430 APPLIED BIOLOGY 



have lost the power of flight (ostrich group), and it has almost 

 disappeared in many birds (ducks, geese, turkeys, chickens, 

 swans) which under domestication have become much larger 

 and heavier than in the wild state. The bodies of eagles, 

 vultures, and the other largest flying birds are not heavy; and 

 the appearance of great size is chiefly due to great expanse 

 of wings. Evidently lightness of body is essential. The 

 air-sacs, already mentioned, help to provide this in small 

 birds, while in larger birds there are also extensive air-spaces 

 in the bones. 



Wings of birds are specially adapted for flight, while the 

 posterior limbs (legs) are adapted for the support and move- 

 ment on land and in water. Since birds are bi-pedal (two- 

 footed), the legs are usually attached comparatively far 

 forward, so that the body is easily balanced. 



Legs. The modifications of these are correlated with 

 the mode of locomotion and other uses of the feet. The 

 following important types may be noted among common 

 birds ; (1) walking feet (ostrich) ; (2) wading feet (herons) ; 

 (3) climbing feet, two toes forward and two backward, 

 (parrots) ; (4) birds of prey (eagle) ; (5) for perching 

 (pigeon) ; (6) swimming feet, with toes partly or entirely 

 joined by webs (ducks, geese). Long wading legs may 

 have swimming feet, thus adapting certain birds to both 

 wading and swimming. However, the swimming feet are 

 usually on birds with relatively short legs. 



The beaks of birds vary extremely in shape, and are 

 adapted to procuring their special kinds of food. Among the 

 most common forms of beaks, some are adapted to hunting 

 for food beneath water, as is the habit of swimming and 

 wading birds ; some are fitted for eating seeds ; and some 

 are for insect catching. The pelican bill is adapted both 

 to catching fish and to storing in the attached pouch. The 

 bird-of-prey type, for tearing flesh, has a short, strong, and 

 hooked upper bill. 



