In this study, Field Sparrows (Spizella pusilla) deserted 46% of nests, parasitized by Brown-headed Cowbirds (Molothrus ater) and only 1% of unparasitized nests suggesting that desertion functions primarily as an antiparasite defense. Sparrows did not desert nests following various clutch manipulations that are often associated with parasitism, indicating that desertion was not in response to the presence of cowbird eggs. Sparrows often deserted nests following encounters with real or mounted cowbirds, suggesting that nest desertion is a response to adult cowbirds. Sparrows deserted nests only in stages most vulnerable to the effects of parasitism. That finding is consistent with the possibility that desertion is a parasite-specific response. Sparrows arrived at nests earlier in the day at our Illinois site, where parasitism was greater, than in Missouri. Our findings confirm that host vigilance can prevent successful parasitism, and we provide the first direct evidence that encounters with cowbirds may cause hosts to desert nests. Our findings may help explain why cowbirds parasitize nests extremely early in the morning and lay rapidly. We suggest that consideration be given to host response following interactions with adult brood parasites because those interactions may have implications for both the ecology and evolution of both the parasite and host.