Using real cowbird eggs, we experimentally parasitized 41 nests of the Warbling Vireo (Vireo gilvus), three each in British Columbia and Colorado, five in Montana, and 30 in Manitoba, and recorded whether the cowbird eggs were accepted or rejected. Cowbird eggs were accepted at all nests tested in British Columbia and Colorado, but both acceptance and rejection were recorded in Montana. In Manitoba, all cowbird eggs were rejected (29 by puncture-ejection, one by desertion). The results suggest acceptance by a western subspecies of the Warbling Vireo, V. g. swainsonii, and rejection by the eastern subspecies, V. g. gilvus. The geographic variability in acceptance/rejection agrees with suggested taxonomic differences for the Warbling Vireo, i.e., that there are two species and that neither appears to vary in response to the presence of cowbird eggs in its nests.