336 ELEMENTARY ZOOLOGY 



OTHER BIRDS. 



Birds are readily and unmistakably distinguishable from 

 all other kinds of animals by their feathers. They are 

 further distinguished from the reptiles on one hand by 

 their possession of a complete double circulation and by 

 their warm blood (normally of a temperature of from 

 100-112 F.), and from the mammals on the other by 

 the absence of milk-glands. There are about io,OOO 

 known species of living birds ; they occur in all countries, 

 being most numerous and varied in the tropics. Birds 

 are exceptionally available animals for the special atten- 

 tion of beginning students, because of their abundance and 

 conspicuousness and the readiness with which their varied 

 and interesting habits may be observed. The bright 

 colors and characteristic manners which make the identifi- 

 cation of the different kinds easy, the songs and flight, 

 and the feeding, nesting and general domestic habits of 

 birds are all excellent subjects for personal field-studies 

 by the students. We shall therefore devote more atten- 

 tion to the birds than to the other classes of vertebrates, 

 just as we selected the insects among the invertebrates for 

 special consideration. 



Body form and structure. The general body form 

 and external appearance of a bird are too familiar to need 

 description. The covering of feathers, the modification 

 of the fore limbs into wings, and the toothless, beaked 

 mouth are characteristic and distinguishing external 

 features. The feathers, although covering the whole of 

 the surface of the body, are not uniformly distributed, but 

 are grouped in tracts called pterylce, separated by bare or 

 downy spaces called apteria. They are of several kinds, 

 the short soft plumules or down-feathers, the large stiffer 

 contour-feathers, whose ends form the outermost covering 

 of the body, the quill-feathers of the wings and tail, and 



