A HISTORY Of VERTEBRATES: BIRDS AND MAMMALS 



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the anterior edge, i.e., increasing its angle of attack. However, increas- 

 ing the angle of attack also disturbs the airstream in such a way that it 

 causes the formation of eddies above the wing. This turbulence produces 

 a drag that tends to reduce lift; however, this can be minimized if the 

 front of the wing is thick and stiff and the posterior margin is thin and 

 trailing. Providing wing slots by spreading the posterior feathers apart 

 slightly, or elevating a group of feathers at the anterior edge of the 

 wrist (the alula), also smooths the airstream and reduces turbulence. 

 When birds are flying rapidly, the speed of the airstream provides suffi- 

 cient lift and the wing need not be tilted greatly. But during takeoff 

 or landing, when speeds are necessarily low, the angle of attack of the 

 wing must be increased and slots must be formed to give increased lift. 

 Some birds obtain additional lift on landing by fanning out the tail 

 feathers and bending them down. The tail, then, acts both as a brake 

 and as high-lift, low-speed airfoil. 



The wings not only provide the lift, but they are also the pro- 

 pellers. In the familiar flapping flight (Fig. 24.2), the up and down 

 movement of the wings relative to the body of the bird is responsible 

 for the forward movement, but the wings do not simply push back 

 against the air as a swimmer would push back against the water. On the 

 downstroke, they move down and forward; on the upstroke, up and 

 back. As a wing moves down, the air pushes up against it and the more 

 flexible posterior margin of the distal part of the wing is twisted up. 

 The distal portion of the wing twists the opposite way on the up- 

 stroke. The twisting of the distal portion of the wing gives it a pitch 

 comparable to that of a propeller and this, together with the movements 

 of this part of the wing, is responsible for the forward motion. In soaring 

 flight, the wings are held still and the bird skillfully makes use of 



Figure 24.1. The effect of wings on the airstream. In A, the wing is held at such 

 an angle that the airstream flows smoothly across it. The air flows more rapidly over 

 the upper surface than across the under surface. This creates a low-pressure area above 

 the wing that provides a lift force. In B, the wing is held at such an angle that lift- 

 reducing turbulence and eddies form above it. (Modified after Young.) 



