34 THE FLIGHT OF BIRDS 



tail completely under command. At a moment's 

 notice they spread it, make it concave, lower it or 

 raise it, lower this side or that. If the tail is spread 

 and lowered, at once the hinder part of the body 

 is lifted (see Pis. in and ix). The opposite effect 

 will follow from the raising of the tail, so that it 

 may catch the wind but little. If a Lark be watched 

 through a field-glass as he rises facing a fair breeze, 

 he will be seen to be perpetually busy with the work 

 of correcting his fore-and-aft balance ; and it is to 

 his tail that he trusts. A Chaffinch, perched on a 

 rail with a fairly high wind blowing in his face, 

 keeps his tail perpetually at work. The tail is, in 

 fact, a fine balancer, even more important in this 

 capacity than it is as a rudder. Were it not for the 

 perfection of this balancer, a small bird could hardly 

 land without danger to his eyes in the chevaux de 

 frise of a furze bush. 



There are other balancing movements still to be 

 considered. From our present point of view, the 

 way the wings are held is of great importance. I 

 have already pointed out that when a boat is 

 tacking (see fig. 4, p. 8) it is the front part of the 

 sail that does most of the work, and that similarly 

 when the gliding bird inclines his wings at a small 

 angle to the horizon, it is the front margin that 

 gives him most support. If, then, he holds his wings 

 fully expanded, so as to have as wide-stretching 

 a front as possible, not only will the lift be greater 

 but the centre of pressure (the point at which we 

 may consider the force of the wind as being, so to 

 speak, focussed) will move forward, and this will 

 tend to give the bird's body an upward incline. 



