WHAT IS A BIRD? 



IN considering the question, What is a 

 Bird ? it is not necessary to extend our 

 comparisons beyond the great division of 

 animals known as vertebrates, or those hav- 

 ing a backbone. 



A bird shares with the mammal, the rep- 

 tile, and the fish, the character of having 

 an internal skeleton, as distinguished from 

 insects and the lower animals generally. 

 An ordinary bird seems very distinct in 

 many features from any other animal, but 

 there are really very few of these that are 

 strictly peculiar to birds. For example, 

 birds have a horny beak, unprovided with 

 teeth, but a horny beak is met with in the 

 duck-billed platypus among mammals, and 

 is as characteristic of turtles as of birds. 

 Some early forms of birds — long since ex- 

 tinct, however — were also well provided with 

 teeth. 



All birds lay eggs, but so do batrachians 

 (toads, frogs and salamanders), all reptiles 

 (snakes, turtles, lizards, etc.), and nearly all 

 fishes, and in some instances these eggs are 

 provided with a shelly covering as they are 

 in birds. But birds' eggs are, as a rule, 

 hatched by being sat upon by the parent 

 bird, which act of brooding provides the 

 heat necessary for their hatching, while in 

 other animals no such care is necessary on 

 the part of the parent. 



Again, birds are warm-blooded, while 

 reptiles, batrachians and fishes are cold- 

 blooded. But mammals are also warm- 

 blooded, and it has been found that a few 

 mammals lay eggs, as the Australian mono- 

 tremes (duck-billed platypus and echidnas). 

 Mammals in general, however, bring forth 

 living young, which are nourished before 

 birth by direct connection with the mother, 

 and for a time after birth are suckled by 

 the mother. Birds are also, as a rule, flying 

 animals, though there are birds unable to 

 fly, in some of which the wings are not only 



useless as organs of flight, but are reduced 

 to mere rudiments. On the other hand, 

 there are flying mammals, as the bats, and 

 also flying fishes and reptiles. So that flight 

 is not distinctive of birds as a class, though 

 so universally a characteristic of those we 

 commonly meet with in our own country. 

 The apparatus of flight in birds is, however, 

 distinctive and peculiar, as we shall see a 

 little later. 



There are many anatomical characters 

 which sharply separate birds from mam- 

 mals, as the presence in the latter of milk 

 glands for the nourishment of the young, 

 a feature not found in birds, and likewise 

 absent in all the other classes of verte- 

 brates. This is, then, a peculiarity of 

 mammals, rather than its absence being a 

 distinctive feature of birds. Birds, also, 

 as distinguished from mammals, share with 

 reptiles the absence of a diaphragm, and 

 the possession of only one occipital con- 

 dyle, or point of articulation of the skull 

 with the neck bones, and of the os quadra- 

 tu7n, a little bone forming the articulation 

 of the lower jaw with the skull, which in 

 mammals hinges directly upon the skull. 

 On the other hand, birds, in common with 

 mammals, are distinguished from reptiles, 

 batrachians and fishes by possessing not 

 only warm blood, but also a complete 

 double circulation. 



The bones of birds are generally hollow, 

 or contain cavities filled with air; and this 

 was formerly thought to be a distinctive 

 feature of birds, but it proves that in some 

 birds these air cavities are lacking, while 

 they are present in the bones of some of 

 the extinct flying reptiles. 



Thus far we have found no single char- 

 acter strictly distinctive of birds, those fea- 

 tures which separate them from mammals 

 being shared also by the other primary di- 

 visions of vertebrates. Indeed, in certain 



