AVES 



227 



assistance to flight most birds are able to decrease the 

 specific gravity of the body by means of a series of air sacs 

 communicating with the lungs. The larger sacs, usually 

 about a dozen hi number, lie in the trunk region, but smaller 

 ones sometimes occur in the bones, which are hollow in^ 

 stead of being filled with marrow 

 as in the mammals. All birds are 

 oviparous (lay eggs), some species 

 depositing as many as two hun- 

 dred eggs in a season, while others 

 lay only one each year. 



No modern bird possesses any 

 teeth, but the fossil remains from 

 Kansas show that the birds living 

 millions of years ago had sharp, 

 reptile-like teeth. The most an- 

 cient bird known is Archceopteryx, 

 about the size of a crow. It pos- 

 sessed a long, jointed tail, numerous 

 sharp, conical teeth, claws on the 

 wing, and many other reptilian 

 characteristics. These facts indi- 

 cate that the birds have descended 

 from a reptile-like ancestor. 



Within the history of man a number of species of Aves 

 have become extinct, and reports from nearly every state 

 show that unless quick and effective protection is afforded 

 our feathered friends, irreparable damage may result to 

 the country. Special investigations made by Hornaday 

 show that the average decrease in bird life throughout the 

 United States from 1883-1898 was forty-six per cent. 



FIG. 262. Diagram of the 

 lungs and air sacs of a pig- 

 eon, p, air sacs ; lu, lung ; 

 t, trachea. 



