The Food of Birds. 
157 
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 Pentatomidac 
(robin one per cent.), of Orthoptera (twenty-one to four) 
and of spiders (eight to a fraction) ; but especially in the 
matter of edible fruits (one to thirty-four). These differ- 
ences are but 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 Lep- 
idoptera, Orthoptera and spiders. 
Economic Relations. 
Mr. B. D. Walsh, the first State Entomologist of Illi- 
nois, reasoning from the comparative numbers of injuri- 
ous and beneficial insects, concludes that a bird must be 
shown to eat at least thirty times as many injurious in- 
dividuals 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 aston- 
ishing that it certainly cannot pass without careful ex- 
amination. In the first playe 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 de- 
ficiency of credit may be due to this neglect of the food of 
the young is compensated in part, at least, by the fact 
that the number of caterpillars eaten is doubtless over- 
estimated in comparison with hard insects, as their flex- 
ible skins remain in the stomachs of birds longer than 
the hard structures of insects. This is exactly contrary 
to the usual supposition, but the frequent occurrence of 
the empty and twisted skins of cutworms in the stom- 
achs of these birds, still recognizable as Noctuidae when 
not even a fragment of a single head remains, is suffi- 
cient evidence that the hard parts break up and disappear 
* “Birds vs. Insects.’* Practical Entomologist, Vol. II, pp. 44-47. 
