26 A BOOK OF BIRDS 



muscular walls, while its cavity is lined with a more or less dense 

 skin, thrown into a series of folds. A stomach of this kind is known 

 as a '' gizzard " ; but in birds which feed on animal food, such as 

 Hawks and Gulls, there is no " gizzard," the walls of this region of 

 the stomach being thin and soft. Where a true gizzard is developed, 

 sharp stones and sand are swallowed by the bird, ^-nd these are stored 

 in the gizzard to aid in the work of grinding up the grain and other 

 hard matter, which form the bird's food. Even flesh-eating birds 

 appear to find it necessary to swallow indigestible matter with their 

 food, and this matter is furnished by the hair and feathers of their 

 victims. Digestion completed, the indigestible residue becomes welded 

 together into a mass, and ejected from the mouth in the form of a 

 '' pellet." From the second stomach the food passes on into the 

 intestine, and here all the nourishing portions are absorbed. 



The air-sacs are found in all birds. They take the form of a 

 number of thin-walled chambers lying on either side of the body, and 

 in front of the merry-thought. During life these chambers are filled 

 with air drawn from the lungs. These chambers, indeed, are really 

 a part of the lungs ; but their precise structure cannot be described 

 here, since this would demand a lengthy description of anatomical 

 details that would be out of place in these pages. At one time it was 

 thought these great air-chambers served the purpose of decreasing the 

 weight of the bird during flight ; but this is now known to be an in- 

 correct view. They serve to assist respiration, and also, probably, to 

 regulate the temperature of the body. 



Concerning Eggs.— Birds, like their cousins the reptiles, and 

 certain lovAy mammals, lay eggs. In birds these are always invested 

 in a hard shell, which is commonly coloured, often very beautifully. 

 How this colouring is formed, and how deposited on the shell, we 

 do not yet know, or at least only very partially so. 



The number of eggs laid by different species of birds varies greatly. 

 Thus some species lay but one egg, as the Guillemot, for example ; 

 some only two, as the Pigeons. Plovers lay four ; while the Game- 

 birds lay from twelve to twenty. 



When first formed the egg of a bird is extremely small, but during 

 its development there is added to the germ, which is to grow into the 

 chick, a great quantity of yolk, to serve as food for the developing 



