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The Effects of Spruce-Fir Disturbances on Bicknell's Thrush (Catharus bicknelli) Occupancy at Mountain Birdwatch Sampling Sites in Vermont
Campbell, Jake
Campbell, Jake
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Abstract
Bicknell’s Thrush (Catharus bicknelli) is one of the most range-restricted forest birds of North America, breeding primarily in the montane spruce-fir forests of the northeastern USA. Population declines over the next decade are expected to be exacerbated by climate change, which is predicted to decrease the amount of spruce-fir forest cover in New England over the next several centuries. The threats faced by Bicknell’s Thrush implore scientists to determine what factors drive its distribution throughout montane spruce-fir forest ecosystems. Of particular interest are the impacts of small-scale canopy gaps on Bicknell’s Thrush. To examine this relationship, I used lidar-derived forest canopy gap measures and other site characteristics to construct a single-season occupancy model of Bicknell’s Thrush based on presence-absence data from 155 Mountain Birdwatch sampling sites in Vermont. I evaluated the relative support of 19 candidate models that included single and additive combinations of elevation, latitude, total canopy gap area, number of canopy gaps, and median canopy gap size. The top ranking model indicated that latitude, elevation, and median gap size had a positive effect on occupancy while number of gaps had a negative effect on occupancy. These results suggest that Bicknell’s Thrush select areas with fewer large gaps over areas with many smaller gaps. This indicates that the additional sunlight received by areas with these canopy gap characteristics could have a positive influence on Bicknell’s Thrush foraging success.
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2022-01-01