Landscape-Level Habitat Use by Brown-Headed Cowbirds in Vermont

Abstract

We studied the distribution of brown-headed cowbirds (Molothrus ater) in a forested landscape in central Vermont to better understand effects of forest disturbance on the occurrence of this species in New England. We mapped 702 patches where chronic disturbance created permanent openings, such as pastures, lawns, and ski areas, and where recent logging had created temporary forest openings. We surveyed 67 randomly selected disturbance patches for cowbird presence during the 1992-93 breeding seasons; we detected cowbirds in 46% of the surveyed patches. We used logistic regression to develop predictive models from variables that described each disturbance patch in terms of size, shape, and position in the landscape relative to livestock areas and other disturbances. The model that best predicted patches used by cowbirds (75-80% correct classification) consisted of variables that described area of the patch, distance to closest chronic disturbance patch, and number of livestock areas within 7 km of the patch. From this model, we concluded that large patches (?= 9.8 ha) that were either chronically disturbed, or close to a chronic disturbance patch, and near a high concentration of livestock areas were most likely to contain cowbirds. We employed a grid-based geographic information system (GIS) procedure to map the probability of cowbird distribution in disturbance patches across the study area. The map illustrates that small (? = 4.0 ha), remote forest openings are unlikely to attract brown-headed cowbirds in Vermont.