The State of Hermit Thrushes
Regionally: Declining
The mean (thick, dark brown line) annual estimate of Hermit Thrush abundance—calculated as the annual sum of estimated Hermit Thrush within the local area (a 4-hectare circle) surrounding all 791 Mountain Birdwatch sampling stations. The lighter vertical bars represent the 95% Bayesian credible interval (a measure of the uncertainty around the abundance estimates).
Hermit Thrush are relatively uncommon in the montane forests surveyed as part of Mountain Birdwatch. By comparison, a visitor to the spruce-fir zone is likely to encounter approximately twice as many Bicknell’s Thrush as Hermit Thrush. The estimated local abundance of Hermit Thrush around Mountain Birdwatch sampling stations, in the figure above, paint an interesting picture. Numbers of Hermit Thrush were clearly on the rise from 2010 to 2014, but then the population suddenly and unexpectedly declined (by nearly 50%) where it has slowly declined since 2015.
Mean annual population trends and population change (with 95% Bayesian credible intervals [CRI]) for Hermit Thrush from 2010 through 2022. A red dot
indicates strong evidence for a negative trend. An orange dot 
indicates weak evidence of a negative trend. Strong evidence is suggested for a trend when the probability of population increase or decrease (from 2010 to 2022) equals or exceeds 95%; conversely, weak evidence is suggested for trends where the probability of change is <95%.
| Region | Mean annual trend (%) | Trend 95% CRI | Probability of decrease | Probability of increase | Population change (%) 2010-2021 | Population change (95% CRI) |
|---|---|---|---|---|---|---|
| All regions | -5.47 | (-7.37, -3.54) | >0.99 | <0.01 | -49.08 | (-60.10, -35.10) |
| New York (state) | -8.09 | (-10.91, -5.18) | >0.99 | <0.01 | -63.66 | (-75.00, -47.17) |
| New York (Catskills) | -7.30 | (-11.30, -2.47) | >0.99 | <0.01 | -59.71 | (-76.27, -25.93) |
| New York (Adirondacks) | -8.64 | (-11.94, -5.16) | >0.99 | <0.01 | -66.17 | (-78.26, -47.06) |
| Vermont | -6.10 | (-9.21, -2.95) | >0.99 | <0.01 | -53.01 | (-68.63, -30.23) |
| New Hampshire |  -1.63 | (-4.70, 1.63) | 0.82 | 0.18 | -17.95 | (-43.90, 21.43) |
| Maine | -5.25 | (-8.14, -2.37) | >0.99 | <0.01 | -47.63 | (-63.89, -25.00) |
Those trends for the spruce-fir zone in the Adirondacks (-8.64% per year) and the Catskills (-7.30% per year) are incredible. At the same time, it’s important to keep in mind that the majority of Hermit Thrush breed at lower elevations, in the hardwood forest. The range of the Hermit Thrush has only recently expanded to include the northeastern U.S. They first expanded into southern New England in the early 1990s (possibly associated with farm abandonment and reforestation), and quickly expanded northward. So while these steep declines for much of the spruce-fir zone are alarming, and parallel state-specific trends from the North American Breeding Bird survey (BBS), they don’t tell the full story.
Hermit Thrush reach the upper limits of their elevational distribution in the lower zone of montane spruce-fir forests. Recent climate modeling by Audubon suggests that Hermit Thrush will disappear from New England over the next 100 years.
Globally: Stable
Data collected by the Breeding Bird Survey indicate that the size of the breeding population of Hermit Thrushes has remained stable. Population gains appear concentrated in eastern North America, exemplified by the ongoing colonization of the southern Appalachians. However, populations in western North America, the Mid-Atlantic and New England states, are likely declining.
