MIGRATION OF BIRDS. 655 



surroundings. Thus eggs laid almost on the ground are often brownish 

 like the soil, those laid in rocky places by the sea often look very 

 like stones, while conspicuous eggs are usually found in covered 

 nests. 



The state of the newly hatched young is very various. Some are 

 born naked, blind, and helpless, and have to be carefully fed by their 

 parents until they are fully fledged. This is true of the thrush and of 

 many other song-birds. Others are born covered with down, but still 

 helpless ; while a few, like the chicks, are able to run about and feed 

 themselves a few minutes after they leave the egg. Those which 

 require to be fed and brooded over are sometimes called Altrices or 

 Insessores, while those which are at once active and able to feed them- 

 selves are called Praecoces or Autophagse. 



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 gradually, 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 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 mottled brown 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 themselves to grain, and these have large crops and strong 

 grinding 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 mammals ; 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 winter diet of fish, and vice versa. In the case of 

 canaries, bullfinches, parrots, etc., it has been noted that the food 

 influences the colouring of the plumage. 



Migration of birds. Migration remains in no small degree a 

 zoological mystery. Oh certain points we need more facts, and even 



