xvi. i 3 ASPECT RATIO 453 



small size is an advantage. On the other hand, a large wing area allows 

 slow flight (lift oc area X speed 2 ) and is found in hawks, vultures, 

 storks, and other birds that fly slowly to hunt, especially if they soar 

 on thermal currents, for which a large lift is necessary (p. 458). 



The loading of the wing varies considerably. Since the weight 

 increases with the cube but the wing area only with the square of the 

 linear dimensions, it follows that large birds must have relatively larger 

 wings than small. However, the larger birds usually have a heavier 

 loading of the wing, for instance, 10 kg./m. 2 in the duck (Anas), 20 in 

 the swan (Cygnns) , 1 in the goldcrest (Regains), 3 in the crow (Corvus). 

 A considerable 'safety margin' remains in most birds; for instance, 

 pigeons were found to be able to fly until as much as 45 per cent of 

 the wing surface was removed ; hawks and owls have an especially high 

 safety margin and they can carry prey almost as heavy as themselves. 



12. Aspect ratio 



Although a small wing area reduces drag, many fast-flying birds 

 have a large wing-span. The aerodynamic advantages of this allow a 

 low rate of descent when gliding, reducing the expenditure of energy 

 necessary to sustain flight. High aspect ratio is therefore found in birds 

 that fly fast by flapping flight (swifts and swallows) and especially in 

 those, such as the albatross, that glide fast in order to obtain sufficient 

 kinetic energy to convert into altitude. However, these wings with very 

 high aspect ratio stall at relatively high speeds and the birds that soar 

 slowly on thermal up-currents over the land mostly have a low aspect 

 ratio. Some figures for aspect ratios are: 



Albatross (Diotnedea) . . .25 



Gull (Lartis) . . . .11 



Swift (Apus) . . . .11 



Shearwater (Puffinus) . . .10 



Vulture {Neophron) . . .6 



Rook (Corvus) . . . .6 



Sparrow (Passer) . . . .5 



13. Wing tips, slots, and camber 



A pointed wing tends to stall first at its tip and is therefore only 

 suitable for fast fliers. Such birds show great development of the hand 

 feathers, producing a long narrow wing, whereas birds built for slower 

 flight and manoeuvre have a shorter broader wing with long arm 

 feathers (Fig. 263). 



The condition of the air around the wing is of first importance for 

 the maintenance of lift; if there is not a smooth stream over the upper 

 and under surfaces the air becomes turbulent, and the aerofoil stalls 

 (Figs. 261-2). This tends to happen either if the speed falls too low or if 

 the angle of the wing relative to the line of motion increases above about 



