SAFETY DEVICES IN" BIKDs' WINGS — GRAHAM 271 



swans, and game birds (pheasants, partridges, and allied species) are 

 some of the more familiar. Without exception they have compara- 

 tively square or rounded wing tips, though the wings themselves 

 are of various shapes. 



Let us consider a group of birds of corresponding size whose 

 feathers do not appear to separate much, if at all, such as the wood- 

 cock, snipe, duck, pigeons, cuckoos, gulls, and nearly all the sea birds. 

 One could almost be sure that, except for their very tips, the feathers 

 of some of these birds always remain packed together. In this 

 group the wing tips are distinctly pointed, though, again, the shape 

 of the wings varies considerably. 



FiGuns 1. — A golden eagle about to alight. Only the tip of the wing is shaded to 

 emphasize the incidence of the separated feathers. (From a photograph by 

 Arthur Brook.) 



As a preliminary basis to work on we might therefore suggest that 

 separation of the flight feathers is more likely to be met with in big 

 than in small birds and in birds that have squarish or rounded wing 

 tips than in those that have pointed ones. 



In view of the wide divergence between the speed of flapping of 

 game birds and of other t^q^es that have separating feathers it 

 would appear that the speed at which the wings are flapped has no 

 direct bearing on the matter. 



II. THE THEORY OP FLIGHT 



Before going into the matter in detail, it is, perhaps, advisable 

 to describe briefly the manner in which a wing derives power or 

 " lift " from the air. First and foremost, it should be borne in mind 

 that, just as a swimmer obtains forward motion by pushing water 

 backward, so does a bird counteract gravity by causing air to move 



