EASTERN BROWN-HEADED COWBIRD 431 



animal may have placed the material there and the cowbird had 

 mistaken it for a bird 's nest. We were too far away from any human 

 habitation for any man or boy to have put it there, so I will let the 

 reader decide how it got there; I offer it only as an interesting 

 suggestion. 



Regardless of what significance the above suggestion may have, our 

 North American cowbirds, as we know them today, are all wholly 

 parasitic, laying their eggs in the nests of other birds and leaving their 

 young to be raised by their foster parents. In the long list of birds 

 thus imposed upon, the vireos, the wood warblers, and the small 

 sparrows figure most prominently. No attempt will be made to list 

 here all of the victims of the cowbirds; this has been well done by 

 Bendire (1895) and more thoroughly done by Friedmann (1929). The 

 latter makes the following general statements as to the families 

 afflicted : 



Most of the victims of the Cowbird are contained in four families — the tyrant 

 flycatchers, the finches, the vireos, and the warblers. Of the thirty-six species 

 and subspecies of tyrant flycatchers in the North American fauna eleven are known 

 to be parasitized, while of the remaining twenty-five, seven do not breed within 

 the breeding range of the Cowbird. * * * The Cowbird is known to victimize 

 sixty-two species and subspecies of finches. The total number of North American 

 forms of this great family is one hundred and ninety-four, of which about a hundred 

 are not known to breed within the range of the Cowbird. Yet the family is one of 

 great importance to the parasite as some of its component species are very fre- 

 quently victimized. The Cowbird is probably one of the chief factors in checking 

 the increase in the smaller Sparrows and Finches. 



The Vireos, while relatively few in species, are nevertheless a very important 

 factor in the natural economy of the Cowbird, and the latter is undoubtedly the 

 most serious single enemy of the birds of this family. No birds are more frequently 

 affected, either absolutely or relatively, and none make less protest at the frequent 

 impositions of the parasite. Of the twelve species of Vireos in the North American 

 Check-List, nine are known to be victimized. Including subspecies, twelve of the 

 twenty-five forms are included in the present list of victims. Of the remaining 

 thirteen, five do not breed within the Cowbird's range and six others have ranges 

 which only slightly coincide with that of Molothrus. 



Of the fifty-four species of Warblers in the North American fauna thirty-six are 

 known to be more or less imposed upon by the Cowbird, and, of the remaining 

 eighteen, at least ten do not breed in any part of the Cowbird's range, or are, at 

 most, so rare, that the absence of records means nothing. The other eight are still 

 little known and very few of their nests have been found. Including subspecies, 

 forty-four, of the seventy-three kinds of Warblers, are included in the list of the 

 victims of the Cowbird. 



Of other families of less importance in the economy of the cowbird, 

 he says that five species of mockingbirds and thrashers, are rarely 

 victimized by the Cowbird ; with only one of the five has the Cowbird 

 definitely known to be successful. He continues: 



The Wrens are almost negligible factors in the ecology of the Cowbird, and the 

 latter is of no great consequence in the life histories of these birds. Four of the 



