BROWN-HEADED COWBIRD PARASITISM RESULTS IN A SINK POPULATION IN WARBLING VIREOS

Abstract

Abstract The Warbling Vireo (Vireo gilvus) is one of the most heavily parasitized host species of the Brown-headed Cowbird (Molothrus ater), suffering up to 80% parasitism in some areas. Warbling Vireo nests that are parasitized by cowbirds typically produce no vireo young. To make predictions about the consequences of brood parasitism on local host populations, we investigated factors that allow such high parasitism to occur. The major factors leading to high levels of brood parasitism on Warbling Vireos in the Okanagan Valley, British Columbia, are habitat overlap with cowbirds and the lack of response of vireos to cowbird eggs. Warbling Vireos responded more strongly to a cowbird dummy placed near the nest than to a dummy of a “neutral” species (junco) or to a common nest predator (crow). Preliminary simulation models based on population parameters obtained from three summers of fieldwork suggest that Warbling Vireos are in danger of extirpation from the Okanagan Valley if little movement of birds occu...