THE AMERICAN ENTOMOLOGIST. 



233 



each a day, would eat in six months about 

 half a million insects. If this number of 

 birds were destroyed, the result would be 

 the preservation, on the area supervised by 

 them, of about 170,000 caterpillars, (90,000 

 of them cutworms), 20,000 leaf-chafers, 

 10,000 curculios and 85,000 crickets, 

 locusts and grasshoppers. How this fright- 

 ful horde of marauders would busy itself 

 if left undisturbed, no one can doubt. 

 It would eat grass and clover and corn 

 and cabbage, inflicting an immense injury 

 itself, and leaving a progeny which would 

 multiply that injury indefinitely. On the 

 other hand, would the 200,000 predaceous 

 beetles and bugs, spiders and ichneumons 

 either prevent or compensate these injur- 

 ies ? I do not believe that we can say 

 positively whether they would or not. 



In a discussion of the natural checks 

 upon the cutworm. Prof. Riley, in his first 

 report as State Entomologist of Missouri, 

 mentions two species of Ichneumon that 

 parasitize the larva, credits the Spined sol- 

 dier bug and the carabid larva, Calosoma 

 calidttm, with its destruction, and says that 

 some kinds of spiders are known to prey 

 upon it. 



From the report of the U. S. Entomolo- 

 gical Commission for 1S77, we learn that 

 the grasshopper is preyed upon at one or 

 the other stage by Agonoderus, Harpalus, 

 Amara and other Carabids ; by soldier 

 beetles, soldier bugs and spiders; and that 

 certain Ichneumonidce parasitize the egg. It 

 seems probable, therefore, that the benefi- 

 cial insects eaten by bluebirds include the 

 special enemies of the cutworms and grass- 

 hoppers it destroys; but he who knows best 

 the small number of reliable observations 

 upon which our general statements of the 

 food of predaceous insects rest, will have 

 the most hesitation in trusting them with- 

 out reserve. The probabilities seem to be 

 against the Bluebird, but the certainties are, 

 as yet, in its favor. 



Finally, I would call attention to the fact 

 that we do not yet know that the normal 

 rate of increase among these carnivorous 

 and parasitic insects is not sufficient to 

 keep their numbers full to the limit of their 



food supply, and to furnish also a surplus 

 for destruction by birds. Just as a tree 

 puts forth more leaves than it needs, and 

 sets more fruit than it can possibly mature, 

 as an offset to the constant, normal depre- 

 dations of insects, so there is much reason 

 to suppose that our insect friends have be- 

 come adjusted to this steady drain on their 

 numbers. There are many considerations 

 involved here which I cannot at present 

 enter into. It will suffice to say that all 

 the evidence we have of the increase and 

 decrease of carnivorous insects attendant 

 upon the increase and decrease of the in- 

 sects upon which they feed, tends to show 

 that the real limit to their multiplication is 

 not destruction by birds, but a deficient 

 food-supply; and that in relieving them 

 from their feathered enemies we should 

 only be giving a portion of them the poor 

 privilege of starving to death instead of 

 being eaten up. 



Considering, therefore, the certainty of 

 the evil consequences of the destruction 

 of the Bluebird, and the uncertainty of the 

 possible good, I believe that, notwithstand- 

 ing the apparent balance against the spe- 

 cies, even the most radical economist, the 

 most indifferent to the beauty and pleasure 

 of the natural world, would have no pre- 

 sent justification for throttling the song of 

 the Bluebird in his garden with the hope 

 of increasing thereby his annual store of 

 hay and cabbage. 



SUMMARY. 



A recapitulation of the above data can 

 best be given in the form of the appended 

 table. The digits indicate the number of 

 birds in which each element of the food 

 was found, — the decimals show the ratio 

 of that element to the entire food of all 

 the birds for the month. The general 

 average and total for each element appear 

 at the right of the table, and the general 

 averages of benefit and injury for the 86 

 birds will be found at the lower right-hand 

 corner. They differ by a unit from the 

 figures given in the text, because the fruits 

 are included in the table, but ignored in 

 the discussions. 



