298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1932 



last thing that is wanted. That being so, the twisting would be 

 reduced, but still a good deal would remain. That it does remain is \ 

 borne out by photographs (fig. 34) , and one can see it, by watching '• 

 closely, with the naked eye. ' 



The quicker the down beat, the steeper will be the gradient of the 

 air stream encountered by points situated near the tip of a wing, 1 

 unless the forward speed is correspondingly increased. Game birds, i 

 such as partridges, usually do fly at great speeds, but for the time i 

 being consider one that has not got up full speed. With its excep- I 

 tionally quick beat, one would expect its wings to be very much ^ 

 twisted in the down beat, but in the few poor photographs which are m 

 obtainable of these birds in flight, there appears to be even less j 

 twisting of the wings than in slower-flapping birds; so one is led to 

 suspect that the action of the slots is to allow the feathers that form ; 

 them to twist individually. This is almost the same action as that of 

 the wing-tip slots of a soaring bird, the main difference being that 



Figure 34.— The down beat seen from behind showing the twist in the 

 wings. Left, fantail pigeon; right, crane. (Sketched from 

 photographs.) 



practically the whole feather (except in the case of the rearmost 

 slotted one) is free to twist, because the unemarginated overlapping 

 parts are so short. 



It appears then that each separate feather works away by itself 

 just like a little wing of a very high aspect ratio (long and narrow)' 

 giving the bird the double advantage of saving wing-tip air spill 

 and weight; for a wing that could compete with the extreme twistin<r 

 that an unslotted partridge's wing would require would have to be 

 very strong indeed, and therefore heavy. 



Figure 35 shows what a section of the wing taken halfway along 

 tlie open slots might be expected to look like under these conditions, 

 rhe pecked lines show the direction of the air flow between the 

 leathers, and the arrows show the probable direction of the resultant 

 force reacting on each feather. They remind one rather of a row 

 of turbine blades. 



This action of the slots in the down beat seems to be applicable to 

 the flapping flight of all birds that have wing-tip slots, for the 

 feathers can easily be seen to separate in each stroke; at any rate 

 in such birds as rooks and crows. By careful watching it can even be 



