Swainson’s Thrush

A regular breeding resident and migrant, accidental in winter. The Swainson’s Thrush was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

The Swainson’s Thrush is found primarily throughout Canada and the western United States, with southern limits in the northeastern and upper midwestern United States. The highest densities are found in western North America from Oregon to British Columbia (Figure 1).

Assigned a Continental Concern Score of 10/20 by Partners in Flight.

Long-distance migrant; overwinters in Mexico, Central America, and South America.

Omnivore; insects and fruit, primarily gathered near the ground.

Cup nest in the understory of a shrub, sapling, or tree.

Roberts (1932) reported that he had limited contact with the Swainson’s Thrush and generally referred to it as “a summer resident in the northern evergreen forests as far south as Pine County and as far west as Itasca Park and Kittson County.” He reported confirmed nesting in Cass, Cook, Itasca, and Kittson Counties; the latter was the first nest found in Minnesota in 1896. All reports were of nests containing eggs, except the Itasca County observation, which was noted as “young feeding out of the nest.” The nests in Cass and Kittson Counties included cowbird young. Roberts reported observations of the Swainson’s Thrush in east-central Minnesota in the 1870s by Trippe as “Rather rare. Does not breed.” However, a report from 1922 referred to the species as “rather common” in Cook County, while Roberts and his colleague, Kilgore, also found it “fairly commonly” in northern Itasca County.

More than forty years later, Green and Janssen (1975) described the breeding distribution of the Swainson’s Thrush as northeastern and north-central Minnesota, plus adjacent northwestern counties. They remarked that the species’ present status in the northwestern counties and as far south as Pine County was unknown. They added confirmed nesting in Crow Wing, Lake, and northern St. Louis Counties. A few years later, Janssen (1987) suggested a more restricted breeding distribution, where the species was confined to the northernmost counties from Clearwater to northern Itasca, most of St. Louis, and all of Lake and Cook Counties. He suggested a northwestern distribution limited to eastern Roseau County. Janssen (1987) and Hertzel and Janssen (1998) only included confirmed nesting in Clearwater and Cook Counties since 1970.

The Minnesota Biological Survey (MBS) included 362 breeding season locations and emphasized the dominant distribution of the Swainson’s Thrush in the eastern portion of St. Louis County and especially throughout Cook and Lake Counties. However, the MBS included observations from one location in Carlton County, five locations in northern Itasca County, two in southern Beltrami County, one in eastern Marshall County, and two in northern Roseau County.

The MNBBA included 859 records that further reinforced the species’ major distribution in Cook, Lake, and northeastern St. Louis Counties, but it also scattered probable and possible nesting throughout Itasca, Koochiching, Lake of the Woods, and Roseau Counties (Figure 2). The breeding distribution of the Swainson’s Thrush was broad but was so scattered that it only appeared in 7.6% of all surveyed blocks (361/4,734) (Figure 3; Table 1). The most southerly observations were from 5 blocks in north-central Minnesota, including southern Cass and Crow Wing Counties and southwestern Aitkin County. Nesting was also confirmed in Crow Wing and northern Cass Counties. Other than the seven confirmed nesting observations in Cook and Lake Counties, confirmed nesting records included one in central Itasca County and two in St. Louis County, The latter included a record in southern St. Louis County in Hermantown, just northwest of Duluth. Possible nesting observations were also recorded in Beltrami, Hubbard, Kittson, and Marshall Counties.

The predicted probability map emphasizes the highest densities of the Swainson’s Thrush from southwestern St. Louis County to extreme eastern Cook County (Figure 4). High densities are also predicted throughout Koochiching County.

Historically, the lack of confirmed nesting and the limited number of confirmed nests was largely due to the difficulty in finding nests of the Swainson’s Thrush (Mack and Yong 2000). This difficulty was also noted in Wisconsin’s breeding bird atlas from 1995 to 2000, where nesting was confirmed in only 10 quads (Cutright et al. 2006). Breeding observations from the MBS and the MNBBA illustrated the breeding distribution in Minnesota is larger than previously outlined by Janssen (1987); but neither source included observations from Pine County, where Roberts (1932) previously recorded the species. There still appeared to be scattered, though sparsely distributed, populations of the Swainson’s Thrush in the northwestern counties of Kittson, Marshall, and Roseau. The intensive MNBBA coverage by astute observers in the Brainerd area also revealed breeding populations in central Minnesota.

