308 



THE VERTEBRATE ANIMALS 



studied carefully, migrations appear to take place along well- 

 defined paths. These paths usually follow the coast very exactly, 

 although in places they may take the line of coast that existed in 

 former geological times. In this country the Mississippi valley, a 

 former arm of the sea, forms one line of migration, while the north 

 Atlantic seacoast forms another route. 1 



It has been shown that the southern movement of migratory 

 birds in the fall of the year is not due entirely to the advent of cold 

 weather, but is largely a matter of adjustment to food supply. 

 A migrant almost always depends upon fruits, seeds, and grains as 



part of its food. Most win- 

 ter residents, as the crow, are 

 omnivorous in diet. Others, 

 as the sparrows, may be seed 

 eaters, but under stress may 

 change their diet to almost 

 Mpt anything in the line of food ; 

 still others, as the wood- 

 peckers, although insect-eat- 

 ing birds, manage to find the 

 desired food tucked away 

 under the bark of trees. 

 Many insect-eating birds, 

 however, because their food 

 is found on green plants, 

 appear to be forced south- 

 ward by the cold weather. 



African ostrich (Struthio camelus). 



Classification of Birds. 

 Birds are divided into two great 

 groups, depending on the de- 

 velopment of the keel; that is, the part of the breastbone to which the 

 muscles used in flight are attached. Hence all flying birds are placed in 

 a group called the Carinatce. 



Birds in which the keel of the breastbone is not well developed, such 



1 There is opportunity for a careful observer to learn much of the spring or fall 

 migrations in the particular part of the country in which he resides. All informa- 

 tion thus obtained should be sent to the secretary of the American Ornithologists' 

 Union or to W. W. Cooke of the Biological Survey, who has done much to estab- 

 lish what we already know about bird migration in this country. 



