MOULTING OF BIRDS. 499 



Moulting. — Every year Birds lose their old feathers. 

 This moulting generally takes place after the fatigue of the 

 breeding season, but in the case of the swallows and the 

 diurnal birds of prey and some others the moult is in mid- 

 winter. The process is comparable to the casting of scales 

 in Reptiles, and to the shedding of hair in Mammals. 

 Feathers are so easily injured that the advantage of the 

 annual renewal is evident, especially when it takes place just 

 before the time at which it may be necessary to set forth on 

 a long migratory flight. 



In moulting, the feathers fall out and are replaced gradu- 

 ally, but sometimes they are shed so rapidly that the bird is 

 left very bare, thus moulting ducks are unable to fly. 

 There are many birds that moult, more or less completely, 

 more than once a year ; thus the garden warbler sheds its 

 feathers twice. The males of many bright birds assume 

 special decorations after a partial moult, which occurs 

 before the time of pairing. Most remarkable is the case of 

 the ptarmigan, which changes its dress three times in the 

 year ; after the breeding season is over the plumage becomes 

 grey ; as the winter sets in it grows white, and suited to the 

 surrounding snow; in the spring, the season of courtship, 

 the wedding-robes are put on. 



Diet. — ^The food of Birds varies greatly, not only in 

 different kinds, but also at different seasons. Many are 

 herbivorous, feeding on the soft green parts of plants, and 

 in these birds the intestine is long. Some confine them- 

 selves to grain, and these have large crops and strong grind- 

 ing gizzards, while those which combine cereals and insects 

 have in most cases no crop. A few sip honey, and may 

 even help in the cross-fertilisation of flowers ; those that 

 feed on fruits play an important part in the dissemination 

 of seeds ; those that devour insects are of great service to 

 man. In fruit-eating and insectivorous birds the crop is 

 usually small, and the gizzard only slightly muscular. But 

 many birds feed on worms, molluscs, fishes, and small mam- 

 mals ; in these the glandular part of the stomach is more 

 developed than the muscular part. It has been shown that 

 the nature of the stomach in the Shetland Gull changes twice 

 a year, as the bird changes a summer diet of grain and seeds 

 for a wintet diet of fish, and vice versa. In the case of 



