VALUE OF BIRDS TO MAN. 63 



the two birds ate only thirty-five hundred an hour for three 

 hours a day, they would consume ten thousand five hundred 

 aphids each day, or seventy-three thousand five hundred in 

 a week. It requires no 

 draft on the imagination 

 to see how such appe- 

 tites may become useful 

 to the farmer if they are 

 satiated on his insect 

 enemies. 



Two Scarlet Tanagers 

 were seen eating very 



Fig. 26. Yellow-throat catohing birch aphids. 



small caterpillars of the 



gipsy moth for eighteen minutes, at the rate of thirty-five 

 a minute. These birds spent much time in that way. If 

 we assume that they ate caterpillars at this rate for only an 

 hour each day, they must have consumed daily twenty-one 

 hundred caterpillars, or fourteen thousand seven hundred 

 in a week. Such a number of caterpillars would be suffi- 

 cient to defoliate two average apple trees, and so prevent 

 fruitage. The removal of these caterpillars might enable the 

 trees to bear a full crop. It is easily possible, therefore, 

 for a single pair of these birds in a week's time to save the 

 fruit of two average apple trees, a crop worth from two 

 to five dollars or more, according to the productiveness of 

 the trees and the price paid for apples. 



BIRDS SAVE TREES AND CROPS FROM DESTRUCTION. 



Since birds evidently operate to check insect outbreaks, it 

 follows that in their capacity of insect destroyers they must 

 in many instances have saved trees and crops from destruc- 

 tion by insect pests. If, however, we turn to the literature 

 of agriculture, entomology, and ornitholog} r , we shall not find 

 it replete with such instances. Still, there are enough on 

 record to show that conspicuous services of birds have been 

 noted occasionally ; and I am convinced by my own experi- 

 ence that such checks to insect increase occur commonly, but 

 escape both observation and record. 



Some brief but striking accounts of this class of occur- 



