Do Attributes of Gray Vireo Nest Sites Influence Brown-headed Cowbird Brood Parasitism?

Brood parasitism can have major detrimental effects on the fitness of reproducing songbirds and often contributes to nest failure. As the Gray Vireo (Vireo vicinior) suffers high rates of brood parasitism by the Brown-headed Cowbird (Molothrus ater), we hypothesized that vireos might choose nest sites selectively to thwart cowbird parasitism. To investigate this question, we measured and compared vegetative and spatial attributes of nest sites in a known breeding population of the Gray Vireo on the Sevilleta National Wildlife Refuge, New Mexico, in 2017 and 2018. Specifically, we compared parasitized and unparasitized nests with respect to the nest’s height and the average height, diameter, and foliage density of the nest tree. We also compared the spatial attributes of the nearest neighboring nest, nearest parasitized nest, and the density of surrounding juniper trees within 50 m of a nest. Interestingly, among none of the variables measured did we find an association with the incidence of brood parasitism, suggesting that nest parasitism might be more strongly linked to other larger-scale ecological or behavioral variables.