The State of Bicknell’s Thrushes
Regionally: Declining
The mean (thick, dark red line) annual estimate of the Bicknell’s Thrush abundance within the immediate areas surrounding ~750 Mountain Birdwatch sampling stations. The lighter vertical bars represent the 95% Bayesian credible interval (a measure of the uncertainty around the abundance estimates).
Mountain Birdwatch data suggest that Bicknell’s Thrush populations have declined by an average of -3.99% per year (95% Bayesian credible interval: -5.45% to -2.46%) in Northern New England and New York over the last decade. Expectedly, the region-wide figure above somewhat masks the trends that are occurring at the state level. Similar to the Blackpoll Warbler and White-throated Sparrow, the Bicknell’s Thrush population has declined steadily across our region since 2010. The steepest declines (>50% decline between 2010 and 2021, 95% Bayesian credible interval: -69.23% to -20.84%, ) were observed in the Catskills: the southernmost portion of the Bicknell’s Thrush global breeding range. The Catskills, however, likely harbor less than 5% of the U.S. Bicknell’s Thrush population. The U.S. population of Bicknell’s Thrush is likely around 70,000, with a global population of less than 120,000 (source: Jason Hill and John Lloyd’s 2017 Ecosphere paper).
Mean annual population trends and population change (with 95% Bayesian credible intervals [CRI]) for Bicknell's Thrush from 2010 through 2021. A red dot
indicates strong evidence for a negative trend.
| Region | Mean annual trend (%) | Trend 95% CRI | Probability of decrease | Probability of increase | Population change (%) 2010-2021 | Population change (95% CRI) |
|---|---|---|---|---|---|---|
| All regions | -3.99 | (-5.45, -2.46) | >0.99 | <0.01 | -35.88 | (-46.02, -23.98) |
| New York (state) | -4.82 | (-7.26, -2.21) | >0.99 | <0.01 | -41.35 | (-56.35, -21.79) |
| New York (Catskills) | -6.51 | (-10.16, -2.10) | >0.99 | <0.01 | -51.02 | (-69.23, -20.84) |
| New York (Adirondacks) | -4.22 | (-6.96, -1.29) | >0.99 | <0.01 | -36.99 | (-54.77, -13.33) |
| Vermont | -2.57 | (-4.57, -0.44) | 0.99 | 0.01 | -24.44 | (-40.24, -4.71) |
| New Hampshire | -4.26 | (-6.21, -2.21) | >0.99 | <0.01 | -37.64 | (-50.63, -21.77) |
| Maine | -4.41 | (-6.50, -2.26) | >0.99 | <0.01 | -38.68 | (-52.26, -22.22) |
Globally: Likely Declining
Mountain Birdwatch data accurately tell the story of Bicknell’s Thrush population health in the northeastern United States. But to understand the global state of Bicknell’s Thrush, we also need to consider data collected in Canada, which is home to a significant number of Bicknell’s Thrush. The Mountain Birdwatch sampling protocol formerly extended throughout Canada, but Bicknell’s Thrush populations tend to be ephemeral in the northern part of their range (Canada). In other words, Bicknell’s Thrush may be locally abundant in Quebec during one year, but largely absent the next year (and locally abundant someplace else) – a boom and bust population dynamic not uncommon for local populations on the edge of a species’ geographic distribution. The Mountain Birdwatch sampling protocol (i.e., fixed locations surveyed every year) is not an ideal sampling methodology for this ephemeral scenario. Two-stage or cluster sampling is a better strategy for the population dynamics of Bicknell’s Thrush in Canada, and indeed, our colleagues in Canada have moved in that direction.
With that said, the North American Breeding Bird survey (which samples fixed locations every year) collects data on Bicknell’s Thrush in Canada, and those data indicate an average decline of -4.1% per year between 1970 and 2019 and an even steeper decline during the most recent decade of that period (-7.4% per year). Data from New Brunswick were too sparse to be included in the Breeding Bird Survey analysis. The number of survey routes on which Bicknell’s Thrush are detected is small and thus confidence in Breeding Bird Survey results is low. However, the second Breeding Bird Atlas of the Maritime Provinces shows recent local extinctions of some coastal populations in Nova Scotia and New Brunswick, which also supports the idea that overall numbers of Bicknell’s Thrush are dwindling. This pattern is similar to research from other animal and plant systems which finds that peripheral populations tend to become extirpated (locally extinct) first when that species is globally declining.
The U.S. Bicknell’s Thrush population was estimated to be 71,318 (95% credible interval: 56,080–89,748) in 2016. Combining those 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. At current levels of warming (and associated changes to moisture regimes), we can expect more than half of the existing Bicknell’s Thrush breeding habitat (spruce-fir forest) in the northeastern U.S. to be replaced by the upslope movement of hardwood forest communities within the next 200 years, further decreasing their population.
Conservation Actions
The International Bicknell’s Thrush Conservation Group outlines a complete discussion of the threats facing Bicknell’s Thrush in the updated Conservation Action Plan for Bicknell’s Thrush.
- Address climate change caused by our activities by supporting government policies and making individual choices that lead to a reduction in greenhouse-gas concentrations.
- Conserve breeding habitat
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- Breeding habitat in the United States are already largely conserved; Hill and Lloyd (2017) estimated that 85% of Bicknell’s Thrush in the U.S. occur on conserved lands like national forests and state-owned conserved areas.
- Breeding habitat in Canadian forests should be conserved by making sure that best-management practices for forestry operations are adhered to whenever logging occurs in areas inhabited by Bicknell’s Thrush (many Bicknell’s Thrush in Canada occur in forests undergoing timber operations).
- Development in mountain-top breeding areas should be avoided wherever possible.
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- Conserve wintering habitat.
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- Winter habitat in the Caribbean should be protected by purchasing it and permanently setting it aside for conservation, by developing easement agreements with willing landowners, or by supporting compatible land uses that provide economic returns while protecting the forest ecosystems required by Bicknell’s Thrush.
- These are obviously complex issues, involving both local and global economic processes, but our colleague Yolanda (Yoli) León is tackling these difficult conservation issues in the Dominican Republic.
- A great case study for integrating conservation into sustainable development is Reserva Zorzal, a farm in the Dominican Republic where seventy percent of the land is set aside to provide habitat for Bicknell’s Thrush and the remainder is used to grow cacao, which generates revenue and jobs.
- Winter habitat in the Caribbean should be protected by purchasing it and permanently setting it aside for conservation, by developing easement agreements with willing landowners, or by supporting compatible land uses that provide economic returns while protecting the forest ecosystems required by Bicknell’s Thrush.
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