The State of Bicknell’s Thrush
Regionally: Likely declining
Mountain Birdwatch data suggest that Bicknell’s Thrush populations have declined by an average 0f -2.41% (95% Bayesian credible interval: -4.44% to -0.45%) per year in the United States over the last decade. Expectedly, the region-wide figure below somewhat masks the trends that are occurring at the state level. Bicknell’s Thrush are clearly not declining steeply and predictably across our region each year like they are for Blackpoll Warbler and White-throated Sparrow. Rather, the figure below suggests a gradual decline in Bicknell’s Thrush with much fluctuation in population size over multiyear periods. The steepest declines (see the below table) were observed in the Catskills: the southernmost portion of the global Bicknell’s Thrush range. The Catskills, however, 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).
The mean (thick, dark orange line) annual estimate of the Bicknell’s Thrush local population size within the immediate areas surrounding ~750 Mountain Birdwatch sampling stations. The vertical orange bars present the 95% Bayesian credible interval (a measure of uncertainty) surrounding those mean annual estimates.
indicates strong evidence for a negative trend. An orange dot 
indicates a likely negative trend. A blue dot 
indicates a likely positive trend. A trend of -2%, for example, indicates that the number of Bicknell's Thrushes in our study area has declined by an average of 2% each year from 2011 through 2019.
| Region | Mean annual trend (%) | 95% credible interval |
|---|---|---|
| All regions combined | -2.41 | (-4.44, -0.45) |
| New York (state) | -3.76 | (-6.30, -1.36) |
| New York (Catskills) | -6.35 | (-10.14, -2.56) |
| New York (Adirondacks) |  -2.83 | (-5.76, 0.15) |
| Vermont |  1.61 | (-1.89, 5.14) |
| New Hampshire | -5.00 | (-7.61, -2.35) |
| Maine | -0.62 | (-4.09, 3.03) |
Globally: Probably 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). These boom and bust population dynamics are not uncommon for local populations on the edge of a species’ geographic distribution (where population densities tend to be lower and subject to small population dynamics [i.e., frequent local extinction and recolonization]).
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 wisely 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 large declines in the number of Bicknell’s Thrush counted in Quebec and Nova Scotia, especially between 1970 and 1990. 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.
Bicknell’s Thrush numbers have declined across Canada according to results from the Breeding Bird Survey. Figure provided by Environment Canada.
Opportunities for Conservation
Opportunities for conservation of Bicknell’s Thrush were identified by the International Bicknell’s Thrush Conservation Group. They include addressing the following:
- The clearing and burning of forests in the Dominican Republic and Haiti to create agricultural fields.
- Climate change.
- Incompatible forestry practices on the breeding grounds, especially in Canada.
A complete discussion of the threats facing Bicknell’s Thrush is provided in the updated Conservation Action Plan for Bicknell’s Thrush.
What We Can Do to Help
Breeding habitat
95% of Bicknell’s Thrush in the U.S. are found at elevations above 805 meters (source: Hill and Lloyd 201&), and Bicknell’s Thrush are most abundant at levels of intermediate (~50%) canopy cover (for example, dense spruce-fir forest intermixed with disturbed areas or openings). The dark blue lines represent the mean prediction, while the numerous thin blue lines show other (less likely) relationships between the abundance of Bicknell’s Thrush around a sampling station and elevation (left panel) and percent canopy coverage (right panel).
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. This is unlikely the case in Canada, where many of their Bicknell’s Thrush occur in forest undergoing timber operations. We recently estimated that breeding habitat in these 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. Development in mountain-top breeding areas should be avoided wherever possible.
Bicknell’s Thrush are a species that favors disturbed breeding areas, not interior forest in the U.S. Past research efforts at VCE have documented that Bicknell’s Thrush tend to be most abundant (and nest more frequently) along the edges of high-elevation ski slopes compared to further away from ski slope edges. Perhaps ski slopes superficially mimic the naturally-occurring disturbances (e.g., fir waves and short regenerating areas caused by microbursts and insect outbreaks) found in their unaltered habitat. This represents a conundrum, because new ski slopes ultimately reduce the acreage of forested habitat available to Bicknell’s Thrush on a mountain, and they fragment the remaining forest habitat. Interestingly, a nest’s distance to the ski edge is not a good predictor of clutch size, nest survival/success, or an adult’s survival, so it’s not clear if there are fitness benefits to nesting along ski runs. We are actively pursuing research in this area.
Wintering habitat
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. Another interesting example of integrating conservation into sustainable development is Reserva Zorzal, a farm in the Dominican Republic. Seventy percent of the land is set aside to provide habitat for Bicknell’s Thrush; the remainder is used to grow cacao, which generates revenue and jobs.
Throughout its range
In the long run, conserving Bicknell’s Thrush will require that we address the climate change caused by our activities. This means supporting government policies – and making individual choices – that lead to a reduction in greenhouse-gas concentrations. 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.
