Avian response to removal of a forest dominant: consequences of hemlock woolly adelgid infestations

Aim This study examines changes in avian community composition associated with the decline and loss of eastern hemlock [Tsuga canadensis (L.) Carr.] resulting from chronic hemlock woolly adelgid (HWA; Adelges tsugae Annand) infestations. Location The study was conducted in a 4900-km(2) study region extending from Long Island Sound northward to the southern border of Massachusetts and including the Connecticut River Valley in Connecticut, USA. Methods Bird surveys were conducted at 40 points in 12 hemlock stands varying in HWA infestation and overstory mortality levels during the avian breeding seasons of 2000 and 2001. Ten-minute, 50-m radius point counts were used to survey all birds seen or heard at each point. Overstory and understory vegetation were sampled at each point. Indicator species analysis and non-metric multidimensional scaling were used to examine relationships between avian community composition and vegetation structure. Results Overstory hemlock mortality was highly correlated with avian community composition. Abundance of eastern wood-pewee (Contopus virens), brown-headed cowbird (Molothrus ater), tufted titmouse (Baeolophus bicolor), white-breasted nuthatch (Sitta carolinensis), red-eyed vireo (Vireo olivaceus), hooded warbler (Wilsonia citrina), and several woodpecker species was highest at points with >60% mortality. Black-throated green warbler (Dendroica virens), Acadian flycatcher (Empidonax virescens), blackburnian warbler (Dendroica fusca), and hermit thrush (Catharus guttatus) were strongly associated with intact hemlock stands that exhibit little or no mortality from HWA. Conclusions Eastern hemlock has unique structural characteristics that provide important habitat for numerous bird species in the north-eastern US. As a result, removal of hemlock by HWA has profound effects on avian communities. Black-throated green warbler, blackburnian warbler, and Acadian flycatcher are very strongly associated with hemlock forests in southern New England and appear to be particularly sensitive to hemlock removal. The hooded warbler, a species whose status is of regional concern, may actually benefit from the development of a dense hardwood seedling layer associated with high hemlock mortality.