94 THE BOOK OF NATURE STUDY 



under the tongue, the skin forming the floor of the mouth being, 

 in this region, more or less distensible. Thereby they are enabled 

 to carry away food either for their young, for the purpose of 

 hiding up for future use, or for greater leisure in eating. In 

 many of the crow tribe and in the swifts such temporary pouches 

 are found. But in the pelicans this pouch assumes huge pro- 

 portions, and is capable of holding a large quantity of fish. 



These varied types of beak have apparently come into being 

 through the action of natural selection, which favours the 

 development and perpetuation of variations in the several direc- 

 tions of these peculiar modifications. 



The development of the beak in certain directions has, in 

 many cases, been accompanied by peculiar modifications in other 

 parts of the body a lengthening of the neck and legs, for ex- 

 ample, or changes in the form of the feet. 



The plover tribe afford some interesting instances of this. 

 The small ringed - plover met with commonly '"among sandy 

 tracks has a short beak and neck and short legs. The curlews 

 and redshanks, which frequent marshy districts and estuaries, 

 procure much of their food by wading in shallow water, and 

 hence have developed long beaks and necks and long legs, and 

 in the stilt-plovers these legs have attained a quite remarkable 

 length. But in the snipe and woodcock the beak is of great 

 length, while the legs are relatively short. These birds, how- 

 ever, are not waders, but use the beak as a probe ; that is to say, 

 as an organ of touch and prehension, when thrust deep down 

 into very soft and often semi-fluid mud. In the eagles and 

 hawks the hook-like beak is seconded by short legs and long 

 toes armed with powerful talons, wherewith the prey is held 

 fast while being torn in pieces. In most water-birds the toes 

 are joined by a web, but this is by no means essential for swim- 

 ming and diving. Thus the common water-hen or " moor-hen " 

 is both an expert swimmer and diver, yet the toes are remark- 

 able for their great length and slenderness, the web being con- 

 spicuous by its absence. In the grebes and coots, which are 

 no less expert divers, the toes are provided with broad lobes. 

 Birds of the kingfisher tribe, which use the feet but little, and 

 only for grasping boughs when at rest ; the three front toes are 



