What is a Bird? 



Z7i 



trels, shearwaters and albatrosses, and in 

 most wading birds, both the wing bones 

 and the flight-feathers are unusually length- 

 ened, giving a great expanse of wing in 

 proportion to the size of the bird;, while in 

 the wrens, many of the sparrows, and in 

 fact the ordinary song birds generally, the 

 partridges, quails and most gallinaceous 

 birds, the wings are small in proportion to 

 the size of the body, and the power of 

 flight correspondingly lessened. 



In fact, the whole structure of the bird 

 is modified with reference to flight. The 

 sternum or breast bone is strongly keeled 

 (as is familiarly seen in the domestic and 

 game birds when served as food), to give 

 attachment to the immense breast muscles 

 which move the wings; the hollow, air- 

 filled bones of all flying birds combine 

 lightness with strength; and even the pro- 

 vision by which birds lay eggs to be hatched 

 by incubation has also reference to the 

 function of flight. If birds carried their 

 young, as mammals do, until ready to be 

 born alive, their weight would so far impair 

 the bird's buoyancy as to prevent flight. 

 How much, indeed, hinges upon the adap- 

 tation of the bird to flight! The eggs are 

 large, to contain, in the large yolk sac and 

 its inclosing white, nourishment for the em- 

 bryo; they are covered by a hard shell, 

 to protect and preserve the precious con- 

 tents; a nest must be provided for them, 

 where they may be successively deposited, 

 and finally brooded and hatched by the 

 parent birds, and where the young (in all 

 the higher forms of bird life) may be fed 

 and reared till able themselves to fly. Con- 

 nected with this is the choice of a home, 

 the character of its location, and the vary- 

 ing degree of skill displayed in nest build- 

 ing, about which so much that is distinctive 

 of bird life, and of our interest in it, cen- 

 ters. 



The power of flight also gives oppor- 

 tunity for the display of their vivacious 

 natures, their restless activity, their intense 



energy of action, their highly nervous tem- 

 perament. It makes them the graceful 

 objects of motion we never weary of watch- 

 ing; it gives us the warbler, flitting nerv- 

 ously from bough to bough in the orchard 

 or forest, the lark singing high in mid-air, 

 the swallow skimming over meadow and 

 river, the terns and gulls coursing along 

 the seashore, the petrel and shearwater 

 skimming the ocean waves, the long lines 

 of wild geese winnowing their swift way 

 through the sky in spring and fall. 



The feathery covering of birds not only 

 gives warmth, lightness, and power of flight, 

 but gracefulness of contour, concealing the 

 angularities of the body. The beauty of 

 birds, particularly their iridescent tints, de- 

 pends upon modifications of the structure 

 of the feathers, as do the various ornamen- 

 tal appendages seen in the delicate plumes 

 of the egret, the ruffs and shields, etc., of 

 the birds of paradise. 



Another striking, if not distinctive fea- 

 ture of birds, is their power of song, par- 

 ticularly as developed in the so-called song 

 birds. The imitative talent of the mocking- 

 bird, the melodious notes of many thrushes, 

 the whistling and vocalization of parrots 

 and other talking birds are without parallel 

 in other classes of animals. Peculiar to 

 birds also, is the structure of the lower 

 larynx seen in the true song birds, which 

 is provided with special vocal muscles. 



From what has been said regarding the 

 distinctive characters of birds, it is evident 

 that they can be unequivocally defined as 

 feathered vertebrates. They are also the 

 only warm-blooded, egg-laying vertebrates, 

 if we except the Australian monotremes. 



Birds, then, may be briefly defined as 

 warm-blooded, egg-laying, feathered ver- 

 tebrates, constructed with especial refer- 

 ence to flight, most of the other character- 

 istics which distinguish birds from mam- 

 mals being shared in common by birds and 

 reptiles. 



J. A. Allen. 



