CHAPTER XXX 



AVES 



Class Aves (A' vez), which includes all birds, has several distinc- 

 tive characteristics. The fundamental structural features of this 

 class are quite closely related to those of reptiles. The outstanding 

 characteristics of birds are as summarized in the following state- 

 ments. (1) The skin is covered with feathers which are exoskeletal 

 outgrowths of it. Birds are the only animals with feathers. (2) The 

 jaws are toothless in the adult and are covered with a hard horny 

 beak. (3) All modern birds are bipedal, the forelimbs being modi- 

 fied into wings, or undeveloped. (4) The pelvic girdle is securely 

 anchored to the vertebral column to support the bird adequately on 

 two legs. These limbs serve the animal in locomotion on land and in 

 water as well as for perching and climbing. (5) The caudal ver- 

 tebrae are greatly reduced in number and all except the free an- 

 terior ones are fused into a single bone, the pygostyle. (6) In most 

 birds the digestive system is modified to provide a crop for storage of 

 food, and a muscular portion of the stomach for chewing food. (7) 

 They are warm-blooded vertebrates. 



Most people associate birds with flight, but there are numerous 

 birds which do not have this power. In fact, several have practically 

 no wings. The wings of the New Zealand kiwi are so rudimentary 

 that they are completely concealed by the body feathers. 



Many birds are migratory, moving north for the summer season 

 and south for the winter. This is a very aristocratic habit around 

 which the living activities are centered. The migration routes are 

 precisely laid out ; however, birds occupying the same general breed- 

 ing range do not necessarily spend the winter in the same region or 

 vice versa, and they do not necessarily travel the same route in the 

 fall as was traveled in the spring. The birds return to the same 

 breeding range and winter home, season after season, which seems to 

 demonstrate a definite homing sense. An example of extreme migra- 

 tion is the Arctic tern which nests within the Arctic circle and win- 

 ters within the Antarctic circle, a distance of 10,000 or 11,000 miles. 



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