*Note that the definition of confirmed nesting of a species is different for Breeding Bird Atlas projects, including the definition used by the Minnesota Breeding Bird Atlas, compared with a more restrictive definition used by the Minnesota Ornithologists’ Union. For details see the Data Methods Section.

In their review of the Swainson’s Thrush in North America, Mack and Yong (2000) described the species’ breeding habitat as “strongly associated with coniferous (e.g., spruce-fir) forests and mixed hardwood-conifer forests across Canada and the northern New England states.” In Minnesota, the species is considered to be conifer-dependent but is also found in mixed coniferous-deciduous upland forests (Figure 5) (Green and Niemi 1978; Niemi and Pfannmuller 1979).

MNBBA data indicated a strong association of the species with bog and boreal coniferous habitats (Figure 6). These associations primarily reflected the species’ occurrence in lowland coniferous, black spruce, tamarack, and white cedar forests, as well as upland coniferous white spruce and balsam fir forests. Habitat analyses in the National Forest Bird (NFB) monitoring program (Niemi et al. 2016) indicated a significant association with aspen-spruce-fir forest cover types but also found the species in mixed swamp coniferous and jack pine forest cover types. In the Agassiz Lowlands Ecological Subsection, the species was most often found in mixed lowland white cedar forests (Bednar et al. 2016).

Partners in Flight (Rosenberg et al. 2016) estimated a global population of 110 million breeding adults in North America but only 100,000 breeding adults in Minnesota (Partners in Flight Science Committee 2013). The MNBBA estimated a population twice that size of 217,000 breeding adults, but the prediction was highly variable. This is still a relatively small proportion of the species’ population, especially because of its wide distribution across the boreal forests of Canada and Alaska, extending southward into the Rocky Mountains and along the Pacific Coast (Figure 1).

The BBS data indicated a nonsignificant trend in the Minnesota breeding population from 1967 to 2015, but sample size was low and only based on 25 BBS routes (Figure 7). Similar nonsignificant trends over the same period were observed in the adjacent Canadian provinces of Manitoba and Ontario. The species was not common enough in Michigan or Wisconsin to confidently estimate a trend. In Minnesota, mean observations of the Swainson’s Thrush were two per route per year. Partners in Flight (Rosenberg et al. 2016) estimated that the North American breeding population has declined by 32% from 1970 to 2014.

The Swainson’s Thrush was not abundant enough in the Chippewa National Forest to detect a trend, but based on 70 forest stands that are sampled each year in the Supeior National Forest, the species is significantly declining by 3.37% per year (Figure 8).  The pattern of the species trend, however, shows an increase from 1995 to 2003 and then a decline from 2003 to 2016.

Mean population densities in the Superior National Forest were 1.0 pair per 40 ha compared with 0.1 pair per 40 ha in the Chippewa National Forest. Within specific habitats, population densities were highest in aspen-spruce-fir mixed forests of the Superior National Forest. In the Agassiz Lowlands Ecological Subsection of northern Minnesota, the species’ densities averaged 1.2 pairs per 40 ha in mixed lowland white cedar forests.

The Partners in Flight continental conservation score of 10/20 indicates some concern for the Swainson’s Thrush in North America (Rosenberg et al. 2016). The species receives minimal conservation attention because of its substantial North American breeding population and wide distribution; however, its current declining population forms the basis of some concern.

Mack and Yong (2000) cited highly variable population fluctuations of the Swainson’s Thrush among different regions of North America. They also identified habitat loss and degradation as well as collisions with towers and buildings as issues that may negatively impact the species’ population. Loss et al. (2014) reinforced the higher-than-average risk of mortality at residences. Bracey et al. (2016) found it to be the most common species subject to window kills in residences at Minnesota Point in Duluth. Longcore et al. (2013) found the species to rank fifth in mortality at communication towers. The increase in both urbanization and the numbers of communication towers and wind turbines could represent a growing concern for this long-distance migrant.

In their summary of bird species susceptible to future climate change, Langham et al. (2015) and the National Audubon Society (2015) do not list the Swainson’s Thrush as vulnerable. This is likely due to the considerable habitat that may be available to the species in the northern portion of its breeding range.