84 ANIMAL BIOLOGY 



lose the birds, we should realize all too well how much they 

 contribute to the happiness of every lover of nature. When, 

 however, we come to discuss the economic value of birds, 

 the good that they do cannot be overestimated. Biologists 

 have carried on long series of studies to determine accurately 

 the food of different kinds of birds. This has been done 



by watching them 

 while they are eating 

 or while feeding their 

 young, and by examin- 

 ing the contents of birds' 

 stomachs. The following 

 paragraphs contain descrip- 

 tions of some of the ways in 

 which birds are of inestima- 

 ble use to man. 



67. Birds as destroyers of harmful 



insects. — Undoubtedly the greatest value 



of birds to man is the good that they do in 



destroying injurious insects. In 13-18, 23, 



and 46, we have described some of the 

 Fig. 72. — Black , , . , j. 



and white warbler, ravages made by our insect foes. 



" But if insects are the natural enemies 



of vegetation, birds are the natural enemies of insects. . . . 



In the air swallows and swifts are coursing rapidly to and fro, 



ever in pursuit of the insects which constitute their sole food. 



When they retire, the nighthawks and whip-poor-wills will take 



up the chase, catching moths and other nocturnal insects 



which would escape day-flying birds. Fly-catchers (Fig. 69) 



lie in wait, darting from ambush at passing prey, and with 



a suggestive click of the bill returning to their post. The 



warblers (Fig. 72), light, active creatures, flutter about the 



