The State of Swainson’s Thrushes
Swainson’s Thrush numbers have likely decreased since Mountain Birdwatch began. Annual fluctuations are evident, but overall counts of this species have steadily declined in the northeast U.S. over the last half-century. Mountain Birdwatch data indicate an average decline of -1.64% per year (95% credible interval = -2.33% to -0.93%) in the mountains of our region; this trend equates to a -17.99% decline in population (95% credible interval = -24.68% to -10.60%) between 2010 and 2022 (see table below).Mean annual population trends and population change (with 95% Bayesian credible intervals [CRI]) for Swainson's Thrush from 2010 through 2022. A red dot indicates strong evidence for a negative trend. An orange dot indicates weak evidence for 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%.
change (95% CRI)
|All regions||-1.64||(-2.33, -0.93)||>0.99||<0.01||-17.99||(-24.68, -10.60)|
|-1.14||(-2.30, 0.13)||0.96||0.04||-12.87||(-24.32, 1.61)|
|-1.21||(-3.48, 1.25)||0.84||0.16||-31.62||(-34.59, 16.13)|
|-1.12||(-2.38, 0.26)||0.94||0.06||-12.69||(-25.09. 3.18)|
|Vermont||-0.97||(-2.7121, 0.22)||0.94||0.06||-11.02||(-22.68, 2.64)|
|New Hampshire||-2.56||(-3.38, -1.72)||>0.99||<0.01||-26.74||(-33.79, -18.77)|
|Maine||-1.18||(-2.13, -0.19)||0.99||0.01||-31.27||(-22.76, -2.29)|
Data collected by the Breeding Bird Survey indicate widespread declines in Swainson’s Thrush numbers throughout North America (~0.5% per year). These declines appear to have begun in the early 1980s, and are most severe across the northern and western portion of the Swainson’s Thrush range. They have disappeared from many locales, such as coastal areas and major interior valleys, that they once inhabited in California. Counts of this species made at several migratory stopover sites (e.g., Long Point Bird Observatory) have declined as well. Areas where population trends are increasing for Swainson’s Thrush include the southern Appalachian Mountains, Eastern Montana and the Western Dakotas, and Alaska.