The State of Swainson’s Thrush
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 a steep overall population decline (-3.5% per year, 95% credible interval = -4.3% to -2.6%) in the mountains of our region; this trend equates to a 24.6% decline (95% credible interval = 18.8% to 29.9%) between 2011 and 2019. These declines are likely prevalent throughout the region, but the steepest declines were observed in the northernmost portion of our region (see table below).Mean annual population trends (below) for Swainson's Thrush from 2011 through 2019. A red dot indicates strong evidence for a negative trend. A blue dot indicates a likely positive trend. A trend of -2%, for example, indicates that the number of Swainson'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||-3.47||(-4.35, -2.56)|
|New York (state)||-5.99||(-7.21, -4.76)|
|New York (Catskills)||1.68||(-0.97, 4.40)|
|New York (Adirondacks)||-7.37||(-8.68, -6.06)|
|New Hampshire||-2.60||(-3.72, -1.45)|
Data collected by the Breeding Bird Survey indicate widespread declines in Swainson’s Thrush numbers throughout North America, although populations appear to have lessened since 2000. 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 that they once inhabited in California, for example in coastal areas and in some of the major interior valleys. 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.
Opportunities for Conservation
A few geographical segments of the North American population of Swainson’s Thrush are increasing, whereas others are decreasing. These kinds of local and regional differences in population trajectories suggests that different populations face different threats. Extensive logging has been implicated as a factor in Swainson’s Thrush population declines out west and in Canadian boreal forests. Recent Wood Thrush (Hylocichla mustelina) research suggests that deforestation on the wintering grounds might be driving overall population declines in that species. Swainson’s Thrush primarily winter in mature forests, which are especially susceptible to logging–might deforestation on the wintering grounds also be a primary threat for Swainson’s Thrushes? Like White-throated Sparrows (Zonotrichia albicollis), Swainson’s Thrush are frequently killed in collisions with buildings, towers and windows during migration; indeed, Swainson’s Thrush are often the most numerous killed bird species during these events. It may be that no single, general factor can explain these observed population declines. Their low reproductive output (generally one brood per season per mating pair), however, may make Swainson’s Thrush populations slow to recover from population declines.
Continued research and monitoring is necessary to understand the factors driving population change in Swainson’s Thrush. Until scientists identify specific opportunities for conservation, Swainson’s Thrush will benefit from the general activities that help all birds: protecting habitat, changing window design and lighting policies to reduce collisions with buildings and other structures during migration, reducing pesticide use, and reducing predation by domestic and feral cats.