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

Abstract

ABSTRACT 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 ...