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PHYLUM CHORDATA : CLASS AVES — BIRDS 



unable to fly. There are many birds that moult, more or less com- 

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

 feathers twice. The males of many birds assume special decorations 

 after a partial or complete moult before the time of pairing (ruff, knot, 

 golden plover). The ptarmigan 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. 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. On certain points we need more facts, and even 

 where facts are abundant we but imperfectly understand them. Let us 

 first state some of the outstanding facts. 



1. Most birds seem to be more or less migratory, but the range 

 differs greatly. It is said that the dotterel may sup on the North 

 African steppe and breakfast next morning on the Arctic tundra : and 

 although the alleged rate may not be demonstrable, there is no doubt 

 that a distance of about 2000 miles is traversed by this bird and by 

 many others. In the Tropics, on the other hand, the migration may 

 simply be from valley' to hillside. 



2. Observers in temperate countries long ago noticed that the birds 

 they saw might be grouped in reference to their migrations. Thus 

 (a) some arrive in spring from the South, remain to breed, and leave for 

 the South in autumn, e.g. swallow and cuckoo in Britain : (ft) some 

 arrive in autumn, chiefly from the North, stay throughout the winter. 

 and flv northwards again in spring, e.g. the fieldfare and the redwimz in 

 Britain ; (c) some— the '• birds of passage " — are seen only for a short 

 time twice a year on their way to colder or warmer countries in sprins; 

 or autumn, e.g. sandpipers ; and [d) some seem to deserve the name of 

 " residents," but reallv exhibit a partial migration, such as the song- 

 thrush and redbreast in Britain. In Europe the spring migration is 

 on the whole northwards and north-eastwards, in autumn southwards 

 and south-eastwards, but the paths are great curves. 



3. There is a striking regularity in the advent and departure of many 



