[CT Birds] Posts on New York Birds re: Swainson's taking over some Bicknell's habitats

David F Provencher david.f.provencher at dom.com
Tue May 15 13:20:19 EDT 2012


As you may know I hike quite a bit in the White Mountains of New Hampshire. I have long experienced the elevation stratification of the different thrushes. Starting at the lowest elevations in the Whites there are Wood Thrush and Veery (and a few Hermit), then moving upward Hermit become the most common, then Swainson's, and finally Bicknell's at the highest elevations. One of the montane effects of climate change is a vertical movement of flora, that is the exploitation by lower elevation flora of increasingly more mild weather conditions higher up. In my hiking I have found that the peaks that have Bicknell's breeding typically have them in a fairly narrow elevation band of appropriate flora. As the flora changes, and lower elevation species push higher, the upper band of flora that holds the Bicknell's isn't likely to successfully move much higher due to the harshness of the highest elevation weather and the intense winds that the highest elevations experience. The growth rate of "trees" at the highest elevations is much slower than the growth rate only a few hundreds of feet lower down. And of course, on the higher peaks, there is an elevation limit above which trees won't grow at all, called "treeline", which is typically at lower elevations in New England (where Bicknell's breed) than in many western mountain ranges. In my experience I can categorically state that on peaks that have both Swainson's and Bicknell's, there are just magnitudes more Swainson's. So as Swainson's habitat, and thus Swainson's Thrushes, move upward, the narrow band of habitat that holds Bicknell's will degrade (change is probably a better word) relatively quickly. So Bicknell's habitat (and Bicknell's Thrushes) will likely disappear from a number of the New England peaks. 


David Provencher
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