HOST RELATIONS OF PARASITIC COWBIRDS 89 



were parasitized. In southern Quebec, Terrill found that 26 out of 63 

 nests, or 41 percent, were so affected. 



In its surveys of the nesting birds of the Detroit region, the Detroit 

 Audubon Society (1953, p. 72; 1954, p. 85; 1956, p. 90) reports that 

 the red-eyed vireo is one of the most heavily parasitized bhds in that 

 area. This conclusion, however, is a comment more upon the vireo as 

 a cowbird host than upon the cowbird as a vireo parasite. The 

 yellow warbler and the song sparrow were the most frequent hosts in 

 the locale; far more of their nests were found with cowbird eggs or 

 young than was the case with the red-eyed vkeo. Only in the per- 

 centage of parasitism of the total number of observed nests did the 

 vireo emerge as a more frequently victimized species. 



In southern Quebec, Terrill (1961, p. 6) examined 64 nests, of which 

 27, or 42.2 percent, were parasitized. He concluded that the red-eyed 

 vireo was victimized more regularly in that area than any other host 

 species. 



To return to Southern's study, we should note his concluding obser- 

 vation that, although both the vireo and the cowbird were common 

 birds in the study area, if "the vireo population were in any way being 

 'harmed' by the cowbird, it was not apparent. Possibly parasitism 

 by the cowbird was a natm'al device for preventing an over-production 

 of vireos. ^Vlien considering the number of species parasitized bj^ the 

 cowbtrds in my study area it does seem possible that, if the cowbird 

 enjoyed equal success with other hosts, it might become too numerous. 

 But I strongly suspect that the cowbu'd has few other hosts in the 

 region that it parasitizes with any comparable success." 



Occasionally, the red-eyed vireo covers over or buries the parasitic 

 eggs in the manner of the yeUow warbler and some other birds, but 

 it has been known to accept and to incubate cowbird eggs even when 

 none of its own were present. Uusally it is an extremely tolerant host. 

 Although in the majority of cases only 1 parasitic egg was present, 

 there are numerous instances of 2, and fewer instances of 3 and even 4, 

 cowbird eggs in a single nest. An extreme case of multiple parasitism, 

 mentioned by Bent (1958, p. 438), was a nest containing 6 cowbii'd 

 eggs and none of the vireo, a phenomenon reported by F. A. E. Starr, 

 who found the vireo, nevertheless, sitting on this unusual clutch. 



Philadelphia Vireo 



Vireo philadelphicus (Cassin) 



The Philadelphia vireo is a rarely reported victim: only two records 

 have turned up. T. E. Randall wrote me that he found a nest of this 

 vireo with an egg of the cowbird (subspecies artemisiae) in Alberta. 

 Bailhe and Harrington (1937, p. 239) recorded that in the Sudbury 

 district, Ontario, on July 18, 1937, C. E. Hope saw a Philadelphia 



