Alteration of a temperate forest invertebrate community by invasive Japanese Barberry (Berberis thunbergii) has limited influence on the diet composition of territorial Ovenbirds (Seiurus aurocapilla)

Invasive species are widely implicated in the decline of songbird populations, potentially via indirect effects such as the restructuring of ecological communities by non-native plants. We used stable isotope analysis to investigate whether ground-foraging, generalist insectivore Ovenbirds (Seiurus aurocapilla Linnaeus, 1766) shifted diets following invasion by a non-native shrub, Japanese Barberry (Berberis thunbergii DC), in a temperate woodland system in New York, USA where barberry was previously linked to trophic downgrading in the leaf litter arthropod community. Assimilated Ovenbird diets were primarily composed of predatory invertebrates followed by sucking herbivores, chewing herbivores, and detritivores. The only notable changes in assimilated Ovenbird diets were a modest 14.7% decrease in chewing herbivore contributions and a minor 7.0% increase in sucking herbivore contributions in forest patches with high barberry densities. Our results indicate that the effects of non-native plants on connections between higher trophic levels are context dependent, and comparisons with other studies suggest that community complexity and time since introduction are key contextual differences that affect the outcome of an invasion. Our results may reflect compensatory shifts in individual foraging effort by Ovenbirds, but, in combination with our previous research, provide little evidence that barberry affects Ovenbird condition and therefore downstream outcomes.