270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1932 



Secondary Feathebs. — The main feathers that spring from the forearm of a 



bird's wing. 

 Shaft. — The horny quill which extends from root to tip of a feather. 

 Slots. — Are of two kinds, the wing-tip slot and the wrist slot. 

 Span. — Of a bird ; the distance between the fully extended wing tips when the 



wings are at full stretch. 

 Stalling. — The process which occurs when an unduly large angle of incidence 



is used. It causes a sudden loss of lift and increase of head resistance. 

 Trailing Edge. — The rear margin of a wing or feather in flying position. 

 Web. — Of a feather ; one of the halves into which the shaft divides the blade. 

 Wing Loading. — The weight carried per unit of wing area with the wings fully 



extended. 



I. SEPARATING WING-TIP FEATHERS 



A noticeable peculiarity in the flight of a certain number of birds 

 is the way their wing-tip feathers separate, both in flapping and in 

 gliding flight. So wide do the gaps between the feathers become at 

 times that the outer parts of the wings take on the appearance of 

 hands with their fingers spread out. One's first thought about the 

 matter is that there is probably nothing in it ; that the feathers sep- 

 arate simply because they are feathers, and, as such, can not help 

 themselves; but, on investigation, this turns out to be one of the 

 most interesting of the many aspects of the flight of birds — interest- 

 ing not only because it brings to light the infinite care and cunning 

 that have been bestowed on the construction of their wings but also 

 because it demonstrates the possibility of applying some of the lessons 

 that birds can teach us to the design of flying machines, gliders in 

 particular. When considering such questions we should always 

 humbly remember that birds are the outcome of the law of the sur- 

 vival of the fittest through countless ages of flying, while we have 

 been at it only for about 30 years. 



It is fairly easy to explain why the wing-tip feathers separate, but 

 the question of the purpose they may serve in doing so is more of a 

 puzzle. As there appear to be several possible and plausible answers 

 to it, I propose to put them down and leave those who are interested 

 in the subject to judge how many, if any, of them are worth consid- 

 ering. Personally I believe that this separation serves different pur- 

 poses in different types of birds and in different phases of flight and 

 that it sometimes serves more than one purpose at a time. 



The fact that the wing-tip feathers of some birds separate widely 

 in flight, while those of others do not appear to do so at all, seems to 

 be about the best clue to follow up. Among the smaller species, such 

 as finches, warblers, tits, swallows, and thrushes, the peculiarity in 

 question is not noticeable to any marked degree; the wings of these 

 birds vary both in general shape and in the pointedness of the tips. 

 It is only in a certain number of the medium and large sized birds 

 that separation is really distinct. Of these, ravens, rooks, eagles, 



