34 THE FLIGHT OF BIRDS 
tail completely under fommand. 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 Pls. m1 and 1x). 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 tor the 
perfection of this balancer, a small bird could hardly 
land without danger to his eyes in the chevaua 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. 
