Spatial segregation between Gray-cheeked Thrush and an introduced nest predator in a managed forest landscape

Introduced species are known for inducing changes in ecosystems and for their impacts on endemic island species. North American red squirrels (Tamiasciurus hudsonicus) were introduced to Newfoundland, Canada, during the 1960s and have been hypothesized as a cause of the precipitous decline of the Newfoundland Gray-cheeked Thrush (Catharus minimus minimus). To test the prediction that the impacts of squirrels led to range contraction by thrushes, we completed 1960 point count surveys for squirrel and thrush over two years (2016 and 2017) in western Newfoundland. Thrushes and squirrels were strongly segregated by elevation, with thrushes now being commonly detected at only high elevation, squirrels being detected most frequently at low elevation, and both species being rare and inversely likely to be present at intermediate elevations. We evaluated local (5.5 ha) and landscape (490.8 ha) scale habitat affiliations of thrushes using landcover data from a provincial forest resource inventory to assess the potential for synergistic impacts on thrush of squirrel invasion and forest management. Gray-cheeked Thrushes were associated with regenerating (10-30 years post-harvest) clear-cuts, conifer forest, and tall scrub at the local scale, but avoided tall scrub, regenerating stands, and second growth forest at the landscape scale. Regenerating clear-cuts and modified strip cuts were selected by thrushes at a local scale. Breeding Bird Survey data show that Gray-cheeked Thrushes were abundant at lower elevations prior to the expansion of squirrels across Newfoundland, so our finding of strong elevational segregation adds to the growing body of evidence suggesting that the introduction of squirrels played an important role in the sudden decline and range contraction of this insular songbird subspecies. Management for squirrels should include efforts to prevent spread upslope and to nearshore islands while more study is done on this and other hypotheses regarding the cause of Gray-cheeked Thrush decline, as well as on relevant aspects of the ecology of this little-studied thrush.

Automated summary

Introduced North American red squirrels have had a significant impact on the decline and range contraction of the endemic Newfoundland Gray-cheeked Thrush. The two species are segregated by elevation, with the thrush now being commonly found at high elevation and the squirrels at low elevation. Thrushes are associated with certain habitat types at both local and landscape scales.