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COPYRIGHT 2005 Wilson Ornithological Society
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 [degrees] 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. Received 9 February 2004, accepted 20 December 2004.
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Bicknell's Thrush (Catharus bicknelli), once considered a subspecies of Gray-cheeked Thrush (C. minimus), gained full species status in 1995 (American Ornithologists' Union 1995). It has since been considered one of the most "at-risk" passerines in eastern North America. Partners in Flight (Pashley et al. 2000) ranks Bicknell's Thrush as the top conservation priority among Neotropical migrants in the Northeast, while the International Union for the Conservation of Nature (BirdLife International 2000) classifies the species as "vulnerable" on its list of threatened species.
Although there is no conclusive evidence of widespread population declines, reports of regional declines (Rompre et al. 1999, Rimmer et al. 2001b) and local extinctions (Christie 1993, Atwood et al. 1996, Nixon 1999, Lambert et al. 2001) have elevated concern for this rare species.
Bicknell's Thrush is a habitat specialist that occupies a naturally fragmented breeding range from the Catskill Mountains of New York to the Gulf of St. Lawrence and Cape Breton Island, Nova Scotia (Atwood et al. 1996, Rimmer et al. 2001a). It is the region's only endemic bird species. In New York, northern New England, and the nearby Estrie region of Quebec, Bicknell's Thrush inhabits montane forests dominated by balsam fir (Abies balsamea), with lesser amounts of spruce (Picea rubens and P. mariana), white birch (Betula papyrifera var. cordifolia), and mountain ash (Sorbus americana and S. decora) (Atwood et al. 1996, Rimmer et al. 2001a, Connolly et al. 2002). Structural attributes of Bicknell's Thrush habitat include a dense understory of softwoods (Sabo 1980, Hale 2001, Pierce-Berrin 2001), low canopy height (Sabo 1980, Noon 1981, Hale 2001), and high incidence of snags, stumps, and dead fallen trees (Connolly 2000). These features typify chronically disturbed sites and regenerating fir waves (Sprugel 1976). Favorable habitat conditions for Bicknell's Thrush also may arise following disturbance by hurricane, ice storm, debris avalanche (Reiners and Lang 1979), or logging (Connolly 2000). Habitat suitability generally decreases with greater prominence of hardwoods (Sabo 1980, Noon 1981, Atwood et al. 1996, Hale 2001, Connolly et al. 2002); however, in the spruce-fir highlands of New Brunswick, Bicknell's Thrush inhabits both young conifer stands and regenerating hardwoods (Nixon 1996, Nixon et al. 2001).
Bicknell's Thrush also occurs in maritime spruce-fir forests at sites scattered along both shores of the St. Lawrence Seaway (Gauthier and Aubry 1996) and throughout the Gulf of St. Lawrence (Nixon 1999). Locations in the Gulf include the western tip of Anticosti Island, the Magdalen Islands (Gauthier and Aubry 1996), Cape Breton Island and small islands offshore of Cape Breton (Erskine 1992; D. Busby pers. comm.). Historic or sporadic records exist for several additional locations around the Bay of Fundy (Erskine 1992, Christie 1993).
In the northeastern United States, climate change could greatly reduce or eliminate balsam fir habitat as growing conditions become more favorable for hardwood species (Iverson and Prasad 2002). Over the long term, a shift in forest composition may impair the viability of Bicknell's Thrush populations in the region. Meanwhile, ski area expansion, communications tower construction, and wind power development incrementally reduce and fragment montane fir forests with unknown consequences for Bicknell's Thrush (Rimmer et al. 2001a). In order to conserve and properly manage remaining Bicknell's Thrush habitat, natural resource managers require reliable, site-specific occurrence information. Because it is not feasible to survey all potential habitats, a predictive habitat map is required for effective conservation planning.
Wildlife habitat maps enable natural resource managers to identify suitable habitat and predict effects of management alternatives. When constructed in a geographic information systems (GIS) environment, such maps can be produced efficiently and applied consistently over large areas; however, the value of a GIS habitat model depends on its predictive capability. Therefore, model validation is a critical step in the habitat mapping process. Validation procedures yield measures of model performance that provide a basis for determining appropriate applications to research and management. An accurate GIS model is a flexible tool that focuses limited resources where they will have the greatest effect.
In a previous study, Atwood et al. (1996) identified forest type, latitude, and elevation as important factors underlying the distribution of Bicknell's Thrush in New England and New York. The goal of our study was to construct and test a predictive distribution...
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