P Rojecting E Ffects of C Limate C Hange on B Icknell ’ S T Hrush H Abitat in the N Ortheastern U Nited S Tates

Abstract

Bicknell’s Thrush (Catharus bicknelli) is a rare habitat specialist that breeds in montane fir-spruce forests of the northeastern United States and adjacent Canada. A warming climate may reduce availability of this forest type by allowing upslope encroachment of mixed and hardwood forests. We used two GIS modeling approaches to assess how elevated temperatures could affect Bicknell’s Thrush habitat in the U.S. Both methods rely on known relationships between forest type, temperature, and elevation. We measured the possible effects of 1° C to 8° C temperature increases on habitat area, number of habitat patches, and mean patch size. Results indicate that a 3° C increase in mean July temperature could result in an 88% to 98% loss of U.S. habitat and extirpations of Bicknell’s Thrush from the Catskill Mountains, the southern Adirondacks, the Green Mountains, and the mountains of western Maine. The same increase could eliminate breeding habitat from up to 144 mountains in New Hampshire. The Adirondack High Peaks, the Presidential Range, and Mount Katahdin are most likely to maintain forest characteristics suitable for Bicknell’s Thrush. The recent disappearance of Bicknell’s Thrush from coastal locations in Canada and from small mountains in the U.S. may signal early effects of climate change. The actual pace and pattern of habitat loss will depend on the rate of temperature change, the influence of site factors (slope, aspect, substrate), and the effect of climate on reproductive rates, resource competition, and natural disturbance.