Red-winged blackbirds (Agelaius phoeniceus) are a polygynous songbird with facultative biparental care, and a common host for brown-headed cowbirds (Molothrus ater), an obligate brood parasite. We examined brood parasitism and paternal care in a long-term study of parental care in red-winged blackbirds. The presence of a cowbird nestling was associated with a higher likelihood of paternal care by the host male redwing in both naturally and experimentally parasitized nests. This result indicates that it was the presence of the brood parasite that was important and not simply that brood parasites chose hosts where paternal care was more likely. Both male and particularly female redwings increased provisioning to parasitized broods. Our work suggests that brood parasites raise the cost of parental care and push a polygynous host species toward monogamy.