138 



BIRDS 



The birds show a 

 number of other special 

 adaptations which are of 

 use to them. These are 

 hollow bones, a keeled 

 sternum (breast bone), 

 and a high body tem- 

 perature. 



The skeleton of a bird 

 shows a prominent ridge 

 on the breast bone. 

 This is the keel of the 

 sternum, which serves 

 as a place of attachment 

 for the large wing mus- 

 cles (Figure 139). The 

 lungs of the bird are 

 small, but air tubes 

 extend into the bones, 



so that the body of the bird is relatively lighter than 



that of animals with solid bones. 



Birds lead an active life, which means that they use a 



great deal of energy. This energy comes 



from the oxidation going on in the body. 



In birds, oxidation is more rapid than in 



other vertebrates, owing to the fact that 



they almost completely change the air 



with each breathing movement and thus 



secure a greater supply of oxygen. The 



rapid oxidation requires a large supply 



of food to be digested and assimilated 



rapidly and it also makes the normal 



J Figure 139. — 



body temperature of birds higher than Skeleton of Mal . 

 that of other vertebrates. lard Duck. 



Figure 138. — Adult Screech Owl. 



