Vegetation around parasitized and non-parasitized nests within deciduous forest

We quantified the vegetation in 0.04-ha circles around 93 nests from 12 forest-dwelling neotropical migrants to determine whether the structure of the vegetation influenced the probability of parasitism. Forty-five nests were parasitized by Brown-headed Cowbirds (Molothrus ater) and 48 were not parasitized. Nests were located within deciduous woods in south-central Wisconsin. We compared 16 variables including percent cover at varying heights, densities of shrubs and trees, and number of dead trees (snags). For all species combined, nests parasitized by cowbirds were characterized by a more open sub-canopy (percent cover 3-10 m) and canopy (percent cover > 10 m) and more ground cover (percent cover 0-0.5 m). Associated with these differences were a higher density of small shrubs and saplings (0.5-1 m) and a lower density of tall shrubs (>5 m) and small trees (7-14.5 cm DBH). Proximity of the nest to a snag did not increase the risk of parasitism. We speculate that the differences in vegetative structure around parasitized and non-parasitized nests were due primarily to differences in the host species composition between the two groups and resulted from cowbird nest-searching strategies and densities of host nests. For Acadian Fly-catchers (Empidonax virescens), the only species for which we had a sufficient sample size to make a within-species comparison, parasitized nests were associated with a more open canopy than non-parasitized nests. None of the other vegetative variables measured differed between the two groups. Our results suggest that species nesting within small forest openings may be particularly vulnerable to cowbird parasitism. However, for a variety of reasons, we do not recommend management at a micro-habitat scale. Instead, we recommend that managers minimize the risk of parasitism through landscape-level management where large areas of contiguous forest are maintained.