A practical model of Bicknell's Thrush distribution in the Northeastern United States

Abstract

Bicknell's Thrush (Catharus bicknelli) is a rare habitat specialist that breeds in dense balsam fir (Abies balsamea) and red spruce (Picea rubens) forests at high elevations in the northeastern United States. Ongoing and projected loss of this forest type has led to increased demand for information on the species' status throughout the region. We used elevation, latitude, and forest type to construct a model of Bicknell's Thrush distribution in New York, Vermont, New Hampshire, and Maine. The model predicts the species to be present in conifer-dominated forests above an elevation threshold that descends with increasing latitude. The slope of the threshold (-81.63 m/l° latitude) reflects climatic effects on forest composition and structure. The distribution model encompasses 136,250 ha of montane forest, including extensive areas of the White Mountains in New Hampshire and Adirondack Mountains in New York. To test model performance, we conducted point count and playback surveys along 1-km routes established in conifer forests above and below the threshold. The model accurately predicted the presence or presumed absence of Bicknell's Thrush on 61 of 72 routes (84.7%). When areas within 50 vertical m of the threshold were excluded, accuracy improved to 98.1%. The distribution model is a practical tool for conservation planning at local and regional levels. Potential applications include projecting effects of climate change on Bicknell's Thrush distribution, assessing risks of habitat alteration, and setting priorities for conservation and management.