294 ANNUAL EEPOBT SMITHSONIAN INSTITUTION, 19 3 2 



It is interesting to note that the slotted areas of a good many of 

 the wings shown in Figure 28 bear a distinct resemblance to the 

 affected areas of wings A and B. 



But why should we confine ourselves to small birds in considering 

 this theory ? Surely all birds that have separating wing-tip feathers 

 must derive a certain amount of this benefit from them. The idea 

 is supported by the fact that some of the really big birds that have 

 long and narrow wings, compared with, say, a wheatear, are well 

 supplied with slots. Vultures, cranes, and swans are good exam- 

 ples (fig. 30). Now compare the shape of their wing tips with 

 those of the big birds that have no slots, the sea birds ; the unslotted 

 wings are, without exception, the more sharply pointed. Square tips 

 are large tips, and the loss from them will be large unless they are 

 slotted. 



The reason why some birds, and not others, can afford to have 

 pointed tips on their wings is not too clear, but it seems that a 



t 



-^ 



Figure 30. — Left, crane; right, swan. (Sketched from photographs) 



pointed tip must be longer than a square slotted one to have the same 

 value; and whereas a bird that always flies in the open, such as a 

 sea gull or a swallow, will not find that his long wings get in the 

 way, one that lives among trees and bushes, and other things that 

 obstruct the air, if so equipped, would find them a decided encum- 

 brance. 



So the root of the matter would appear to be this : That if his 

 method of living will permit, a bird will have long, narrow, pointed 

 wings of efficient airfoil shape because that is the nearest he can 

 get to the ideal wing; but if he must have shorter ones to suit his 

 environment, he can not afford to have them pointed, because such 

 a shape would deprive him of some of his wing area; therefore, in 

 order to prevent the great waste of surface that the spilling of the 

 air over a broad wing tip occasions, he must have it split up into 

 a number of small airfoils of efficient shape. 



Incidentally, this " shaping " of the wing is known in aeronautical 

 circles as the aspect ratio. A long, narrow wing is said to have a 

 high aspect ratio, and a short-broad one a low aspect ratio. The 



