92 THE BIOLOGY OF BIRDS 



F. M. Ogilvie (1921) suggested that the explanation of this 

 storing instinct is to be found in the fact that gannets feed 

 on surface-swimming fish and are therefore dependent for 

 their food supply on the weather. When it is stormy the 

 fish swim at a greater depth. " If the gale continues for 

 three or four days, during the whole of that time the bird 

 will catch nothing, and it is possible that the fear of such a 

 catastrophe occurring is at the root of the habit, and that 

 the bird's instinct teaches him always to keep a day or two's 

 supplies in hand, as long as he is able to do so." Perhaps, 

 since we do not know much about a bird's foresight or fear, 

 it would be safer to say that the frequently recurring vicissi- 

 tude of storm has served to sift out variations in the direction 

 of a storing instinct. 



§ 2. Adaptations of Bills, Tongue, and Feet 



The absence of teeth in modern birds is compensated 

 for, as in tortoises and turtles, by the development of the 

 horny bill covering the jaws. It has often a sharp edge 

 suited for cutting and tearing, and it goes beyond the 

 function of teeth in its adaptations to capturing the prey. 

 There is not much real chewing or mastication in birds, but 

 parrots gnaw their nuts, and a captive golden eagle will 

 with very rapid movements of its beak dislocate the vertebrae 

 of a trout from end to end. It might be convenient to 

 keep the word " bill " for the horny sheath and the word 

 " beak " for bill and jaws combined. 



A generalised bill may be illustrated by that of the 

 crow and allied birds — strong, pointed, somewhat triangular 

 in section, suited for many different purposes. Shortening, 

 sharpening, and curving such a bill results in the effective 

 instrument of the bird of prey ; it is quick to kill by piercing 

 the skull or breaking the neck or cutting the jugular vein, 

 it is also well suited for the rapid skinning or plucking of 

 the victim. On the same line of evolution are the bills of 

 the carnivorous owls and the vegetarian parrots, a peculiarity 

 in the latter being the movable hinging of the premaxillae 



