xl INTRODUCTION 



at each stroke. Compared 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 mem- 

 brane stretched between long, slender fingers, so that 

 any serious injury to this membrane permanently dis- 

 ables 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 and Air-sacs. — Though once 

 upon a time birds had teeth, they long ago solved the 

 problem of doing without them. Like the Tortoise 

 and the Turtle among the reptiles, birds have re- 

 placed 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 that of 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 King- 

 fishers; and a similar weapon is developed where a 

 rapid succession of blows is rained upon hard, unyield- 

 ing surfaces, as in the beaks of Woodpeckers, which, 

 moreover, have the density of the horn immensely in- 

 creased. On the other hand, these sheaths are often 

 of great delicacy, as in the case of the Snipe and 

 Woodcock, the fragile, upturned beak of the Avocet, 

 or the long, rod-like probe of many Humming-birds. 



In the so-called soft-billed birds, the jaws serve 



