Proliferation of invasive plants in forest understories throughout North America has prompted restoration efforts focused on removal of invasive vegetation. Although the negative impacts of invasives on native plant communities are well documented, effects on forest bird communities remain largely unknown. To address this issue, we examined the response of avian communities to invasive plants in forest fragments across 4 counties in northeastern Illinois, a region characterized by extensive urbanization. We surveyed breeding bird communities in 46 forest plots representing a gradient in abundance of invasive woody plants. We quantified vegetation structure and composition within plots, as well as landscape context. Exotic trees and shrubs were present on all but 3 plots. Although native trees were common, native species represented <7% of total stem density of shrubs. Measures of invasion were weakly correlated with those representing urbanization, yet broad-scale measures of urbanization such as building density and urban cover were strongly associated with avian community structure. At finer scales, measures of invasion were important predictors of the relative abundance of birds in several nesting and foraging guilds. Shrub nesters showed a positive response to invasive vegetation, whereas the relative abundance of aerial salliers and ground nesters decreased with increased proportions of invasive trees. Because restoration strategies aimed at the complete removal of invasive shrubs could diminish habitat quality for some species, thinning of understory vegetation or the removal of invasive trees may confer the greatest benefits to avian communities when few native understory plants are present.
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Vol. 116 • No. 3