Nest desertion by Field Sparrows and its possible influence on the evolution of Cowbird behavior

In this study, Field Sparrows (Spizella pusilla) deserted 46%. of nests, pa rasitized by Brown-headed Cowbirds (Molothrus ater) and only Po of unparasi tized nests suggesting that desertion functions primarily as an antiparasit e defense. Sparrows did not desert nests following various clutch manipulat ions that are often associated with parasitism, indicating that desertion w as not in response to the presence of cowbird eggs. Sparrows often deserted nests following encounters vith real or mounted cowbirds, suggesting that nest desertion is a response to adult cowbirds. Sparrows deserted nests onl y in stages most vulnerable to the effects of parasitism. That finding is c onsistent with the possibility that desertion is a parasite-specific respon se. Sparrows arrived at nests earlier in the da, at our Illinois site, wher e parasitism was greater, than in Missouri. Our findings confirm that host vigilance can prevent successful parasitism, and we provide the first direc t evidence that encounters with cowbirds may cause hosts to desert nests. O ur 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 tho se interactions may have implications for both the ecology and evolution of both the parasite and host.