BIRDS ORGANIZATIONDIAGRAM 4. 57 



CLASS BIRDS. 



[ DIAGRAM 4. ] 



BIRDS are vertebrate animals, which are always easily recog- 

 nized by having only two legs and the body covered with 

 feathers. They have also very warm blood. Nearly all birds 

 fly, and their body seems formed to cleave the air. The beak 

 forms a point in front, like the prow of a ship, and the body 

 ends behind in a tail, by which the bird directs its flight, as a 

 boat is guided by the rudder. The whole structure of the bird 

 is arranged for flight ; the feathers overlap one another like the 

 tiles of a roof, for gliding through the air; the wings, which 

 correspond with the arms, spread out, and fold back against the 

 body ; they are moved by very large muscles, which form the 

 greater part of the flesh of the bird. These are those found on 

 each side of the breast attached to a bone called the sternum, 

 which is furnished with a sort of keel in the middle. A fowl has 

 a higher keel to the sternum than a duck. The height of 

 the keel is not the most important item ; but the width and 

 length of the sternum and corresponding muscle, and the shape 

 of the wing, determine the powers of flight. The wings are 

 provided with long feathers which spread out like a fan 

 when the bird opens its wing, and fold one over the other when 

 it closes it. These feathers are sometimes very long, and 



