292 BIRDS 



Chief characteristics of the birds. -They are all covered 

 witli feather,s. The queer "kiwi," or Apteryx, of New 

 Zealand, possesses only hairlike feathers. No living birds 

 possess teeth, but the jaws are modified into a beak incased 

 in a horny covering. Tlie heart is f(jur chaniljered, and 

 there is a consequent double circulation, — that is, one set 

 of vessels carrying impure (venous) blood, antl another set 

 carrying pure (arterial) blood, ^^'hich is like the circulation 

 in man. The front pair of limits is modified into organs of 

 flight, which in some are nevertheless useless as such. The 

 temperature of tlie jjody is higher tlian in any other animals. 

 Therefore, for the first time we meet with warm-blooded 

 animals. The l)oncs of many birds are lioUow and filled 

 with air. ]\Iore(jver, there are often air sacs in the body 

 for the purpose of increasing the buoyancy of the animal. 



Molting of birds. — A feather does not continue, like a 

 hair, to grow indefinitely: but after once attaining its 

 growth, it remains unchanged until shed, when a new 

 feather grows in its ]dace. Generally speaking, birds slied 

 all their feathers, or molt, once a year, after the breeding 

 season is over. Some birds pass through two molts a year, 

 one in the autumn and another in tlie spring. The feathers 

 foll(j\\'ing the autumnal molt may l)e of one color, whWe 

 those following the spring molt may be of anotlier. Hence 

 such birds possess a certain color in winter and another in 

 summer, or, as we say, hiivc seasonal colors. 



Incubation of birds. — All bii'ds are d(>veloped from_ eij;os 

 laid l)y the fouiaJe jKU'cut, usually in a nest of lier own 

 building. The time necessary for the incubatiini of (he 

 eggs varies with dirfercnt liirds. For tlie majority of bii-.ls 

 the eggs are iiicuJKited from ten <o (liirt)' days, but an 

 ostrich egg requiri^s neaily fifty days of inculiation. Some 



