BIRDS 



395 



the presence of its natural 

 enemies. This condition 

 is spoken of as the " bal- 

 ance of nature," and 

 when it is destroyed by 

 the elimination of one 

 side of the balance, of 

 the birds, only disaster 

 can result. In similar 

 fashion, birds keep down 

 the mice and other ro- 

 dents, and hinder the in- 

 crease of weeds by con- 

 suming vast quantities of 

 their seeds. The preser- 

 vation of birds thus becomes not merely a matter of 

 sentiment but a public policy of the highest importance. 



Photograph by E. R. Warren 

 FIG. 168. Western tree sparrow (Spizella 

 monticola ochracea), Colorado Springs, 

 Colorado. A winter resident in Colorado, 

 and a typical member of the large family 

 Fringillidae, which includes the sparrows, 

 finches, etc. Note the short, thick bill, 

 adapted for feeding on seeds, etc. 



References 



BEEBE, C. WILLIAM. The Bird; Its Form and Function. Henry Holt & Co., 



1906. 



KNOWLTON, F. H. Birds of the World. Henry Holt & Co., 1909. 

 RIDGWAY, R. Birds of North and Middle America. (United States 



National Museum.) This is the standard work on the classification 



of American birds, but is severely technical. 



NEWTON, ALFRED. A Dictionary of Birds. A. C. Black, 1893-96. 

 WEED, C. M., and DEARBORN, N. Birds in Their Relations to Man. J. B. 



Lippincott Company, 1903. 

 United States Department of Agriculture. Many valuable bulletins on 



economic ornithology. 



