A practical model of Bicknell

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 meters per 1 degree 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. This shapefile represents the polygons of potential habitat derived from a raster of this model, which is available upon request. Full details can be found in Lambert, J. D., K. P. McFarland, C. C. Rimmer, S. D. Faccio, and J. L. Atwood. 2005. A practical model of Bicknell's Thrush distribution in the northeastern United States. Wilson Bulletin 117:1-11.

比氏夜鸫（Bicknell’s Thrush，学名*Catharus bicknelli*）是一种稀有的生境特化物种，在美国东北部高海拔的密集香脂冷杉（Abies balsamea）与红云杉（Picea rubens）林中繁殖。该森林类型的现有丧失与预估丧失，使得整个区域内对该物种种群现状信息的需求日益攀升。本研究基于海拔、纬度与森林类型，构建了美国纽约州、佛蒙特州、新罕布什尔州及缅因州范围内比氏夜鸫的分布模型。该模型预测，该物种会出现在随纬度升高而降低的海拔阈值以上的以针叶林为主的森林中；该阈值的坡度（每1纬度变化对应81.63米的海拔变化），反映了气候对森林组成与结构的调控作用。该分布模型涵盖136250公顷山地森林，包括新罕布什尔州怀特山脉与纽约州阿迪朗达克山脉的大片区域。为验证模型性能，本研究在阈值上下的针叶林内设置1公里样线，开展定点计数调查与回放调查。模型对72条样线中61条（84.7%）的比氏夜鸫存在与否或推定缺失情况做出了准确预测；若排除阈值上下50垂直米范围内的区域，预测准确率提升至98.1%。该分布模型是开展地方与区域层面保护规划的实用工具，其潜在应用场景包括预测气候变化对比氏夜鸫分布的影响、评估生境改变的风险，以及确定保护与管理工作的优先级。本形状文件（shapefile）包含由该模型栅格数据导出的潜在生境多边形，该栅格数据可按需获取。详细研究细节可参见Lambert JD、McFarland KP、Rimmer CC、Faccio SD及Atwood JL于2005年发表的文献"A practical model of Bicknell's Thrush distribution in the northeastern United States"，刊载于《威尔逊通报》117卷：1-11。