SAFETY DEVICES IN BIEDS' WINGS GRAHAM 275 



The emargination obeys certain definite rules. The front web of 

 the first flight feather, the first visible primary, is never emarginated ; 

 where any marked separation takes place, the rear web is. Then, 

 in some wings the front web of the second feather is the only other 

 emarginated one, but in others varying numbers of feathers have 

 steps in both webs — five appears to be the greatest number — while in 

 all cases the hindmost feather that has a step has it only in the front 

 web. The result of this arrangement is that when the wing is fully 

 spread the outer parts of the feathers do not overlap and gaps or 

 " slots " form between them as shown in Figure 4.* 



For some reason, in the wings of certain species of birds nature 

 has taken particular care that these slots shall be of fair width, even 

 at their inner extremities, where they might reasonably be expected 

 to form very acute angles, owing to the fact that the feathers radiate 

 from a fairly small center, the hand of the wing. She has achieved 



Figure 4. — Right wing-tip of a buzzard seen from below. 

 The front webs of the feathers are drawn black for 

 emphasis 



this result by making the webs of the feathers narrower just outside 

 the steps than they are farther out toward the wing tips. The 

 effect is that the margins of the slots are more nearly parallel than 

 they would otherwise be and the inner extremities squarer (fig. 5). 

 Though not found in quite all emarginated feathers, this re- 

 markably careful shaping is often to be seen in the feathers of birds 

 with well-developed slots. Its purpose has perhaps something to 

 do with the " drag " that would be induced by air rushing at high 

 speed through a narrow space, with silence in flight, or with the 

 need for a good flow of air throughout the whole length of a slot 

 in order that the full benefit may be derived from it. The slots in 

 the wings of pigeons are good examples of the type that lacks this 

 careful shaping, and these birds are noticeably more noisy in flight 

 than many others. (For comparison see fig. 36.) It also looks as if 



* In some birds the emargination of the rear webs is very indistinct, particularly in the 

 feathers that form the hindmost slots. The pheasant's wiug is a good example. 



149571—33 19 



