THE ALLIES OF THE PIGEON 397 



northerly and southerly movements then begun among the 

 birds have been continued till today. 



Bird-Banding. In the past few years much valuable in- 

 formation about movement and migration of birds has been 

 obtained by banding. Small metal bands are placed on the 

 feet of young birds before they leave the nest and of old birds 

 captured in traps. These bands bear numbers and instruc- 

 tions to notify the Biological Survey at Washington, D.C., 

 in case the bird is killed or trapped. By this means it has 

 been determined that in many species individuals return 

 year after year to exactly the same nesting site. Migration 

 routes are also studied in this manner. 



Economic Importance of Birds. Leaving out of considera- 

 tion their value to man as a source of food, birds are chiefly 

 important economically in connection with their destruction 

 of insects injurious to vegetation. Of course not all birds 

 are beneficial in this respect ; whether they are beneficial or 

 injurious depends largely on the character of their food. 

 What we know of the food of birds has come not only from 

 the observation of the birds in the field but also from the 

 examination of the contents of their stomachs. The Bureau 

 of Biological Survey of the Department of Agriculture has 

 performed a most useful task in collecting, tabulating, and 

 publishing observations from all parts of the country. The 

 diagram (Fig. 210) shows the proportion of different sorts 

 of food in the young and the adult of the common crow. 



Some of the conclusions of Dr. Judd, who studied the 

 birds of a Maryland farm for the Biological Survey, are as 

 follows : 



. . . the English sparrow, the sharp-shinned and Cooper hawks, 

 and the great horned owl are, as everywhere, inimical to the 

 farmers' interests and should be killed at every opportunity. The 

 sapsucker punctures orchard trees extensively and should be shot. 

 The study of the crow is unfavorable in results so far as these 



