From May through July, 1993–1998 and 2001, effects of Brown-headed Cowbird (Molothrus ater) parasitism on 36 Warbling Vireo (Vireo gilvus) nests were documented in southwest Colorado. Overall parasitism was 75.0% and was the major source of nest failure. Among vireo nests that were not depredated, a significantly lower proportion of parasitized nests (10.2%) than nonparasitized nests (85.7%) fledged host young. Significantly fewer vireos hatched and fledged per nest and per egg in parasitized nests compared with nonparasitized nests. We observed no significant differences in mortality rates during the incubation stage between parasitized and nonparasitized nests. However, during the nestling stage, mortality rate was significantly higher in parasitized nests (0.17 nests lost per observation day) than nonparasitized nests (0.01 nests lost per observation day). Cowbirds almost always hatched 3 to 5 days before vireo nestlings. In two cases, vireo nestlings fledged from parasitized nests; in one nest the cowbird egg failed to hatch, and in the other nest the vireo hatched simultaneously with the cowbird. The low vireo production at that study site suggests that that area may be a sink population for Warbling Vireos.