734 EVOLUTION OF ANIMALS Part V 



body is left bare of feathers. Its details vary in different species and within one 

 species with age, sex, and other physiological conditions. The molt of a 

 feather is the stimulus to the growth of another in its place, but the succeeding 

 one may be different from its predecessor. In its first winter plumage, the male 

 scarlet tanager is olive with brown wings and tail. In the following spring, 

 these feathers are replaced by scarlet and black ones. All adults of the smaller 

 land birds undergo at least one annual molt at the end of the breeding season 

 when their plumage is entirely renewed. The large and important wing feathers 



Fig. 36.5. Cross sections of a barb from a blue feather of an Ant Thrush (7) 

 and Cotinga (2), greatly magnified. A layer of reflecting cells on the upper sur- 

 face of the barb is backed by cells containing black pigment. Whether shades of 

 blue are light or dark depends upon the amount of black pigment that is present 

 and how it is distributed in the cells. (Courtesy, Allen: Birds and Their At- 

 tributes. Boston, Marshall Jones Co., 1925.) 



are molted less often than any others. Those of the wings and tail are typically 

 molted in symmetrical pairs making the least disturbance to flight. Molting is 

 mainly under the influence of the thyroid and pituitary glands. 



Special Adjustments 



Bill and Food. A bird's bill is its mouth, lips, teeth, and nose, and in use 

 takes the place of hands. With it birds get their own food, feed their young, 

 preen and oil their feathers, defend themselves and build their nests (Fig. 

 36.9). 



Of all uses of bills, feeding is the most important. Birds are high-geared 

 engines running at a rate that in mammals would be fever heat and only 

 plenty of the most nutritious foods is adequate for them. These are mainly 

 seeds and animal tissues. Seeds are stored with oil and starch. The meat is of 

 many sorts, worms and insects, fish, mice and other small mammals, all of it 

 high in protein. 



Bills tell what birds eat and where they find it (Figs. 36.6, 36.7). A crow's 

 bill is an all-round tool for miscellaneous food. Crows dig up corn, crack nuts, 

 break eggs, and pick and tear at various refuse. With the same kind of bills, 

 starlings are also markedly successful in getting a living. 



Many of the carnivorous birds are fish eaters. The American bittern of the 

 watery bogs spears both frogs and fishes. The edges of the bill of the fish- 

 eating merganser duck are deeply saw-toothed, once in its grip the most slip- 



