Brood parasitism by Brown-headed Cowbirds (Molothrus ater; hereafter “cowbirds”) is an important factor contributing to the endangered status of the Southwestern Willow Flycatcher (Empidonax traillii extimus, hereafter “flycatcher”). We report on factors that influence brood parasitism on the flycatcher using seven seasons of nest data (n = 491 nests) collected in riparian patches comprising cottonwood (Populus fremontii), willow (Salix spp.), and boxelder (Acer negundo) along the Gila River in southwest New Mexico. We quantified habitat and landscape characteristics that might be associated with higher levels of parasitism and assessed the correlation between those and the observed incidence of nest parasitism. Given that cowbirds associate with cattle, we were particularly interested to determine whether the distance to active summer grazing would influence the frequency of brood parasitism. We found an apparent negative trend between the distance to grazing and brood parasitism, but this trend was not statistically significant. Rather, brood parasitism was more strongly correlated with characteristics of the habitat. Specifically, nests in larger patches, and nests built near the patch edge adjacent to the river, were more susceptible to parasitism, which suggests that these areas are preferred nesting habitat for cowbirds because of a potentially greater abundance of hosts. Parasitism was significantly lower within the core of large patches, but the insulating effect was not evident in small and medium-sized patches. Higher nest height was strongly correlated with lower probability of parasitism, most notably for nests in boxelders. Nests in boxelders were less susceptible to parasitism, whereas nests in willows were more susceptible to parasitism. We discuss the results in the context of other studies. We recommend that management efforts to recover the flycatcher should focus on increasing quality habitat, and we suggest that cattle management should focus on eliminating the adverse effects of grazing on riparian health as a more feasible option than removing cattle far enough from riparian corridors so as to preclude parasitism.
Empidonax traillii extimus en un Paisaje con Pastoreo: Factores que Influyen sobre el Parasitismo de la Nidada