13° 



TRANSACTIONS OF THE ILLINOIS 



In June five specimens were taken. While we find the ratio of cater- 

 pillars about the same (twenty-nine per cent.), the eye runs wonderingly 

 down the June column of the table, past the crane-flies and the preda- 

 ceous beetles and the dung-beetles and the wire-worms and the curculios 

 and the grasshoppers and the bugs and the spiders and the myriapods, 

 encountering only empty spaces or the most insignificant figures, until 

 one begins to think the birds must have lived on air or the beautiful June 

 weather, reaching finally the very bottom of the list, when the sinister 

 heading " fruits " is found, and opposite this are fifty-five of the missing 

 units. Seventeen per cent, of the food was raspberries and thirty-eight 

 per cent, cherries. Besides the above twenty-nine per cent, of cater- 

 pillars, these birds had eaten six per cent, of other injurious insects, 

 making a total of thirty-five per cent, to their credit, against fifty-five per 

 cent, of stolen fruit. 



And here we come to the most difficult feature of this whole research. 



How shall we balance the different elements of food against each 

 other ? Shall we simply measure bulk against bulk, subtracting the 

 thirty-five per cent, of caterpillars, etc., from the fifty-five per cent, of 

 raspberries and cherries, and say that these birds are to be charged with 

 a balance of evil amounting to twenty per cent, of their food? That is 

 to say, is a quart of caterpillars only the fair equivalent of a quart of 

 cherries? Or, to put a more evident case as an example, suppose that 

 the injury done had been the destruction of ichneumon parasites, should 

 we use these percentages without modification, and balance a quart of 

 ichneumons against a quart of caterpillars? The absurdity of this is 

 evident. A quart of ichneumons would destroy bushels of caterpillars. 

 Similarly, if not equally absurd, would be the conclusion that our bird 

 is injurious because it has eaten more fruit than insects. A quart of 

 cherries is not worth more than ten cents on the tree. Does any one 

 suppose that a quart of average caterpillars would do no more than a 

 dime's worth of damage in a summer season ? Scatter a pint of canker- 

 worms over an apple-tree, of leaf-rollers in a strawberry field, of green 

 caterpillars on a maple, of army-worms in a meadow, and let them work 

 until they *' spin up," and see if a dime will mend the damages? 



And then we must remember this most important fact, in this 

 connection : that when a bird eats fruit the injury done stops right there, 

 there are no accumulating consequences ; but when it destroys a noxious 

 insect, it checks the increase of the species, it destroys not only the one 

 actual insect, but an indefinitely numerous host of potential ones. 

 The benefit done is so much capital invested at an enormous rate of 

 compound interest. 



I believe that this one consideration will prove sufficient to settle the 

 question with respect to many insectivorous birds ; and it certainly indi- 

 cates to my mind that, notwithstanding the apparent balance against 

 them, our robins were very largely beneficial in June. This is especially 

 evident when we recall the fact that in this month the robins chiefly raise 

 their families, so that we have left out of account the vast amount of 

 insect-food fed to their young. Indeed, the search for soft food which 



