292 BIRDS 



Chief characteristics of the birds. — They are all covered 

 with feathers. The queer "kiwi," or Apteryx, of New 

 Zealand, possesses only hairhke feathers. No living birds 

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

 in a horny covering. The heart is four chambered, and 

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

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

 carrying pure (arterial) blood, which is like the circulation 

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

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

 temperature of the body is higher than in any other animals. 

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

 animals. The bones of many birds are hollow and filled 

 with air. Moreover, 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, Uke a 

 hair, to grow indefinitely; but after once attaining its 

 growth, it remains unchanged until shed, when a new 

 feather grows in its place. Generally speaking, birds shed 

 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 the spring. The feathers 

 following the autumnal molt may be of one color, while 

 those following the spring molt may be of another. Hence 

 such birds possess a certain color in winter and another in 

 sunmier, or, as we say, have seasonal colors. 



Incubation of birds. — All birds are developed from eggs 

 laid by the female parent, usually in a nest of her own 

 building. The tim.e necessary for the incubation of the 

 eggs varies with different birds. For the majority of birds 

 the eggs are incubated from ten to tliirty days, but an 

 ostrich egg requires nearly fifty days of incubation. Some 



