Interspecific aggression by the Swainson's Thrush (Catharus ustulatus) may limit the distribution of the threatened Bicknell's Thrush (Catharus bicknelli) in the Adirondack Mountains

Bicknell's Thrush (Catharus bicknelli) is a threatened species that inhabits high-elevation spruce-fir forests in the northeastern United States. Bicknell's Thrush populations are predicted to shift upslope in response to global warming, leading to local extinctions on mountains that lack the elevational extent to accommodate such shifts. However, biotic interactions may influence how montane species respond to changing abiotic conditions. In territorial songbirds, for example, aggression from dominant low-elevation species has been hypothesized to combine with warming temperatures to push'' populations of subordinate montane species upslope at a faster rate than in the absence of interspecific aggression. This push'' hypothesis could apply to Bicknell's Thrushes: the related Swainson's Thrush (Catharus ustulatus) inhabits lower elevations, Bicknell's Thrushes are largely confined to mountaintops, and interspecific aggression by Swainson's Thrushes toward Bicknell's Thrushes has been anecdotally reported. We used reciprocal playback experiments to test whether patterns of interspecific aggression in Swainson's and Bicknell's thrushes along an elevational gradient in the Adirondack Mountains were consistent with the push'' hypothesis. We found weak asymmetrical interspecific aggression: Some Swainson's Thrush individuals at high elevations where the 2 species co-occurred responded aggressively to Bicknell's Thrush playback, and Bicknell's Thrushes did not respond aggressively to Swainson's Thrush playback. While Swainson's and Bicknell's thrushes appeared to regularly occupy overlapping home ranges, asymmetrical interspecific aggression from a lower-elevation species is consistent with the push'' hypothesis and suggests that interspecific aggression from Swainson's Thrushes may be a factor limiting Bicknell's Thrush populations in the Adirondack Mountains. More broadly, this case example supports the recent call to incorporate biotic interactions in models that predict montane species' responses to climate change.