SAFETY DEVICES IN BIRDS WINGS — GRAHAM 



283 



so low as to have but little effect upon it). That means a large 

 angle of incidence and the need for the slot once more to prevent 

 the stalling of the wing tips. 



It is worth noting here that the wind tips are doing nearly all 

 the work, because the inner parts, being close to the body, can not be 

 flapjjed through an arc large enough to produce the air speed which 

 is essential for the production of force from the air; therefore, it 

 is doubly important that the best should be got out of the tips. The 

 slot allows this to be done by permitting the use of a large angle of 

 incidence. 



VI. OPENING AND CLOSING OF WING-TIP SLOTS 



The study of how wing-tip slots are opened and closed is most 

 interesting, because it discloses the presence in a bird's wing of one 

 of the most cunning, economical, and amaz- 

 ingly effective devices imaginable. 



A bird at rest can spread its wings sufficiently 

 for the slots to open fully ; one can see the great 

 birds of prey at the zoo doing it almost any 

 day. That is evidence that birds certainly are 

 provided with the necessary muscular equip- 

 ment for the movement, but it does not follow 

 that the}^ use it for that purpose in flight. Here 

 is evidence that they do not. If one takes the 

 wing of a freshly-killed rook, for example, 

 spreading it so that the feather tips are just not 

 separating, and holding it at a large angle of 

 incidence (in the nature of 25°) to the draught 

 from a powerful electric fan, the air stream 

 will open the slots by blowing up the broader 

 rear webs of the emarginated parts of the 

 feathers. If, on the other hand, this wing is 

 held with the feathers loose and not pressed together, it can easily 

 be spread so far that gaps appear between the broad parts of the 

 feathers on the body side of the steps in the webs, as in Figure 17. 

 That is, one can overspread it. 



That is how a bird at rest appears to stretch its wings — with the 

 feathers not pressed together. But, if one personally takes the 

 place of the air stream which would be met in flight, and holds the 

 wing so that each feather is pressing up against that which over- 

 laps it, and then one tries to spread the wing as far, it will be found 

 that a brake is quite suddenly put on which seems to lock all the 

 emarginated feathers in the " slot-fully-open " position. Only by 

 tearing apart the barbs of the front webs, where they still overlap 

 the rear webs, can one effect any further spreading. The secret of 



FiGORE 17. — Lower sur- 

 face of a song-thrush's 

 right wing-tip, with 

 the slots more than 

 fully opened. Gaps 

 appearing beyond the 

 inner limits of the 

 slots and barbs being 

 torn apart from each 

 other are shown 



