INTRODUCTION 



xli 



Fig. 9. — Head of a Hawk, showing 

 the hook-shaped beak used for 

 tearing prey. 



merely as light forceps, and, consequently, they and 

 their sheaths offer no very striking characters ; while 

 in Swifts, Swallows, and Nightjars the beak has be- 

 come reduced to the 

 smallest possible lim- 

 its because the jaws 

 perform but little 

 work in seizing the 

 food. When slip- 

 pery victims have to 

 be held, such as fish, 

 the edges of these 

 horny sheaths are 

 armed with saw-like 

 teeth, as in the Mer- 

 gansers 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, resembling 

 the baleen-plates of "whalebone" Whales, are devel- 

 oped, 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 determined by the 

 nature of the bird's food. 



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. 



