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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



The important thing to note is that though the air stream is still 

 being slow in filling the furrow cut by the wing, it is now being 

 slow in moving forwards and upwards, and the pull of the air on 

 the wing is in the reverse direction — backwards and downwards. So 

 now, a large part of the force of air reaction is in a backward and 

 downward direction, instead of being at 90° to the wing surface; 

 just what is not wanted as a rule. This state of affairs is known as 

 a " stall," and it always comes into existence when an unduly large 

 angle of incidence is used, either in gliding or flapping flight. 



An airplane whose wings are stalled commences to fall, owing to 

 the lack of "lift," and then to spin, on account of certain little- 

 known aerodynamic laws. But, in some of the more modern types, 

 this stalled descent can be controlled in such a way that, instead of 

 spinning, the machine descends on an even keel. This is precisely 

 what some species of birds can do by virtue of their separating 

 feathers, as I hope to show in this article. Without this separation 

 of the feathers they could not do it. 



III. EMARGINATION 



That nature had a definite purpose in view when she provided some 

 birds and not others with separating flight feathers becomes appar- 



FiGURE 3. — Above, 3rd flight feather of a buzzard ; below, of a 

 golden plover 



ent if the shape of such feathers is compared with that of corre- 

 sponding ones taken from a wing in which separation does not take 

 place. Figure 3 illustrates this comparison. Observe how the buz- 

 zard's feather (a separating one) is reduced in width from a broad 

 base to a much narrower tip, not gradually, but in a distinct step; 

 whereas the golden plover's (a nonseparating one) only narrows 

 down gentlj^ the whole way. The feathers illustrated are taken from 

 similar positions in the wing. 



This stepping down in width, known to ornithologists as " emar- 

 gination," is always present in the feathers of birds that have 

 separating flight feathers, sometimes on both webs of the feathers, 

 sometimes only on the front webs, while in certain feathers in any 

 particular wing it is confined to the rear edges. The terms " front " 

 and " rear " are used here, rather than the usual " outer " and 

 " inner," because we are considering the wing in its working position, 

 fully spread. 



