CHAPTER IX 

 BIRDS 



BIRDS are feathered creatures. This is a sufficient defini- 

 tion. Flight is generally associated with the thought of birds, 

 but this power is not peculiar to them, for many other creat- 

 ures besides birds fly, such as insects and bats, and formerly 

 there were reptiles that flew. Flying squirrels, flying fishes, 

 and flying toads soar on membranous expansions of the skin. 

 But none of these animals fly, like the birds, by means of 

 feathers. 



The feet of loons, ducks, and geese are covered with a 

 scaly skin, reminding one forcibly of the skin on the feet of a 

 turtle. In the lower orders of the birds these scales are 

 small and reptilian, but they coalesce more and more into 

 larger and more horny plates as we go up the scale, till in 

 the robin and bluebird the tarsus is covered with a continuous 

 plate along the front. 



The homology of a bird's wing is instructive. In general, 

 it corresponds to our arm. The quill feathers are attached 

 to the forearm and to the hand. The bones of the latter are 

 generally grown together and unjointed, the thumb alone 

 being free. 



The leg bones also correspond to those in the mammal. 

 The prominent bone, usually unfeathered, is the tarsus or 

 ankle bone, the heel being just above it. The knee bends 



