280 Trees, Stars, and Birds 



had common ancestors. In like manner naturalists be- 

 lieve that the members of the thrush family are all re- 



FIG. 167. A, foot of a perching bird; B, foot of a wading bird; C, foot of 

 a swimming bird ; D, foot of a bird of prey. 



lated and that if we could trace their ancestry far enough 

 back we should find that robins, bluebirds, and thrushes 

 had a common ancestor ; that is, if we had lived at some 

 period in the history of the world, perhaps many thou- 

 sands of years ago, we would not have seen robins, blue- 

 birds, and thrushes, but some one bird from which they 

 have all sprung. 



It is true also that robins and birds we classify in other 

 families, such as sparrows and crows, had common ances- 

 tors, but their relationships are not so close. In other 

 words, the common ancestors of crows, sparrows, and 

 robins did not live so recently as the common ancestors 

 of thrushes, bluebirds, and robins, but it is believed that 

 all birds have descended from one kind of bird, the re- 

 mains of which have been found in rocks formed millions 

 of years ago. 



Guides in classifying. In classifying birds, naturalists 

 are guided by the structure of the different parts. This 

 includes such characters as the general form of the bird, 

 the shape and length of the beak, the arrangement of 

 the toes on the foot, or the thickness of the walls of the 

 gizzard. They are guided also by a study of the young 



