SAFETY DEVICES IN" BIRDS ' WINGS GRAHAM 281 



tain their " lift," ^ and they are not of the most suitable type for 

 the work, because the teal has far higher wing loading (smaller 

 wings for his weight) than most birds. 



The production of a big braking effect requires a strong back- 

 ward-inclined reaction from the air; that means a large angle of 

 incidence (fig. 14), almost certainly larger than the stalling angle of 

 the wings. In these circumstances, even if they do stall, it does not 

 matter much as regards " braking," for though the total resultant 

 produced may fall in value, it will be all in the right direction — 

 backwards. But it does matter from the point of view of " control," 

 and that is where the slots come in. They insure that part, at 

 least, of the wing, and that the most important part for controlling, 

 will not stall. So this preliminary glide down toward the water 

 appears to develop into a stalled, yet controlled descent, with the 

 body in a horizontal position, or even slightly tilted up in front, 



'l3ir«.cr.'orx o| ^K^kf ^ 



Figure 14. — Diagram showing liovv braking eflfect increases witli ttie 

 angle of incidence. Force R2 points more backward than Rl because 

 angle alpha is greater than beta 



not inclined downwards as in a true glide or dive. One can often 

 see commoner birds, notably rooks {Corvus frugilegus), carrying 

 out the same maneuver. 



Having, in this way, reduced his speed somewhat, the teal finds 

 that the controlled stalled descent is going to bring him on to the 

 water with too much downward speed for comfort. To overcome 

 this trouble he starts flapping his wings, at first with very small 

 strokes, little more than a quivering of the wing tips, then gradually 

 increasing the movement until it is almost as vigorous as when he is 

 getting under way at the beginning of a flight. The true reason 

 for the need to start flapping is that he has reduced 

 the speed of the air past his wings so much that they are un- 

 able to derive from it the necessary force to obtain braking effect 

 and " lift," and that therefore the wings themselves must be moved 

 to increase the lift. The movement which has to be made up for is a 

 forward one, so the wings must be moved forward; that means a 

 forward and backward flap, which is the form of flapping flight 

 often used by birds when alighting on windless days. 



^ Probably the spreading of the webbed feet, ready to continue the " braking " In the 

 water, assists the wings slightly in this. 



