VALUE OF BIRDS T0 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 
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. 
Fig. 26.— Yellow-throat catching birch aphids. 
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 ornithology, 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- 
