286 THE STRUCTURE AND LIFE OF BIRDS CHAP. 
they are 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 
