THE CLASSIFICATION OF BIRDS 193 



it goes to sleep. But it is really in no danger of falling, for the 

 tendon which governs the movements of its toes is so constructed 

 that when the bird sits down they catch a firm hold of the perch, 

 from which they cannot be loosened till it stands up again. So 

 while it is asleep a bird is actually fixed to its support. 



Birds, like mammals, have a four-chambered heart, and are 

 warm - blooded. The skeleton is very lightly constructed, the 

 bones being generally hollow; and many birds are able to pass air 

 to various parts of the body, and even into their bones. Their 

 digestive powers are marvellous, as no doubt some have thought 

 when they have seen them swallowing small pebbles. It is said 

 that the ostrich considers nails a special dainty. But birds do not 

 take these things as food, neither do they digest them. They 

 know, however, that such hard substances will help to prepare for 

 digestion their ordinary food, which often consists of corn and other 

 hard seeds. 



Most birds are gifted with very good memories. Those that 

 migrate are known to come back to the same nesting-place year 

 after year, while many can be taught to speak and even to whistle 

 a whole melody. 



THE CLASSIFICATION OF BIRDS 



Naturalists have found it so exceedingly difficult to classify 

 birds that even up to this day no perfectly satisfactory arrange- 

 ment of the birds has been fixed upon. 



The great difficulty of arranging the birds into natural 

 orders, families, genera, and species, arises out of the one fact 

 that they are as a class, i.e. as a whole group of animals, so very 

 much alike. They are all covered with feathers, all have wings, 

 more or less developed, and two legs. They are all without teeth, 

 all have beaks, all lay eggs, none suckle their young, and almost all 

 of them fly. So while it is easy to know a bird from a mammal 

 or from a reptile, it is not easy to make distinctive divisions among 

 the birds themselves, not nearly so easy as it has been to so divide 



(M868) ^ 



