Class 5. Aves. 451 



The plumage, which replaces the down feathers, is usually essentially 

 different from that of the parent. 



Many yotmg Birds, like young Ohelonia and Crocodilia, possess, on tte upper 

 side of the beak, a small hard outicularised knob, witli wMcb they break through 

 the egg (neb). 



Whilst some Birds inhabit the same confined locality throughout 

 the year, and may therefore be called residents, others undertake 

 longer or shorter excursions or true migrations. Some wander about 

 through a large district in search of food, and are most like the 

 residents ; others, according to the season, leave the mountains and 

 betake themselves to the neighbouring valleys, or, impelled by 

 necessity, exchange the forest for the open country, etc. The more 

 conspicuous migrants travel greater distances, for they breed 

 every year in a cold climate, and spend the winter in a warm 

 country some way off. They follow special routes, which are arranged 

 so that, as far as possible, only those countries which resemble 

 the native habitat shall be touched at. Coast-birds usually take a 

 course along the sea-coast, or, if necessary, along rivers ; Marsh-birds 

 go over marshy-ground, along rivers, etc. The same route is generally 

 followed, whether going or returning. Most travel in large flocks, 

 sometimes several species in company, and as old and young fly 

 together, the knowledge of the way is always handed on to, and 

 preserved by, successive generations j Birds cannot find the way 

 " instinctively," although in Birds of Passage an inherited indefinite 

 instinct to wander may be noticed, which shows itself in restlessness 

 in young caged individuals, at the time when migration occurs. 

 Migrations from colder regions take place at different, but for each 

 species definite, times, usually in the autumn, for some species even 

 in August and July ; the return occurs from February to May, those 

 forms which are among the earliest to go away, returning last. Most 

 of the Birds of Passage, which breed in Britain, winter in South 

 Europe or North Africa. 



It is easy to understand that wanderings in general, and also ti-ue migration, 

 originated in the need for food ; it has been noticed that cei^tain forms which do 

 not usually migrate, journey South in severe winters in seai-oh of food, whilst, 

 in mild winters, some Birds of Passage remain in their breeding place : on the 

 other hand, it may be observed, that migration is undertaken so instinctively 

 by most Birds that they will start even in time of plenty, so that their 

 wandering is no longer directly dependent upon the food supply. 



Birds occur wherever there is life, although they are most 

 abundant in the Tropics, and constitute at the present time a very 

 numerous but fairly uniform class. Geologically this is the youngest of 

 all vertebrate classes, since the oldest avian form known, a single species 

 only, comes from the Jurassic, so that they were certainly very scarce 

 at that time ; a larger number occurs in the Cretaceous (all Odont- 

 ornithes) and many in Tertiary formations. 



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