266 



HOW DO WE CLASSIFY ANIMALS? 



nest, but lays its eggs on the ground. Birds immune from the 

 attacks of enemies because of their isolation or their protective 

 coloration (as the gulls and terns) build a rough nest among the 

 rocks or on the beach. The eggs, especially those of the tern, are 

 marked and colored so as to be almost indistinguishable from the 

 rocks or sand on which they rest. Other birds have made their nest 

 a place of refuge as well as a place to hatch the eggs. 



Care of the young. After the eggs have been hatched, the young 

 birds in most cases are quite dependent upon the parents for 

 food. Most young birds are prodigious eaters. As a result they 

 grow very rapidly. It has been estimated that a young robin 

 eats two or three times its own weight of food every day. In the 

 case of the pigeons and some other birds, food is swallowed by 

 the mother, partially digested in the crop, and then regurgitated 



into the mouths of 

 the young nestlings. 

 Relationship of 

 birds and reptiles. 

 The birds afford an 

 interesting example 

 of how the history 

 of past ages of the 

 earth has given a 

 clue to the struc- 

 tural relation which 

 birds bear to other 

 animals. Several 

 years ago, two fossil 

 skeletons were found 

 in Europe of a bird- 

 like creature which 

 had not only wings 

 and feathers, but 

 also teeth and a lizardlike tail. From these fossil remains and 

 certain structures (as scales) and habits (as the egg-laying habits), 

 naturalists have concluded that birds and reptiles in distant 

 times were closely related and that our existing birds probably 



Museum of Natural History 



The eggs of a dinosaur, a large land reptile which lived mil- 

 lions of years ago. This nest of eighteen eggs was found in 

 Mongolia. 



