FOES AMONG THE BIRDS 107 



It does not always follow that birds are doing 

 harm when destroying these beneficial insects. In 

 the case of the small parasites, their presence in the 

 bird's stomach suggests that probably these insects 

 at the time were unusually abundant and probably 

 little harm was done by destroying their excess. In 

 the case of parasitic insects eaten during the latter 

 half of the summer, there would not have been suffi- 

 cient time for these to become effective even had 

 they lived. There is also to be considered the fact 

 that the parasites often destroy useful insects or are 

 destroyed by other insects. So that when all things 

 are taken into consideration in this complicated 

 problem, the amount of harm done by birds in eat- 

 ing beneficial insects is not so great as might at first 

 appear. 



Injury to valuable birds. Some birds may be 

 harmful by injuring other birds which are distinctly 

 valuable. In some cases the adult birds may be 

 killed and used for food. The worst offenders are 

 the sharp-shinned hawk, Cooper's hawk, pigeon 

 hawk, and the great horned owl, especially the 

 sharp-shinned hawk, more than half of whose food 

 consists of small valuable birds. This hawk has 

 been known to feed on forty-six kinds of birds. 

 Other hawks and owls, but to a much less extent, 

 may occasionally feed on song-birds. Beneficial 

 birds form about one fourth of the food of the 

 shrike, or butcher-bird. In other cases birds may 



