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The State of The Mountain Birds Report: 2021

The latest State of the Mountains Bird Report indicates steep and consistent declines for six of the 10 bird species monitored through Mountain Birdwatch.

More than a decade of monitoring by many hundreds of community scientists has revealed that our mountain birds are facing challenging times in the northeastern United States. More than half of the 10 bird species (Winter Wren, Bicknell’s Thrush, Swainson’s Thrush, Hermit Thrush, Blackpoll Warbler, and White-throated Sparrow) monitored by Mountain Birdwatch have experienced average declines of greater than -39% since 2010. Regionally, the most persistent and widespread declines occurred in the Catskills, where 7 of the 8 monitored bird species declined by an average of -48% since 2010. Note, Fox Sparrow and Boreal Chickadee do not breed in the Catskills, and only Black-capped Chickadees increased there.

A plot showing Mountain Birdwatch populations by year for each species from 2010 to 2021. Each species is shown in a different color. Fox Sparrow, Black-capped Chickadee, and Boreal Chickadee have positive population changes since 2010. Yellow-bellied Flycatcher has had almost no overall population change since 2010. Swainson's Thrush, Bicknell's Thrush, Hermit Thrush, Winter Wren, Blackpoll Warbler, and White-throated Sparrow have had negative population changes since 2010.

The estimated percent population change since 2010 for all 10 bird species monitored via Mountain Birdwatch. Positive values indicate a population increase, while negative values indicate a decline compared to 2010. For example, an increase of 100% indicates a doubling of the population size since 2010. The 95% Bayesian credible intervals are omitted for clarity. (Click to enlarge).

The 2021 report is alarming, because it indicates substantial declines for most of the Mountain Birdwatch monitored species whose populations predominantly occur in the spruce-fir zone and indicates declines for Hermit Thrush whose core population occurs further downslope in the hardwood zone. All bird species and population segments that nest within the high elevation spruce-fir zone, however, are highly susceptible to the effects of global climate change. As temperatures continue to rise this century, quantitative ecologists predict that boreal species will shift their breeding ranges upslope and northward. By the end of this century, it is likely that the breeding ranges of most of our boreal forest breeders (e.g., Blackpoll Warblers and White-throated Sparrows) will be entirely restricted to the boreal forests of Canada and Alaska. Check out Audubon’s Survival by Degrees website to interact with these climate and species’ distribution models.

Mean annual population trends and population change (with 95% Bayesian credible intervals [CRI]) for the 10 avian species and red squirrel monitored through Mountain Birdwatch from 2010 through 2021. The color of the dot proceeding the mean annual trend estimate indicates the direction and confidence of the trend: (strong evidence for a negative trend), (weak evidence for a negative trend), (weak evidence for a positive trend), or (strong evidence for a positive trend).
SpeciesMean annual
trend (%)
Trend
95% CRI
Probability of
decrease
Probability of
increase
Population
change (%)
2010-2021
Population
change (95% CRI)
Yellow-bellied Flycatcher 0.06(-1.28, 1.44)0.460.540.92(-13.21, 17.04)
Black-capped Chickadee 4.02(1.13, 6.96)<0.01>0.9956.06(13.20, 109.56)
Boreal Chickadee 4.19(0.24, 8.48)0.020.9860.57(2.70, 144.83)
Winter Wren -4.12(-4.85, -3.36)>0.99<0.01-36.98(-42.11, -31.36)
Bicknell's Thrush -3.99(-5.45, -2.46)>0.99<0.01-35.88(-46.02, -23.98)
Swainson's Thrush -1.38(-2.06, -0.69)>0.99<0.01-14.08(-20.45, -7.29)
Hermit Thrush -6.92(-8.97, -4.76)>0.99<0.01-54.22(-64.42, -41.53)
Blackpoll Warbler -5.34(-6.04, -4.65)>0.99<0.01-45.29(-49.59, -40.78)
White-throated Sparrow -5.96(-6.76, -5.17)>0.99<0.01-49.07(-53.67, -44.20)
Fox Sparrow 5.59(2.34, 8.87)<0.01>0.9984.51(29.00, 154.79)
Red Squirrel 3.99(0.40, 7.48)0.010.9956.43(4.51, 121.09)

Given these forecasted changes, it is imperative that we continue to monitor these species through Mountain Birdwatch and document their responses to climate change, and other human-induced planetary changes like habitat loss and degradation. Here at VCE, and elsewhere, scientists are racing to find ways to slow and reverse these declining trends and northward shifts through such actions as habitat modification and (potentially) through reintroductions, translocation, and assisted migration. Without the baseline data of Mountain Birdwatch, we will not be in a strong position to fully understand how (or if) these management actions positively benefit these avian populations.

You can find the remainder of the 2021 report (e.g., more species-specific results, Methods, and Conservation Implications) via the web pages linked to the menu headings atop each page. If you find this report useful, please cite it (see sidebar for suggested citation) and the open access data and drop us a line (jhill at vtecostudies.org) to let us know how you used these data. Thank you.

State of the Mountain Birds