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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



this careful shaping is designed to prevent wear on the edges of the 

 feathers, for it must cause the whole slot to open at almost the same 

 moment, instead of the separation starting at the tip and working 

 inward, with consequent chafing. 



IV. BENDING AND TWISTING OF SEPARATED FEATHERS 



Figures 6 and 7 show examples of separation. These birds may 

 either be making a downward beat or just gliding, the camera does 

 not tell us for certain, but for the present purpose that does not mat- 



Under-surface of right 



wing-tip of a Ruddy 



Sheld-Duck. 



2nd mght- 

 feather of a 

 Partridge, 



Figure 5.— Typical examples of emargination being 

 greater near the step than at the wing-tip. Both 

 feathers narrower at A than at B 



ter. All that is required is to know that the air stream is striking 

 the wings from below. That it is doing so is quite evident. 



In both birds the separated feathers are distinctly bent in an 

 upward and forward direction, and at the same time twisted in such 

 a way that their leading edges lie lower than their rear or trailing 

 edges. First consider the twisting alone. Since no feather has 

 muscular power in itself, this effect must be due to the reaction of 

 the air which the feathers are displacing. The wing itself is held 

 or moved by its owner in such a way that the air stream is striking 

 it at an angle from below and in front, the angle of incidence. 

 The roots and the overlapping parts of the feathers are embodied 

 in and supported by this main part of the wing, but the separated 

 outer portions lack mutual support and are to a certain extent at 



