Red-winged Blackbirds (Agelaius phoeniceus) may be more likely to desert nests parasitized by Brown-headed Cowbirds (Molothrus ater) when they interact with the parasite, because desertion frequencies are higher at naturally parasitized nests than at artificially parasitized nests in some brood-parasitic hosts. We experimentally parasitized blackbird nests in southern Manitoba with cowbird model eggs and presented taxidermic mounts of adult female cowbirds near their nests. We observed no desertion, which is possibly attributable to constraints of a shorter breeding season at this latitude. We also compared rejection frequencies for blackbirds when a mounted female cowbird was placed near the nest to rejection frequencies recorded when (1) no model egg was added, (2) a model egg was added but no mounted cowbird was presented, and (3) a model egg was added plus presentation of a mount of a non-parasitic and non-threatening species, the Fox Sparrow (Passerella iliaca). Blackbirds struck the cowbird mount more often than the sparrow mount, but responses to the cowbird mount did not influence acceptance or rejection of the model egg. Blackbirds apparently engage in the least costly form of anti-parasite defense (aggression) and accept the costs of acceptance instead of those of renesting during a short breeding season.