Swainson's Thrush

The State of Swainson’s Thrushes

Regionally: Declining

Swainson’s Thrush study area abundance from 2010 to 2023.

The mean (thick, dark line) annual estimate of Swainson’s Thrush abundance—calculated as the annual sum of estimated Swainson’s Thrush within the local area (a 4-hectare circle) surrounding all 791 Mountain Birdwatch sampling stations. The thinner, lighter lines represent less probable estimates of the annual abundance.

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 northeastern U.S. over the last half-century. Mountain Birdwatch data indicate an average decline of -2.04% per year (80% credible interval = -2.91% to -1.17%) in the mountains of our region; this trend equates to a -23.48% decline in population (80% credible interval = -31.86% to -14.17%) between 2010 and 2023.

Mean annual population trends and population change (with 80% Bayesian credible intervals [CRI]) for Swainson's Thrush from 2010 through 2023. 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 80% CRI does not contain zero.

Region	Mean annual trend (%)	Trend (80% CRI)	Probability of decrease	Probability of increase	Population change (%) 2010-2023	Population change (80% CRI)
All regions	-2.04	(-2.91, -1.17)	0.99	0.01	-23.48	(-31.86, -14.17)
New York (Catskills)	-1.63	(-2.90, -0.28)	0.94	0.06	-19.23	(-31.81, -3.53)
New York (Adirondacks)	-2.73	(-3.87, -1.59)	>0.99	<0.01	-30.19	(-40.12, -18.78)
Vermont	-1.65	(-2.67, -0.62)	0.98	0.02	-19.45	(-29.69, -7.75)
New Hampshire	-2.16	(-3.06, -1.23)	>0.99	<0.01	-24.68	(-33.24, -14.86)
Maine	-1.59	(-2.58, -0.61)	0.98	0.02	-18.79	(-28.84, -7.62)

Globally: Declining

Trend estimates from the North American Breeding Bird Survey and eBird largely agree with each other and Mountain Birdwatch data. These data sources indicate modest, widespread declines in Swainson’s Thrush numbers throughout North America (~0.4% per year). These declines appear to have begun in the early 1980s, and are most severe across the northeastern U.S. and southeastern Canada. 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 the British Columbia coastal forests and Alaska.

**Perhaps best known for its silvery, ascending song, Swainson’s Thrush (Catharus ustulatus) is an increasingly uncommon bird of our northeastern mountain forests.**

Range and habitat

The breeding range (green) of Swainson’s Thrush extends from the high mountains of the northeastern United States, across the boreal forest of Canada and Alaska, and into the mountains and coastal forests of western North America. Swainson’s Thrush winter (red) in tropical forests of Mexico, Central America, and South America.

Swainson’s Thrush occupies a very broad geographic range, and individuals from different parts of the range look and behave quite differently. Scientists recognize two subspecies groups of Swainson’s Thrush: the typical olive-backed form whose breeding range includes the northeastern United States, the Rocky Mountains, and the boreal forest of Canada and Alaska, and the russet-backed form that breeds only in forests along the Pacific coast of North America. In the northeastern United States, Swainson’s Thrushes prefer northern latitudes (see green plot below).

The mean relationship between abundance and latitude (dark green line) of Swainson’s Thrushes within the Mountain Birdwatch study area and the uncertainty of those estimates (i.e., the 95% Bayesian credible interval, represented by thin lines). (Click to enlarge).

Evergreen forests are the most common breeding habitat for both subspecies groups of Swainson’s Thrush. Swainson’s Thrush breed in northern hardwood forests of New York and New England, too, but are far more common in the mid-elevation forests of spruce and fir at elevations between 700 – 1,000 meters (see blue plot below).

The mean relationship between abundance and elevation (dark blue line) of Swainson’s Thrushes within the Mountain Birdwatch study area and the uncertainty of those estimates (i.e., the 95% Bayesian credible interval, represented by thin lines). (Click to enlarge).

The two different forms of Swainson’s Thrush also use different migratory pathways. Individuals of the olive-backed form migrate south through eastern North America and then cross over the Gulf of Mexico to reach their wintering grounds in northern South America. Interestingly, this means that a Swainson’s Thrush breeding in the boreal forest of northeast Alaska migrates east all the way across the continent before turning south, much like a Blackpoll Warbler.

The russet-backed form of Swainson’s Thrush breed along the Pacific coast of North America and follow a migratory pathway (brown) south along the coast. The olive-backed form breeds across the boreal forest and mountains of North America, and migrate through eastern and central North America (gray line).

More Natural History