60 THE FLIGHT OF BIRDS 



at a moment when they were not held symmetrically. 

 Another plan is to bend sideways at the waist, 

 so as to move the centre of gravity towards one 

 side. I was long doubtful whether this plan was 

 actually adopted. At length I have obtained a 

 photograph of a Gull, taken from below, where the 

 waist, thus bent, is clearly shown, but unfortunately 

 this photograph will not stand reproduction. Below 

 I give some measurements which show that birds 

 that are good steerers have greater suppleness of 

 waist than clumsy steerers. 



The tail also is undoubtedly used for steering, 

 though, I believe, it is more frequently of service 

 in maintaining or restoring equilibrium. The rule 

 is, as I have shown, that if the legs are long the 

 tail is small, and as it is indisputable that long 

 legs are very useful for balancing and of but little 

 use in steering, we may infer that the tail is more 

 a balancer than a rudder. Nevertheless, it is of 

 much use in steering. Though its edges look to 

 right and left, it can be made an effective rudder 

 by the lowering of one side more than the other. 



Web-footed birds probably use their feet occa- 

 sionally for steering purposes, but I doubt if their 

 action counts for very much. When a Duck, for 

 example, is hurtling through the air, if he lowers 

 one foot, the resistance of the air will double it up, 

 not expand it. It is very different when a Duck 

 in swimming kicks backward ; the water acts 

 upon the under-surface and spreads the webs to 

 their fullest extent. Some birds when alighting 

 use their feet in the same way. A Gannet, for in- 

 stance, kicks hard in order to correct his balance 



