FEET OF BIRDS. 519 



of gravity is thus transferred alternately from one 

 foot to the other : hence, in walking, the head of a 

 bird is in constant motion ; whilst the duck and 

 other birds, whose legs are ver}^ short, have a 

 waddling gait. It may be observed that the more 

 perfectly predaceous birds are not the best formed 

 for walking ; because, were they to use their feet 

 for that purpose, their talons, which are required 

 to be kept sharp for seizing and tearing their prey, 

 would be blunted ; and accordingly the Eagle, 

 when moving along the ground, supports itself 

 partly by the motion of its wings. 



In roosting, birds often support themselves on 

 their perch by means of one leg only, the other 

 being folded close to the body. They even main- 

 tain this attitude with greater ease and security 

 than if they rested upon both feet. The true expla- 

 nation of this curious fact was long ago given by 

 Borelli. On tracing the tendons (t, t. Fig. 233) of 

 the muscles (m, m) which bend the claws, and 

 enable them to grasp an object, we find them 

 passing over the outer angles of each of the inter- 

 vening joints, so that whenever these joints are 

 bent, as shown in Fig. 234, those tendons are put 

 upon the stretch, and mechanically, or without 

 any action of the muscles, tend to close the foot. 

 When the bird is on its perch, this effect is pro- 

 duced by the mere weight of the body, which, of 

 course, tends to bend all the joints of the limb on 

 which it rests ; so that the greater that weight, the 

 greater is the force with which the toes grasp the 

 perch. All this takes place without muscular effort 

 or volition on the part of the bird. It remains in 

 this position with more security on one foot than it 



