5 
ogist; and to give you some further proofs of the power exercised by the birds I can 
do no better than quote from one of his papers published in 1880. Taking three 
birds to the acre as a basis he continues : “ It is my own opinion that about two- 
t irds of the food of birds consists of insects, and that tliis insect food will avemge, 
dt t e lowest reasonable estimate, twenty insects or insects’ egg's per day for each 
individual of the two-thirds, giving a total for the year of 7,200 per acre, or about 
two hundred and fifty billions for the state. 
Estimates of the average number of insects per square yard give us, at the 
arthest. ten thousand per acre for our whole area. On this basis, if the opera¬ 
tions of the birds were to be suspended, the rate of increase of the.se insect hosts 
would be accelerated by about seventy per cent ; and their numbers instead of re¬ 
maining year by year at the present average figure, would be increased over two- 
t irds each year. Anyone familiar with geometrical 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 was not otherwise 
c ecked, we should have the whole state carpeted with insects, one to the square 
inch over our whole territory. I have so arranged this computation as to exclude 
the insolvable question of the relative value of the birds and predaceous or para¬ 
sitic ins-^cts, unless we suppose that birds eat an undue proportion of beneficial 
species. 
“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 Illi¬ 
nois 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 
t eir probability. At any rate, few intelligent farmers or gardener would refuse an 
ofter to insure complete protection year after year, against insects of all sorts for 
twenty-five cents an acre per annum; and we will, therefore, place the damage at 
one-half the above amount—ten million dollars per annum. 
Supposing that, as a consequence of this investigation, we are able to take 
nmasiires which shall result in the increase 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 per annum, equivalent to the 
addition of over one and one-half million dollars to the permanent value of our 
property; or if. as ni fact a most moderate estimate, we should succeed in increasing 
the efficiency of birds five per cent, we should therefore add eight and one-fourth 
million dollars to the permanent wealth of the state, provided, as before, that birds 
do not eat unduly of beneficial species.’’ 
Profes.sor Forbes adds that “these figures will be at once rejected by most 
naturalists as absurdly low’’ and in this also I must heartily agree with him. But 
whether low or not I hope what I have said will convince you that in the birds we 
have one of the great and constantly acting forces of nature; that any disturbance in 
the action of this force must have widespread consequences, and if the interference is 
of any magnitude, such as a marked diminution in the number of birds in a given 
locality, the consequences would inevitably be serious and would probably be mani¬ 
fested in a disaster to ourselves. 
The results that would follow such a disturbance of nature’s economy as here 
suggested rest on no fine drawn theories but on actual historic experiences.’ Before 
the days of scientific investigation enactments against the birds as destroyers of fruit 
and gram were not rare. In 1820, Bridgewater, Mass., successfully exterminated 
