INTRODUCTORY 25 



succession of blows is rained upon hard, unyielding surfaces, as in 



the beaks of Woodpeckers, which, moreover, have the density of the 



horn immensely increased. 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 merely as light 



forceps, and, consequently, they and their sheaths offer no very 



striking characters ; while in Swifts, Sv^allows, and Nightjars the 



beak has become reduced to the smallest possible limits — and this 



because the jaws perform but little work in seizing the food. When 



slippery victims have to be held, 



such as fish, the edges of these 



horny sheaths are armed with more 



or fewer saw-like '* teeth," as in the 



Mergansers among the Ducks ; or 



these "teeth " may take the form of 



needle-like spines, as in the Darters. 



In the Ducks and certain Petrels, 



horny plates, recalling the baleen- Fig. lo.-Beak of a Hawk to show 

 '^ ' the hook=shaped beak used for 



plates of ** Whale-bone " Whales, tearing prey. 



are developed, and these serve as 



sieves, or strainers, allowing the water taken into the mouth with 



the food to escape, leaving the solid matter behind. 



This horn-encased region of the jaws forms the "beak," and the 

 shape of this is, as we have just indicated, determined by the nature 

 of the food it is required to manipulate. 



From the mouth the food is passed down the gullet, or oesophagus, 

 until, in many birds, such as Pigeons and Fowls, it reaches a special 

 dilatation of the gullet known as the *' crop." This is a thin-walled 

 bag, wherein the food is stored and softened, preparatory to being 

 passed on to the stomach. This, in birds, consists of two parts, one 

 lying in front of the other. The first, which is superficially hardly 

 distinguishable from the gullet, is known as the " proventriculus." 

 The walls thereof are richly supplied with digestive glands. From 

 this first stomach the food passes into the second, which, in birds 

 such as Fowls and Pigeons, for example, has extremely thick and 



4 



