460 



THE BIRDS 



xvi. 16- 



low aspect ratio, and wings broad at the tip and usually provided 

 with well-marked slots. 



17. Use of vertical wind variations 



The decreasing effect of surface friction causes the wind to blow 

 faster at greater heights and this phenomenon is used by some birds 

 at sea, where conditions are presumably more uniform than over the 



Fig. 273. Wings of the albatross (Diomeded), used for soaring flight. Wing very 

 narrow, with long upper arm region. (Drawn from photograph by Aymar.) 



land. The albatross (Diomeded) (Fig. 273) is the classic example of this 

 type of bird, proceeding in a regular series of movements, without 

 flapping the wings, downwind losing height and gaining speed and 

 then upwind gaining height and passing into a faster-moving layer. 

 Each downwind tack is longer than the upwind one. During the upwind 

 tack the wings are spread forwards, downwind backwards. The alba- 

 tross remains all the time within 50 ft of the sea surface, because the 

 variations in wind velocity are marked only at low levels. The 

 albatross shows the characteristics suitable for this type of flight, 

 namely, large size (it is the largest of all flying birds), great wing span 

 (11 ft), high aspect ratio (25), and pointed wing tips, without slots. 

 Other sea birds, such as the gulls, though not so highly specialized, 



