A fine‐scale U.S. population estimate of a montane spruce–fir bird species of conservation concern

Abstract

Citation: Hill, J. M., and J. D. Lloyd. 2017. A fine-scale U.S. population estimate of a montane spruce-fir bird species of conservation concern. Ecosphere 8(8):e01921. 10. 1002/ecs2.1921 Abstract. Bicknell's Thrush (Catharus bicknelli) is one of the most range-restricted bird species in North America. In the United States (U.S.), it occurs only in the disturbed montane forests of fir (Abies balsamea) and spruce (Picea spp.) in the northeastern states of New York, Vermont, New Hampshire, and Maine. Climate change is expected to substantially diminish this vegetation community (>50% extinction from current range) by 2300. Despite the increasingly tenuous conservation status of the species, fundamental demographic parameters remain unknown and overall population size is uncertain. We used N-mixture models in a hierarchical Bayesian framework to predict population size and to elucidate spatial and temporal patterns of Bicknell's Thrush abundance in the northeastern United States. From 2011 to 2016, as part of Mountain Birdwatch, citizen-scientists conducted 14,552 five-minute point counts at 747 sampling locations along hiking trails. Observers detected Bicknell's Thrush at 49.4% of points and tallied 1079 observations of the species. Bicknell's Thrush abundance was parsimoniously modeled as a complex function of elevation, forest canopy cover, and latitude. Using our model results, and the estimated U.S. range of the Bicknell's Thrush, we produced the first fine-scale (<1.0 ha resolution) abundance estimate and map of Bicknell's Thrush density across their U.S. range. We predicted the U.S. Bicknell's Thrush population in 2016 as 71,318 (95% credible interval: 56,080-89,748), and 95% of that population occurred above 805 m. Combining our results with existing estimates of population size in Canada suggests a global population size of <120,000; Bicknell's Thrush likely have one of the smallest population sizes of regularly occurring bird species within the contiguous United States and Canada. By cross-referencing the U.S. Geological Survey Protected Areas Database, we estimated that 76.6% of Bicknell's Thrush habitat occurred on conserved lands across the United States and that this habitat supported 84.6% of the predicted population. The White Mountain National Forest (New Hampshire and Maine) is the largest conservator of Bicknell's Thrush habitat in the United States and supports~31% of the predicted U.S. population. Our model provides a testable framework for assessing the success of future conservation and management actions on Bicknell's Thrush populations throughout their U.S. range.

Direct and indirect effects of climate on bird abundance along elevation gradients in the Northern Appalachian mountains

“…Recent changes in passerine distributions and abundances have been observed occurring across latitude and elevation gradients across the world (Auer & King, ; Hickling, Roy, Hill, Fox, & Thomas, ; Parmesan, ; Parmesan & Yohe, ; Tingley, Monahan, Beissinger, & Moritz, ). In the Northeastern United States, there is evidence that populations of high‐elevation montane bird species have been declining for decades (Hill & Lloyd, ; King, Lambert, Buonaccorsi, & Prout, ; Lambert, King, Buonaccorsi, & Prout, ; Studds, McFarland, et al, ), including state‐level extirpation of high‐elevation endemic species (Rimmer & Mcfarland, ). Moreover, in the White Mountains of New Hampshire, low‐elevation birds have been shown to have shifted their range over 100 m upslope while high‐elevation birds have shifted their ranges downslope over the same 19 year period (DeLuca & King, ).…”

Bicknell&#8217;s Thrush (Catharus bicknelli) habitat occupancy in Québec&#8217;s Laurentian Highlands

“…During the study period, in New Hampshire (1993Lambert et al 2008), an annual decline (-7%) was reported while recent analyses from data across its United States range (2011-2016) produced a nonsignificant decline (Hill and Lloyd 2017). Bicknell's Thrush numbers were declining in the Canadian Maritimes and southeastern Québec where the species became extirpated from several locations (Whittam 2015, Québec Breeding Bird Atlas 2018.…”

“…Without discontinuing efforts to conserve breeding habitat, greater attention should be paid to other potential limiting factors. Current challenges for the species outside the breeding range are habitat loss and degradation due to anthropogenic and natural causes on its wintering grounds in the Greater Antilles (Hill and).…”

“…Bicknell's Thrush (Catharus bicknelli) has one of the most restricted breeding ranges of all North American forest-breeding migratory birds (COSEWIC 2009, McFarland et al 2013, Townsend et al 2015, Hill and Lloyd 2017. At the continental scale, the species ranks as one of the highest conservation priorities (Rich et al 2004, Rosenberg et al 2014, while it is legally considered as threatened in Canada (Government of Canada 2012) and vulnerable in Québec (Gouvernement du Québec 2009).…”

Apparent survival of a range-restricted montane forest bird species is influenced by weather throughout the annual cycle

“…Bicknell's Thrush (Catharus bicknelli) is a rare, range-restricted songbird, breeding only in high-elevation fir (Abies balsamea) forests of the northeastern United States and southeastern Canada and wintering predominantly in wet, broadleaf primary forests of the Dominican Republic (Townsend et al 2015). With a recent global population estimate of < 120,000, Bicknell's Thrush likely have one of the smallest population sizes of any regularly occurring migratory songbird species within the contiguous U.S. and Canada (Hill and Lloyd 2017), and is considered one of the Nearctic-Neotropical migrants at greatest risk of extinction and thus of highest continental conservation concern (Rich et al 2004, Wells 2007, Townsend et al 2015. In the northeastern U.S., 95% of the breeding population is found above 805 m elevation in naturally fragmented "sky islands" of montane fir forest (Hill and Lloyd 2017).…”