. Food habits of the grosbeaks . are not wholly beneficial, therefore, although probablythe balance is in their favor. If it is the rule, however, as appearsfrom the present examination, that not more than 1 blue grosbeak in50 takes a robber fly, the resulting damage need not disturb us. Besides the insects of predatory habits, there are others which areuseful as parasites. None of these were taken directly by any of theblue grosbeaks examined, but about 20 eggs of a parasite, probablya tachina fly, were attached to the body of a purslane caterpillar eatenby. one of the birds. Had these parasi
. Food habits of the grosbeaks . are not wholly beneficial, therefore, although probablythe balance is in their favor. If it is the rule, however, as appearsfrom the present examination, that not more than 1 blue grosbeak in50 takes a robber fly, the resulting damage need not disturb us. Besides the insects of predatory habits, there are others which areuseful as parasites. None of these were taken directly by any of theblue grosbeaks examined, but about 20 eggs of a parasite, probablya tachina fly, were attached to the body of a purslane caterpillar eatenby. one of the birds. Had these parasitic flies been allowed to com-plete development they would have attacked and destroyed othercaterpillars, but probably not so many as the grosbeak which fed uponthem, one-tenth of whose food would have consisted of these creatures. Caterpillars and adult lepidoptera (moths) compose percentof the food of the blue grosbeak. The purslane caterpillar (fig. 38)mentioned above is eaten by is fed also to the 82 FOOD HABITS OF THE GROSBEAKS. Ordinarily this insect is harmless, but occasionally it attacks gardenand fruit crops, especially the sugar beet. Another caterpillar also,the cotton cutworm {Prodenia ornithogalli, fig. 9), which attacksthe latter crop as well as the tomato and cotton, is devoured by theblue grosbeak. Four birds, taken in the. cotton fields of Texas inMay, had eaten 9 cotton cutworms, which constituted more than 40percent of their food. Many other caterpillars also of the samefamily (Sphingidae), as the purslane feeder, and two moths wereconsumed by the blue grosbeaks examined. Because of the injurious
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