The feet of these birds are grasping rather than 

 climbing feet. The exterior front toe is reversed, so 

 that the toes act two against two, which is the reason 

 why the birds are called zygodactylic, or yoke-toed. 

 In some of the species the feet are used in the same 

 manner as the grasping feet or hands of the tree 

 mammalia ; and as some of these climbing mammalia 

 use the prehensile tail as a fifth hand, some of the 

 climbing birds use the bill or beak as a third one. 

 Those which have that habit, have the upper mandi- 

 ble hooked at the tip ; and the acting parts of both 

 mandibles are short, but very strong, and act with 

 much force, like nutcrackers. The muscles which 

 move these powerful mandibles, give considerable 

 enlargement to the sides of the head. Those which 



have this habit are mostly vegetable feeders, and live 

 among the twigs, nestling in the holes of old trees. 



