Song Sparrows vs. cowbird brood parasites: Impacts of forest structure and nest-site selection

We studied Song Sparrow (Melospiza melodia) reproductive ecology in natural and restored riparian forest to evaluate the influence of habitat structure and nest-site selection on parasitism by Brown-headed Cowbirds (Molothrus ater). Logistic regression revealed that habitat structure affected the probability of parasitism at two scales, BROAD (within 11.3 m of the nest) and LOCAL (within 5 m of the nest). Nests with abundant lateral cover at a height of < 1 m (BROAD) were less likely to be parasitized, suggesting that lateral cover at ground level may reduce parasitism by concealing host movements in the vicinity of the nest. in contrast, nests with abundant foliage cover at a height of 2-3 m (LOCAL) were more likely to be parasitized, suggesting that foliage cover above the nest may increase parasitism by providing perches from which female cowbirds can watch host activities and find nests. Sparrow nest-site selection varied among forest types, but lateral cover at ground level and foliage cover above the nest were similar for nests in each forest type. Results suggest that further research on the effects of habitat structure on nest-site selection and cowbird parasitism could lead to methods of reducing parasitism through appropriate habitat management and restoration efforts.