62 THE STRUCTURE AND LIFE OF BIRDS cHap. 
comparatively big! In all birds the gullet or 
cesophagus 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 prey, 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. The 
South American bird, the Hoatzin, so remarkable for 
the two claws on each of its wings and for having the 
1His 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 Thder-Reich, vol. Aves, p. 672. 
