262 



HOW DO WE CLASSIFY ANIMALS? 



feathers, give the rounded contour to the body. In the wings we 

 find quill feathers ; these are adapted for service in flight by having 



long hollow shafts, 



the whole making a 



rttofri . light structure and 



^^ offering considerable 



resistance to the air. 

 Feathers are devel- 

 oped from the outer 

 layer of the skin, and 

 are formed in almost 

 exactly the same 

 manner as are the 

 scales of a fish or a 

 lizard. Feathers are 

 shed or moulted one 

 or more times dur- 

 ing the year, and lost 

 or broken feathers 

 are replaced. Some 

 birds moult twice a 

 year, having differ- 

 ent colored feathers 

 in the summer and 

 winter seasons. 

 Many bones are hollow or have large spongy cavities. Some 

 bones, notably the breastbone, are greatly developed in flying 

 birds for the attachment of the muscles used in flight. 



The ankle of a bird is long and reptile-like and, like the foot, is 

 covered with scales. The most extraordinary adaptations are 

 found in the feet of various birds : some for perching, others for 

 swimming, others for scratching, etc. By looking at the feet of a 

 bird we are able to decide almost certainly its habitat, method of 

 life, and perhaps its food. 



In the perching birds we find three toes in front and one behind, 

 the hind toe playing an important part in clinging to the perch. 

 The three toes in front curve around the perch^ often meeting the 



Wright Pierce 



Compare this bird with an airplane. In what ways are they 

 similar, and in what ways different ? 



