We studied effects of parasitism by brown-headed cowbirds (Molothrus ater) on three frequent hosts from a pinyon-juniper (Pinus-Juniperus) community in New Mexico. During 1993–1999 and 2001–2002, we measured frequencies of parasitism and examined how differences in size of host, life-history traits, and behavior, as well as aspects of behavior of brown-headed cowbirds (i.e., removal of eggs of host and multiple parasitism), influenced reproductive success of blue-gray gnatcatchers (Polioptila caerulea), plumbeous vireos (Vireo plumbeus), and western tanagers (Piranga ludoviciana). Brown-headed cowbirds parasitized 79% of 288 nests of blue-gray gnatcatchers, 84% of 170 nests of plumbeous vireos, and 76% of 129 nests of western tanagers. Parasitism reduced productivity of hosts through removal of eggs, reduced hatching success, reduced survival of nestlings, and desertion of nest. Relative to unparasitized nests, size of clutch of singly parasitized nests was reduced, on average, by 0.3 eggs for the plumbeous vireo and by 0.8 eggs for the two other species. Both blue-gray gnatcatchers and plumbeous vireos rarely fledged any of their own young from parasitized nests, whereas western tanagers did so regularly. All three species deserted some nests in association with parasitism, but the primary response to parasitism was acceptance of eggs from brown-headed cowbirds. Multiple parasitism was common, however, mean number of young of hosts that fledged only differed between singly and multiply parasitized nests for the western tanager due to high costs of single parasitism for the blue-gray gnatcatcher and plumbeous vireo. Overall, costs of parasitism were negatively correlated to body size of host; parasitism caused failure of 58% of nests of blue-gray gnatcatchers, 49% of nests of plumbeous vireos, and 22% of nests of western tanagers.