BIRDS . 
205 
materially to the weight. The whole plumage of a 
common fowl weighs only about three ounces. 
Having plumage filled with air heated by the body, 
carrying air-sacs in viscera and bones, breathing faster 
than other animals, pumping 
blood more rapidly to all parts 
of its body, and with it, oxygen 
to replenish the internal fires, 
the bird easily rises on the atmos¬ 
phere much as an iron ship floats 
on the ocean. 
Other Voluntary Movements. 
Swimming birds have the toes 
webbed to serve as paddles and 
the form of the body is modified 
to fit it for motion through the 
water. The legs are placed 
farther back so that the propel¬ 
ling power may act from behind. 
In running and wading the struc¬ 
ture of legs, feet, wings and beak 
are wonderfully fitted for their 
peculiar work. 
Not only can birds move from 
place to place in search of food, 
or in undertaking their long 
migrations, but by voluntary movements they are also 
able to perform skilled labor in the building of nests, 
to carry on warfare, and to express the most varied 
emotions. 
Birds and Reptiles. Birds resemble reptiles in 
many ways. They have epidermal scales on some part 
of the body, the digits end in claws, the lower jaw is 
connected to the upper jaw by a quadrate bone, the 
skull is fastened to the first vertebra by a single con¬ 
dyle, true ribs are present, there are no gills, the eggs 
are large, and the digestive, reproductive, and excre¬ 
tory organs empty into a single cavity, the cloaca. 
Fig. 162.—The Sternum 
of a Shrike. 
