Brown-Headed Cowbird parasitism results in a sink population in Warbling Vireos

The Warbling Vireo (Vireo gilvus) is one of the most heavily parasitized ho st species of the Brown-headed Cowbird (Molothrus ater), suffering up to 80 % parasitism in some areas. Warbling Vireo nests that are parasitized by co wbirds typically produce no vireo young. To make predictions about the cons equences of brood parasitism on local host populations, we investigated fac tors Chat 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 respons e of vireos to cowbird eggs. Warbling Vireos responded more strongly to a c owbird dummy placed near the nest than to a dummy of a "neutral" species (j unco) or to a common nest predator (crow). Preliminary simulation models ba sed on population parameters obtained from three summers of fieldwork sugge st that Warbling Vireos are in danger of extirpation from the Okanagan Vall ey if little movement of birds occurs between areas with different levers o f brood parasitism. The models also indicate the sensitivity of this specie s to variation in adult and juvenile survival rates and number of successfu l broods produced per season. Our results emphasize the need for a metapopu lation analysis that compares the population ecology of Warbling Vireos at high elevations land probably with low parasitism) with that at low elevati ons thigh parasitism) and that ascertains whether sufficient dispersal occu rs between these populations to prevent extinction at lower elevations. Suf ficient dispersal is indicated by apparently stable vireo populations in th e Okanagan Valley and by Breeding Bird Survey data that show an overall inc reasing trend for this species in British Columbia.