Invasive insects pose a significant threat to biodiversity, often affecting entire communities through the destruction of foundation species. Eastern Hemlock (Tsuga canadensis [L.] Carr.) forests, which are unique habitats in eastern North America, are threatened by an introduced insect, the Hemlock Woolly Adelgid (Adelges tsugae Annand). Previous studies have found declines in the abundance of some forest bird species, including the Acadian Flycatcher (Empidonax virescens), with increasing infestation by Hemlock Woolly Adelgids. To identify potential mechanisms behind these declines, we studied abundance, breeding biology, and habitat selection of Acadian Flycatchers in 11 Eastern Hemlock-dominated riparian sites in Pennsylvania and New Jersey, representing a continuum of infestation levels, in 2001–2002 and 2006–2007. Eastern Hemlock supported 90% of all nests and was used more as a nesting substrate than expected at most sites. We found ∼70% fewer breeding pairs at heavily infested sites, though nest survival rates were not affected. The results suggest that Acadian Flycatcher populations in Eastern Hemlock forests may decrease with continued decline of Eastern Hemlocks. Populations in the Appalachian Highlands, where the species' association with Eastern Hemlock is most pronounced, may be at greatest risk.