A HISTORY OF VERTEBRATES: BIRDS AND MAMMALS 471 



Stream across the wing results in greater lift. Soaring birds, in contrast, 

 need larger wing areas. 



Most of the features of bird wings also apply to the wings of air- 

 planes. But a bird's wings and tail have one great advantage over those 

 of an airplane in that they can be varied considerably to adjust to dif- 

 ferent speeds and types of flight, for different angles of attack, and for 

 many other variables. An individual bird is much more versatile than a 

 single type of airplane. 



208. Structure of Birds 



There are few features of the anatomy of birds that are not directly 

 or indirectly related to flight. They are adapted structurally and func- 

 tionally to provide a high energy output in a body of low weight. 



Scales and Feathers. Birds have retained the horny scales of rep- 

 tiles on parts of their legs, on their feet, and, in modified form, as a 

 covering for their beaks, but the scales that cover the rest of the reptilian 

 body have been transformed into feathers. Feathers, like horny scales, 

 are epidermal outgrowths whose cells have accumulated large amounts 

 of keratin and are no longer living. Pigment deposited in these cells 

 during the development of the feather, together with surface modifica- 

 tions that reflect certain light rays, is responsible for the brilliant colors 

 of birds. Feathers, more than any other single feature, characterize birds, 

 for they are found only in members of this class. They overlap, entrap 

 air and form an insulating layer that reduces loss of body heat and helps 

 to make a high body temperature possible. Those on the tail and wings 

 form the primary flying surfaces. 



The contour feathers that cover the body or provide the flying 

 surface consist of a stilt, central shaft bearing numerous parallel side 

 branches, the barbs, which collectively form the vane (Fig. 24.3). Each 

 barb bears minute hooked branches, barbules, along its side, which 

 interlock with the barbules of adjacent barbs to hold the barbs together. 

 If the barbs separate, the bird can preen the feather with its bill until 

 they hook together again; thus the vane is a strong and easily repaired 

 surface ideal'for flight. In birds that have lost the power of flight, such 

 as the ostrich, booklets are not present upon the barbules, and the 

 feather is very fluffy. The proximal end of the shaft does not have barbs 

 and is known as the quill; much of it is lodged within an epidermal 

 follicle in the skin. The quill is hollow and blood vessels enter it during 

 the development of the feather. A small aftershaft, bearing a few barbs, 

 may arise from the distal end of the quill. 



Other types of feathers include the hairlike flloplumes, sometimes 

 visible on plucked fowl, and down feathers. Down covers young birds, 

 and is found under the contour feathers in the adults of certain species, 

 particularly aquatic ones. It is unusually good insulation for it has a 

 reduced shaft and long, fluffy barbs arising directly from the distal end 

 of the quill. 



Birds molt periodically. They lose their worn feathers and new 

 ones grow out from germinal tissue at the base of the follicles. Molting 



