PIGEONS 197 



4. The beak, which is weak and cornified only at the tip, is adapted 

 only for picking up small grain, and not for dividing these into smaller 

 portions — a task which is performed by 



5. The organs of digestion (see above illustration), viz. : 



(a) The crop, consisting of two lateral dilatations of the oesophagus, 

 and 



(b) The glandular stomach, or proventriculus, a dilatation of the lower 

 portion of the oesophagus, in which the food is softened by the water 

 which is taken in (see Section 6), and by the slimy secretions of mucus 

 glands. 



(c) The grinding of the grain swallowed for food takes place in the 

 gizzard, the walls of which consist of very thick muscles. It always 

 contains small stones or pebbles (quartz grains), which are swallowed 

 with the food. By the contraction and subsequent dilatation of the mus- 

 cular walls of the gizzard (see Part I., p. 5), the internal cavity of this 

 organ is alternately enlarged and diminished, whereby the food is sub- 

 jected to a grinding motion, and gradually crushed up into finer and 

 finer particles, the pebbles being at the same time gradually worn down 

 and continually replaced by other sharp-edged ones. (The grating noise 

 produced by these foreign bodies in the gizzard may be heard from 

 outside in some gallinaceous birds — e.g., a turkey — by applying the ear 

 to the bird's body.) To protect the gizzard from being injured by these 

 stones, its inner surface is lined with a strong horny membrane (see 

 Section 8, p. 147). 



The cavity of the stomach being, however, small, only a small 

 quantity of grain can be ground up at a time ; and as the bird needs 

 much more food than the stomach can hold for the time being, the 

 surplus is stored up in the crop, which thus acts as a store-chamber. 

 (Compare ruminants.) 



6. The pigeon requires a good deal of water, for the purpose of 

 softening the hard-shelled grains which constitute its food. In drinking 

 the bird pushes its beak to a fair depth into the water, the slit-like 

 nostrils being closed at the same time by the cartilaginous scales. In 

 this way the beak is converted into a tube, as it were, by which the 

 water is sucked in. 



C. Enemies. 



1. The pigeon is a defenceless bird (compare beak, feet, and claws), 

 and consequently shy and timid. 



2. From hawks, falcons, crows, it is protected by its sharp sight 

 and rapid flight. 



3. By building its nest in caves and other inaccessible spots, the rock- 



