CLASS AVES 



399 



Air sacs do not lighten the bird's body to any appreciable extent when 

 the bird is in the air, as is often stated, though they do lighten it when 

 the bird is swimming. Another requirement of an airplane is the posses- 

 sion of planes for support; in a bird the wings and tail furnish such planes, 

 giving a broad surface for support and also being capable of adjustment 



Fig. 274. — Beaks of birds. From mounted specimens. A, generalized beak of ring- 

 necked pheasant. X J^. B, straining beak of canvasback duck. X %. C, spearing 

 beak of bittern. X }4- D, probing beak of greater yellowlegs. X J^. E, beak of 

 brown pelican. X J^. F, chisel -like beak of hairy woodpecker. X /i- G, carnivorous 

 beak of Swainson hawk. X 3i- H, insectivorous beak of nighthawk. X ^i- I, 

 insectivorous beak of myrtle warbler. X %■ J, graminivorous beak of black-headed 

 grosbeak. X ,?^. K. beak of red crossbill. X %. 



to variations in the direction and strength of the wind. A third necessity 

 in such a machine is the development of a large amount of sustained 

 power; this is secured by the great size of the flight muscles and by the 

 very effective aeration of the blood, which results in rapid and continuous 

 oxidation. The large size of the heart and the relatively great capacity 

 of the blood vessels also contribute to the same end. Still other char- 

 acteristics which contribute to the considerable and constant production 

 of energy are the effectiveness of the processes of digestion and elimina- 



