

 their feathers, it would be possible to construct a fairly 

 accurate classification. The colors of feathers, like 

 those of the scales of butterflies, are due partly to pig- 

 ment and. partly to structure. Pigment is coloring 

 matter which may be extracted, corresponding to a dye. 



