Invasive insects pose a significant threat to biodiversity, often affecting entire ecological communities through destruction of foundation species. This situation is currently unfolding in eastern hemlock (Tsuga canadensis) forests of the northeastern United States which are severely threatened by the invasive hemlock woolly adelgid (Adelges tsugae). Among the many organisms dependent on hemlock stands are several species of birds including Acadian flycatcher (Empidonax virescens), blue-headed vireo (Vireo solitarius), black-throated green warbler (Dendroica virens) and blackburnian warbler (Dendroica fusca). The extent of their dependency varies and is at least partially the result of foraging and/or nesting requirements. Here we ask to what extent will the degradation and perhaps eventual elimination of hemlocks affect populations of these bird species in the northeastern United States using studies from infested and pristine hemlock stands across Pennsylvania. Bird population levels were determined using detections along linear transects and spot-mapping techniques in hemlock stands with varying levels of infestation. Studies involving the Acadian flycatcher also investigated nesting success parameters in relation to severity of infestation.  
Results/Conclusions

Correlations between detections of vireo and warbler species and measures of adelgid impact involving branch terminal infestation were not significant. Infestation levels using measures of branch condition were significantly positively correlated with detections of blue-headed vireo and black-throated green warbler. This unexpected positive relationship between detections of these species and more severe infestation levels could be the result of the influence of secondary insect infestation on foraging opportunities but  this remains a topic for future exploration. Conversely, Acadian flycatcher breeding density declined by 70% in heavily infested sites although nest survival rates were not affected. The apparent differences in response to varying infestation levels between vireos/warblers and the Acadian flycatcher may be the result of different strategies of exploitation of the hemlock-dominated environment.