62 THE STRUCTURE AND LIFE OF BIRDS chap. 



comparatively big. 1 In all birds the gullet or 

 oesophagus is large. In many, especially in seed- 

 eaters, it opens out into a great expansion, with 

 thickened walls, called the crop, which reaches a 

 high development in the pigeon. In this bird it is 

 marked by irregular ridges, and in the breeding season 

 the cells of the mucous membrane that line it give off 

 the peculiar cheesy substance known as " pigeon's 

 milk," with which the young are fed. The crop secretes 

 no special digestive fluid : it is mainly a storehouse 

 in which the food is kept till the stomach is ready to 

 deal with it. The glands found there are only the 

 ordinary glands of the mucous membrane. Still it 

 must not be supposed that the food which passes 

 from the crop is in the same condition as that which 

 enters it. During its stay there it is acted on by what 

 saliva has been shot upon it, by water, by the watery 

 secretion of the mucous membrane, and by the warmth 

 of the body. Though the crop is not nearly so much 

 developed in birds of pre\ , yet in some of them it has 

 been found equal to hard work. In owls, for instance, 

 the contraction of the walls strips the skin off their 

 prey after the under-skin has been weakened by the 

 secretions, and then the well-known pellets consisting 

 of hair, feathers, and bones are thrown up. 2 The 

 South American bird, the Hoatzin, so remarkable for 

 the two claws on each of its wings and for having the 



1 His tongue, which he can shoot out almost as far as a 

 chameleon shoots his, is armed with backward-pointing bristles, 

 and the sticky saliva poured upon it adapts it still further for 

 fishing out insects from under bark. 



2 See Bronn"s Thier-Reich, vol. Ai'tS, p. 672. 



