290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



itself at a similar angle of incidence. In this way the direction of 

 flow of the air stream is changed step by step through a greater 

 angle than the stalling angle, without the burbling that would cer- 

 tainly occur if the attempt were made to do it all in one act. 



Each feather is acting for the benefit of its " next astern " in the 

 same way that a Handley-Page auxiliary winglet does for an air- 

 plane's wing; while it is, at the same time, producing a useful reac- 

 tion in an upward direction, with either a slightly backward or 

 slightly forward inclination, depending upon its position in the wing. 



The reason for the bending up of separated feather tips has 

 been discussed, but the question whether they serve any useful purpose 

 in so doing still remains. There can be little doubt that when so 

 bent they improve stability at low air-speeds. The surfaces of the 

 blades of the feathers, instead of facing upwards and downwards, 

 point more or less sideways, and so they become little keel surfaces, 

 and, placed as they are at the ends of the long levers of the wings, 

 their effect must be considerable. Keally, they serve the same pur- 

 pose as the " dihedral angle " (upward inclination of the wings from 

 root to tip) used by aircraft designers to give lateral stability. 



VIII. THE RELATION BETWEEN SLOTS AND THE SHAPE OF WINGS 



It was observed at the beginning of this paper that slots are not 

 particularly noticeable in the wings of small birds in flight. The 

 reason for this is that, the eye fails to see them because they are 

 very small, and the wings usually move at a great speed. The truth 

 is that man}' of the small birds are very well equipped with slots. 

 A blue tit, for instance, has five; a song thrush {Turdus philo77ielus) 

 three; the robin {Erithacus rubecula), tree creeper {Gerthia fa- 

 miliaris) and long-tailed tit {Aegithalos caudatus) have four; but in 

 none of these birds is their development so marked as in some of 

 their large relations. Figure 26 shows two views of a thrush's wing 

 with its slots fully opened, and Figure 27 similar views for com- 

 parison of the unslotted wing of a swallow at full spread. 



As a rule, the slots of small birds are formed more by the emargi- 

 nation of the front webs of the feathers than of the rear ones, but 

 these rear webs are usually so thin and flexible that they must be 

 very easily persuaded to blow upwards, in such a way as to clear the 

 leading edge of the next feather behind. In molted feathers, one 

 often finds that the trailing edges have been worn to shreds opposite 

 the emarginated front web of the next feather in rear by continual 

 engagement and release with it. 



Small birds probably derive a certain improvement in lateral 

 control from their slots, but they do not often appear to carry out 

 the stalled descent, and they certainly never do anything in the 



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