The Food of Birds . 
87 
Estimates of the average number of insects per square 
yard in this State gives us, at farthest, ten thousand per 
acre for our whole area. On this basis, if the operations 
of the birds were to be suspended, the rate of increase of 
these insect hosts would be accelerated about seventy per 
cent., and their numbers, instead of remaining year by 
year at the present average figure, would be increased 
over two-thirds each year. Anyone familiar with geo- 
metrical ratios will understand the inevitable result. In 
the second year we should find insects nearly, three times 
as numerous as now, and, in about twelve years, if this 
increase were not otherwise checked, we should have the 
entire State carpeted with insects, one to the square inch 
over our whole territory. I have so arranged this com- 
putation as to exclude the insoluble question of the rela- 
tive value of birds and predaceous or parasitic insects, 
unless we suppose that birds eat an undue proportion of 
beneficial species. 
This is intended only as an illustration of the great 
power of birds for good or evil, and not as a prediction 
of the consequences of their total destruction. These 
consequences would not be by any means so simple, but 
would apparently be fully as grave. 
Let us take another view of this matter. According to 
the computation of our first State Entomologist, Mr. 
Walsh, the average damage done by insects in Illinois 
amounts to twenty million dollars a year. These are 
large figures, certainly; but when we find that this means 
only about fifty-six cents an acre, we begin to see their 
probability. At any rate, few intelligent farmers or 
gardeners would refuse an offer to insure complete pro- 
tection, year after year, against insects of all sorts for 
twenty-five cents an acre per annum; and we will, there- 
fore place the damage at one-lialf of the above amount — 
ten million dollars per annum. 
Supposing that, as a consequence of this investigation, 
we are able to take measures which shall result in the in- 
crease, by so much as one per cent., of the efficiency of 
birds as an insect police, the effect would be a diminution 
of the above injury to the amount of sixty-six thousand 
