HOST RELATIONS OF PARASITIC COWBIRDS 9 



however, the catbu'd and the brown thrasher are the only species 

 for which there are numerous records. 



Of the 18 North American members of the thrush family, 8 are 

 known to be parasitized; 6 of the others do not breed in the cowbird's 

 range. Some of the brown thrushes (Hylocichla) are important 

 hosts. 



The vireos are severely parasitized. There are 12 species in North 

 America, one of which, the black-whiskered vireo, is allopatric with 

 the cowbu'd, but all the others are imposed upon, many of them 

 extensively. 



The near relatives of the vii'eos, the wood warblers, are also very 

 important as hosts and include some of the most frequently victimized 

 bi'"ds. There are records of parasitism on 45 of the 57 species of 

 wood warblers m North Ajuerica, and of the unused minority, about 

 half are protected by theu" geogTaphic or ecologic allopatry with 

 the parasite. 



The blackbirds and orioles of North America comprise 18 species, 

 not counting the 2 species of cowbii'ds, and of these 18, 13 are included 

 among the victims of the brown-headed cowbird. The redwinged 

 blackbu'd is the only member of the family that is generally and 

 frequently parasitized; some of the others are local in their sympatry 

 and availability. 



The tanagers are a small group in North America, comprising only 

 four species, of which thi"ee are known to be victimized by the parasite. 



The largest avian family in North America, the finches and their 

 allies, comprises 84 species, of which 56 are known to be parasitized, 

 including a good number that are used extensively. Of the remaining 

 28 species, 5 are only accidental in North America, not counted as 

 breeding birds, and 16 others are allopatric with the cowbird. It is 

 possible that, as the range of the parasite expands, some of these 

 may become sympatric and may then be imposed upon. The north- 

 western extension of the cowbu'd's breedmg range in British Columbia 

 has resulted recently in the addition of the redpoll to the list of its 

 victims. 



In their comprehensive summary of the bu'ds of Maryland and the 

 District of Columbia, a very well-studied area, Stewart and Robbins 

 (1958, p. 329) noted 223 cases of cowbird parasitism. Of these, 74 

 involved species of fuiches, 53 were with warblers, 44 with vireos, 12 

 with thrushes, 10 with blackbirds and orioles, 10 with flycatchers, 8 

 with tanagers, and 12 with various other hosts. 



Changes in Host Selection 



Comparison of the host preferences of Molothrus ater with those of 

 its neotropical relative, M. bonariensis, shows some interesting differ- 



