256 THE STRUCTURE AND LIFE OF BIRDS chap. 



they arc mainly beneath the level of the body, this 

 last means of balancing is out of the question, and the 

 bird must trust to head, waist, or tail. 



The question at what point between the head and 

 tail the centre of gravity lies is a very important one. 

 Probably if we divide the keel into three equal divisions 

 it will fall in the middle one, in a vertical line passing 

 a little behind the point at which the second rib joins 

 the trunk. It is impossible to decide exactly. It 

 has been supposed that birds can at pleasure shift the 

 centre of gravity backwards and forwards. To some 

 extent no doubt they can, but their power of doing so 

 has been much exaggerated. Any bird, but especially 

 the long-necked ones, can move the centre of gravity 

 forward by extending the neck to its full length. But 

 owing to the lightness of the skull this has not so 

 much effect as might be thought, except perhaps with 

 large-headed birds like ducks. Then again a long- 

 legged bird as he flies may move it in a backward 

 direction by trailing his legs full length behind him. 

 Many of them do habitually carry their necks or legs 

 stretched out during flight — their necks, possibly, in 

 order to shift the centre of gravity. The filling of 

 the air-sacks will have little effect, since they extend 

 before and behind the important point. The move- 

 ment of the wings forward or backward will, no doubt, 

 carry the centre of gravity to some extent with it. 

 These small changes, however, do not help us to 

 understand how the bird is able during flight to 

 maintain a horizontal position, for the main weight 

 still lies behind the points where the wings attach. 



Of the many means that have been suggested for 



