GENERAL CHARACTERS OF BIRDS. 7 



THE GENERAL CHARACTERS OF BIRDS. 



Every bird has an ovoid trunk, developing forwards a more or less elongated 

 neck terminating in a well diflferentiated head. This head has lateral eyes, 

 sometimes inclining forwards, and its jaws, generally abruptly differentiated 

 from the rest of the head, are covered with horny pieces, constituting what is 

 called a bill or beak, and this is the most variable hard part, externally, in the 

 class; the nostrils are two and symmetrical, but vary in position and develop- 

 ment, and are sometimes (rarely) almost entirely obliterated. There are no 

 external ears or ear-conchs. 



The legs are essentially similar in all the members of the class but their 

 mode of insertion and the length of the femora entail variations in the pose 

 and carriage of the trunk; this is generally oblique, but in some (as the duck) 

 it is nearly horizontal or even droops forwards while in others (as the auks and 

 penguins) it is almost or quite vertical. The femora are completely enveloped 

 within the musculature and integument; the tibia; in most birds are more or 

 loss hidden proximally but in the long-legged forms they are almost com- 

 pletely exserted, the musculature reduced to a minimum, and the distal ends 

 covered with scutella. The heel joint is between the proximal and distal rows of 

 carpal bones which, as already indicated, are coalesced with the adjoining 

 long bones; as a rule the heel is upraised and the tarsi (tarso-metatarsi) carried 

 at more or less of an angle from the ground; the toes radiate from the tarso- 

 metatarsi and are usually four in number, three directed forwards and one 

 backwards, but there are numerous and various deviations from this form and 

 in one type (the ostriches) there are only two toes; the toes generally have 

 a characteristic number of joints, a tridactylous or tetradactylous foot having 

 three joints or phalanges in the inside toe, four in the middle, and five in the 

 outside, but there are a number of exceptions, as in the typical swifts, some 

 kingfishers, the typical goatsuckers and various others. 



The contrast between the constancy in the number of toe joints, and the 

 irrelevancy or want of co-ordination of the deviations, in the types mentioned, 

 with other characters is remarkable. The deviations are unaccompanied by 

 other modifications of structure to such an extent that they are of little, if any, 

 more than generic importance. 



The tarsi and the toes are almost always covered with scutella or scales of 

 some kind or other. The toes are terminated by claws which vary in shape 

 and function. 



The wings vary greatly in development. In almost all species they are 

 extended sufficiently for flight but there are various species, which are nearly 

 related to swift-flying birds, that have lost the power of flight, such as the ex- 

 tinct great auk and the adult steamer duck of South America; others have not 



