The Food of Birds. 157 



it differs principall}^ in the lar2;er number of Hymenop- 

 tera (seven to four) and Lepidoptera (twenty-six to sev- 

 enteen), the lack of Diptera (robin seventeen per cent.), 

 the excess of Aphodii (six to two), of Pentatomid;v (robin 

 one per cent.), of Orthoptera (twenty-one to four) and of 

 spiders (ei^ht to a fraction) ; .but especially in the 

 matter of edible fruits (one to thirty-four). These differ- 

 ences are l)ut little greater, however, than those among the 

 thrushes themselves. Compared with the thrush family as a 

 whole, its salient peculiarities are its neglect of Diptera 

 and garden fruits and its preference for Lepidoptera, Or- 

 thoptera and spiders. 



Economic Relations. 



Mr. B. D. Walsh, the first State Entomologist of Illinois, 

 reasoning from the comparative numbers of injurious and 

 beneficial insects, concludes that a bird must be shown to 

 eat at least thirty times as many injurious individuals as 

 beneficial before it can be considered useful.* 



According to this estimate, the bluebird does at least 

 thirteen times as much harm as good; that is to say, the 

 beneficial insects eaten would themselves have destroyed 

 thirteen times as many injurious insects as the birds have 

 eaten. This conclusion is so unexpected and astonishing 

 that it certainly cannot pass without careful examination. 

 In the first place we should bear in mind that nothing has 

 yet been learned of the food of the young, and there is some 

 reason for supposing that birds select the softer insects for 

 their young. Whatever deficiency of credit may be due 

 to this neglect of the food of the youn g is compensated 

 in j)art, at least, by the fact that the number of caterpil- 

 lars eaten is doubtless overestimated in comparison with 

 hard insects, as their flexible skins remain in the stomachs 

 of birds longer than the hard structures of insects. This 

 is exactly contrary to the usual supposition, but the fre- 

 quent occurrence of the empty and twisted skins of cut- 

 worms in the stomachs of these birds, still recognizal^le as 

 Noctuidae when not even a fragment of a single head re- 

 mains, is sufficient evidence that the hard parts break up 



*"Birds vs. Insects." Practical Entomologist, Vol. II., pp. 44-47. 



