302 



ZOOLOGY 



largely a matter of adjustment to food supply. A migrant almost 

 always depends to a large extent upon fruits, seeds, and grains as 

 part of its food. Most winter residents, as the crow, are omniv- 

 orous in diet. Others, as the sparrows, may be seed eaters, but 

 under stress may change their diet to almost anything in the line 

 of food; still others, as the woodpeckers, although insect-eating 

 birds, manage to find the desired food tucked away under the 

 bark of trees. Most insect-eating birds, however, because their 

 food is found on green plants, are forced southward by the cold 

 weather. Migrations are almost entirely due to need of food 

 which cannot be obtained during a time when vegetation is 

 dormant and the ground is frozen. 



Classification of Birds. — Birds are divided into two great groups, de- 

 pending on the development of the keel, that is, the part of the sternum to 



which the muscles used in flight 

 are attached. This bone is well 

 known to every one who has 

 ever picked the breastbone of 

 a chicken. Hence all flying 

 birds are placed in a group 

 called the Carinatce. 



Birds in which the keel of 

 the breastbone is not well de- 

 veloped, such as the ostrich and 

 cassowary, are said to belong to 

 the Ratitce. These birds make 

 up for their lack of wing de- 

 velopment by having the legs 

 strong and long. 



The flying birds are further 

 subdivided into a number of 

 orders, the classification based 

 upon the adaptations of differ- 

 ent parts of the bird, especially 

 the legs and feet, the wings 

 and the bill, to different func- 

 tions. We shall not trouble 

 ourselves to learn all the differ- 

 ent groups, but shall content 

 ourselves with picking out some of the more evident and important ones. 

 I. Perching Birds. — To this order belong most of our common birds. 



African ostrich {Struthio camelvs). 



