The Food of Birds. 87 



Estiniates 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 ])asis, if the opera- 

 tions of the birds were to be suspended, the rate of in- 

 crease of these insect hosts would be accelerated about 

 seventy per cent., and their numbers, instead of remain- 

 ing year by year at the present average figure, would be 

 increased over two-thirds each year. Any one familiar 

 with geometrical ratios will understand the inevitable re- 

 sult. 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 

 computation as to exclude the insoluble question of the 

 relative 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 pow- 

 er of birds for good or evil, and not as a prediction of the 

 consequences of their total destruction. These consequen- 

 ces would not be by any means so simple, but would ap- 

 parently 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 re- 

 fuse an otfer to insure complete protection, year after year, 

 against insects of all sorts for twenty-Uve cents an acre per 

 annum; and we will, therefore, place the damage at one- 

 half 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 

 dollars i^er annum, equivalent to the addition of over one 



