24 A BOOK OF BIRDS 



sake, been removed in the accompanying diagram. The row marked 

 t.m. is formed by the major coverts, or tectrices majores. 



The manner in which the feathers of the wing overlap one another 

 is a matter of some importance to those who are engaged in the work 

 of classifying birds, and in the mechanism of flight is even more im- 

 portant. If the wing of any bird be examined, it will be found that the 

 free edges of the '' quill " feathers and of the major coverts are turned 

 outwards — towards the tip of the wing ; while more or fewer of 

 the other coverts have the free edges turned towards the body. In the 

 first case the overlap is said to be ** distal," in the second *' proximal." 

 The significance this overlap bears in the arrangement of the quill 

 feathers is easily apparent. Having a distal overlap, when the wing 

 is raised the wind forces its way easily between the feathers, and so 

 offers the least possible resistance to the upstroke ; but during 

 the downstroke this peculiar overlap affords an unyielding surface, 

 and so forces the body upwards and forwards at each stroke. Com- 

 pared with the bat's wing, the wing of the bird is a vastly superior 

 organ ; and for this reason : the wing of the bat is formed by a 

 membrane stretched between long, slender fingers, so that any serious 

 injury to this membrane permanently disables the creature. The 

 bird's wing, on the other hand, has its flying membrane formed of a 

 number of overlapping, elastic, ribbon-like structures — the quills — 

 which are periodically renewed, and can be replaced if injured. 



Organs of Digestion andAir=sacs. With a brief survey of the 

 organs of digestion and the peculiar structures known as the air- 

 sacs, our introductory chapter may well close. 



Though once upon a time birds were burdened with teeth, they 

 long ago solved the problem of doing without them, and thereby are 

 to be envied. Like the Tortoise and the Turtle among the reptiles, 

 the birds have replaced their teeth by horny sheaths which encase 

 the jaws, and these sheaths take many forms. As a rule, that of the 

 upper differs but little from the under jaw ; but in many birds this 

 is not so. In the birds of prey, for example, which tear their food 

 in pieces, the sheath of the upper jaw is hook-shaped ; and this is 

 true also of the parrots. Birds which seize rapidly moving and 

 slippery prey, such as fish, have spear-shaped beaks, as in Herons 

 and Kingfishers ; and a similar weapon is developed where a rapid 



