BIRDS. 



559 



sistance of the air causes it to be supported by 

 the wings. When the bird is resting upon its 

 legs, the centre of gravity must, in like manner, 



be brought immediately over the base of support 

 formed by the toes : it becomes necessary, there- 

 fore, to provide means for shifting the centre of 

 gravity from one place to another, according to 



