Pii: S0169-2046(02)00220-7

10 We examined the impact of the urban environment on the spatial and temporal activity of brood parasitic Brown-headed Cowbirds (Molothrus ater) in Boulder County, CO, USA. We found that cowbirds used the urban areas for foraging and roosting and traveled into the 3240 ha wildland preserve of ponderosa pine in the mornings to parasitize songbird hosts. Cowbird abundance decreased with distance from the urban/wildland boundary, and Plumbeous Vireo (Vireo plumbeus) nests closer to the urban/wildland boundary were more likely to be parasitized by cowbirds than those farther away. A linear regression accurately predicted the relative abundance of cowbirds based on parameters of distance from residential areas, and distance from roads and trails within the wildland preserve. For species of concern that are known cowbird hosts, creating larger preserves, reducing residential encroachment, and reducing preserve perforation by roads and trails might alleviate high frequencies of parasitism for a portion of the host population. However, even large preserves, such as found in Boulder, CO, USA cannot insulate all focal nesting species from the urban effect of increased brood parasitism. Efforts to reduce food resources and cover for cowbirds in the urban areas might prove to ameliorate host reproductive success close to the urban/wildland boundary through decreases in cowbird abundance. 11