The State of Hermit Thrushes in the Northeast
Regionally: Declining
The mean (thick, yellow line) annual estimate of Hermit Thrush abundance within the immediate area surrounding all 803 Mountain Birdwatch sampling locations, with a 95% Bayesian credible interval (blue polygon, representing estimate uncertainty).
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 for the time being. 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 overall declined since 2015.
Predicted Hermit Thrush adult density per approximate hectare in an average year (between 2010 and 2024), as estimated from Mountain Birdwatch data. The base map shows the extent of the Mountain Birdwatch region: eastern New York, Vermont, New Hampshire, and western Maine.
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 80% CRI does not contain zero. The trend is the annual percentage that a population has (on average) changed each year since 2010, while the population change is the total estimated population percentage change over all years. These estimates have uncertainty, of course, which one can evaluate by examining the credible intervals and the probability that the population has declined probability of decrease) or increased (probability of increase) since 2010.
| Region | Mean annual trend (%) | Trend (80% CRI) | Probability of decrease | Probability of increase | Population change (%) 2010-2024 | Population change (80% CRI) |
|---|---|---|---|---|---|---|
| All regions | -6.44 | (-8.70, -4.21) | >0.99 | <0.01 | -60.61 | -60.61 |
| Maine | -8.94 | (-11.91, -6.00) | >0.99 | <0.01 | -73.04 | -73.04 |
| New Hampshire | -4.79 | (-7.36, -2.18) | 0.99 | 0.01 | -49.73 | -49.73 |
| New York (entire region) | -3.75 | (-6.49, -0.99) | 0.96 | 0.04 | -41.43 | -41.43 |
| New York (Adirondacks only) | -3.61 | (-6.51, -0.72) | 0.94 | 0.06 | -40.23 | -40.23 |
| New York (Catskills only) | -4.62 | (-7.65, -1.59) | 0.97 | 0.03 | -48.46 | -48.46 |
| Vermont | -10.07 | (-12.73, -7.44) | >0.99 | <0.01 | -77.37 | -77.37 |
Globally: Declining
Mountain Birdwatch data suggest that Hermit Thrush populations have undergone tremendous declines over the past 15 year, with Vermont’s population of Hermit Thrush (within the spruce-fir zone) having declined by >77%. 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 may not tell the full story. eBird trends, however, suggest that Hermit Thrush populations have declined over the last decade from across their entire breeding range, with the exception of Nova Scotia. The driving force behind these continental downward trends for Hermit Thrush is unknown at the moment. Hermit Thrush reach the upper limits of their elevational distribution in the lower zone of montane spruce-fir forests. Recent climate modeling suggests that Hermit Thrush will disappear from New England over the next 100 years.
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.
