We studied populations of Song Sparrows (Melospiza melodia) on 11 islands near Victoria, British Columbia, to evaluate the relationship between nest site characteristics and parasitism by Brown-headed Cowbirds (Molothrus ater). For all nests we recorded their height, the height of vegetation above the nest, the percentage of overhead and side exposure, the number of perches in trees and shrubs within 10 m of the nest, and the distance to the nearest tree or shrub perch. Song Sparrow nests within 10 m of trees were parasitized more often than those farther from trees. Estimates of overhead and side exposure also were lower for parasitized nests than unparasitized nests. Our results support the hypotheses that the proximity of trees, and the amount of concealing cover around host nests, influence the success of cowbirds searching for nests to parasitize.