INSECT FOOD OF THE BLUE GROSBEAK. 81 



weeds, a work of which the blue grosbeak assumes a proper share 

 and the value of which in the aggregate is immense. 



Before leaving the subject of vegetable food, it should be mentioned 

 that one blue grosbeak, collected in the District of Columbia, had fed 

 largely upon the seeds of wild rice (Zizania aquatica). This plant 

 is an important source of food for many larger birds, especially ducks 

 and geese, but is not generally sought by the smaller species. 



Animal Food. 



The blue grosbeak consumes more than twice as much animal as 

 vegetable food, and it consists of snails, spiders, and various insects. 

 The latter constitute 65.7 out of 67.6 percent, the entire amount, and 

 while comprising for the most part injurious species, include a few 

 forms generally considered useful. These will be discussed at once. 



A certain family of ground-inhabiting beetles, the Carabida?, on 

 account of their predaceous habits, are 

 usually classed as beneficial. They have been 

 detected, however ; feeding upon vegetable 

 matter, and in many places even upon culti- 

 vated crops. These facts should make us 

 slow to condemn a bird for picking up a few 

 of them. The present species manifests very 

 little liking for ground-beetles, onlv 4 birds 



e M . A . to b . _ . ■' /; _ Fig. 37.— Seeds of green fox- 



of the entire number having taken them, and tali (chwtochioa viridis). 

 in no case did they constitute more than 10 ( From Hillman > Nevada 



■l •- ■ , , , , Experiment Station.) 



per cent of the stomach contents. 



One other predaceous insect was captured. This was a robber fly 

 (Asilidse). Flies of this family capture grasshoppers, beetles, other 

 flies and bees, and at times do considerable mischief among honey 

 bees. They are not wholly beneficial, therefore, although probably 

 the balance is in their favor. If it is the rule, however, as appears 

 from the present examination, that not more than 1 blue grosbeak in 

 50 takes a robber fly, the resulting damage need not disturb us. 



Besides the insects of predatory habits, there are others which are 

 useful as parasites. None of these were taken directly by any of the 

 blue grosbeaks examined, but about 20 eggs of a parasite, probably 

 a tachina fly, were attached to the body of a purslane caterpillar eaten 

 by one of the birds. Had these parasitic flies been allowed to com- 

 plete development they would have attacked and destroyed other 

 caterpillars, but probably not so many as the grosbeak which fed upon 

 them, one-tenth of whose food would have consisted of these creatures. 



Caterpillars and adult lepidoptera (moths) compose 9.77 percent 

 of the food of the blue grosbeak. The purslane caterpillar (fig. 38) 

 mentioned above is eaten by adults and is fed also to the nestlings. 



