452 THE BIRDS xvi. 9- 



disturbed at its hinder ('trailing') edge and by eddies round the end 

 ('tip vortex'). The proportion of length to breadth (aspect ratio) in the 

 wing suitable for a particular type of flight depends on the need to 

 provide a sufficiently large undisturbed area. 



The shape of the aerofoil is of critical importance in determining its 

 aerodynamic capacities. For birds, as for aeroplanes, there are differing 

 shapes, suitable for various types of flight. To understand them we 

 must classify the means by which birds attain the necessary forward 

 velocity. First and most obvious is flapping flight. Though the details 

 of this are varied and not fully understood, it can be regarded as a 

 screw-like motion of the wings, providing forward and upward com- 

 ponents (p. 455). 



In still air the only alternative to flapping flight is to glide down- 

 wards, which obviously cannot continue indefinitely. Yet some birds, 

 such as the gulls, and especially the albatrosses and the buzzards, 

 condors, and other birds of prey, can be seen to soar for many minutes, 

 gaining height without flapping the wings. Lord Rayleigh showed how 

 they can do this by making use of the fact that the air is seldom still. 

 Theoretically the bird can use three types of air movement : (1) ascend- 

 ing currents, usually thermal ; (2) variations in the wind velocity at any 

 one level (gusts); (3) differences in wind velocity at different levels. 

 The first method is that used by human sail-planes and is certainly 

 adopted by many soaring land birds. The gustiness of the wind is 

 probably turned to advantage by gulls, rooks, and many other birds, 

 and the decrease in wind velocity near the sea surface is used by marine 

 soaring birds, notably the albatross. 



10. Wing shape 



A wing of the shape that allows an albatross or swift to make its 

 superb manoeuvres would stall immediately at the speed of flight 

 adopted by a crow. In discussing wing shape the chief factors to be 

 considered are (1) the wing area, (2) the aspect ratio (wing length/ 

 breadth), (3) the w 7 ing outline and taper, (4) the presence of holes or 

 slots, (5) the camber or curvature of the wing. 



1 1 . Wing area and loading 



A small wing area is necessary for fast flight, since the dragoc areaX 

 speed 2 , at least for high speeds. For this reason fast aeroplanes and 

 birds have small wings, but in the bird the fact that the wing provides 

 the forward momentum as well as the lift greatly complicates matters. 

 For flapping flight the wings must be moved relatively fast and for this 



