THE STRUCTURE AND ACTIVITIES OF BIRDS 343 



easily. An examination of the bones in a bird's foot shows at 

 once that birds walk on their toes, that is, are digitigrade, and 

 not on the sole (plantigrade), as in man. 



ACTION or TOES WHILE PERCHING. - - When at rest, the birds 

 often maintain themselves for hours perched on a limb, with the 

 toes holding the body upright. This would soon tire the muscles 

 if it were not for a special mechanism which automatically causes 

 the toes to grasp the perch. The tendon which bends the toes 

 passes over the 

 back of the ankle 

 joint. The weight 

 of the body bends 

 this joint, draws 

 the toes around 

 the perch, and au- 

 tomatically holds 

 the bird firmly in 

 place. 



How the Beak 

 is Used. - - Birds, 

 like turtles, are 

 toothless, and the 

 jaws are covered 

 by horny sheaths 

 which constitute 

 the beak. The 

 beak of the bird performs many of the functions of human 

 hands : it is used to obtain food, build nests, preen the 

 feathers, care for the young, etc. The bird must be able to move 

 its head freely if the beak is to succeed in accomplishing ail 

 these duties. It is able to do this because the neck is compara- 

 tively long and the vertebrae in it move easily upon one another. 



BEAKS ARE ADAPTED TO VARIOUS PURPOSES. Just as the 

 feet differ in different birds according to the habits of the species, 

 so the beaks are much modified for particular purposes (Fig. 215). 



FIG. 221. Eggs of whippoorwill laid on dead leaves 

 on the ground in the woods. (Photo, by Hegner.) 



