A Feathered Parasite 79 



nests of the red-headed woodpecker and the rock wrens 

 are not exempt. Some species, notably the summer 

 warblers, detect the imposture and set about defeating 

 the purposes of the interloper by building another story 

 to their little cottage, leaving the obtruded eggs in the 

 cellar, where they do not receive enough warmth to 

 develop the embryo. 



While it is surprising that acute birds should allow 

 themselves to be imposed on in this way, perhaps, after 

 all, they look upon the cowbird as a kind of blessing in 

 disguise ; at least, he may not be an unmixed evil. They 

 may act on the principle of reciprocity that " one good 

 turn deserves another." What I mean is this: In my 

 rambles I have often found the cowbirds the first to give 

 warning of the approach of a supposed danger. Having 

 no domestic duties of their own, they can well secrete 

 themselves in a tall tree 'overlooking the entire premises, 

 and thus play the useful role of sentinel. This, I am 

 disposed to believe, is one of the compensating uses of 

 this parasite, and may furnish the reason for his being 

 tolerated in birdland. And he is tolerated. Has any 

 one ever seen other birds driving the cowbird away from 

 their breeding precincts, or charging him with desperate 

 courage, as they do the blue jays, the hawks, the owls, 

 and other predatory species? He evidently subserves 

 some useful purpose in the avian community, or he would 

 not be treated with so much consideration. 



A young cowbird that I purloined from the nest and 

 tried to rear bv hand did not prove a pleasant pet. He 



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