This secretive denizen of forests and woodlands across the northern portion of the continent is perhaps best known by its flutelike, upwardly spiraling song that lingers well into the fading summer twilight. It is a Nearctic-Neotropical migrant that breeds as far north as Alaska and northern Canada and winters primarily in South America. One of the most characteristic birds of montane fir forests over much of its range, Swainson's Thrush fills a different niche in coastal California, where it inhabits riparian woodlands. These 2 groups, the "olive-backed" and coastal breeding "russet-backed" thrushes, also differ in morphology, in some aspects of their vocalizations, and in their wintering areas, with the russet-backed group wintering primarily in Middle America and the olive-backed in South America. Adults of both sexes often return to previous breeding sites, which may be an advantage in northern, higher-elevation forests where persistent snow packs foreshorten the breeding season.
Originally classified in the genus Turdus from specimens on the Columbia River in Washington and the Saskatchewan River in Canada, Swainson's Thrush is now included in the genus Catharus with the Veery (C. fuscescens) and Hermit (C. guttatus), Gray-cheeked (C. minimus), and Bicknell's (C. bicknelli) thrushes. Among these and other closely related, spotted North American thrushes, it forages higher off the ground than its relatives and uses more aerial, fly-catching techniques to obtain insect prey, a characteristic that earned it the name "mosquito thrush" in Maine (Bent 1949). Eight forms of Swainson's Thrush have been described based on geographical differences in coloration, with six currently recognized as subspecies. The confused historical taxonomy of this species and currently recognized differences may yet prompt further reclassification.
Although Swainson's Thrush is still considered one of the most common birds of northern spruce-fir forests, populations are declining even where abundant, particularly in Alaska and the Northeast. In California, the breeding range of this species has contracted during the last century, and "the disappearance of the Swainson's Thrush from Yosemite Valley is one of the unsolved mysteries of Sierran ornithology" (Marshall 1988b: 367, citing Beedy and Granholm 1985). The trend is clear, but the reasons are not. Nests are not besieged by cowbirds, but predation rates are high, resulting in low nest-success rates overall, critically low in some locales. Loss of breeding habitat may be a contributing factor, including loss of mature conifer forest and forest fragmentation, but Swainson's Thrushes are relatively abundant in some early-successional habitats, such as conifer plantations; this fact clouds the forest-management issue. On the Pacific Coast, loss of riparian habitat to development and grazing have likely contributed to declines. The impact of habitat changes on wintering grounds is unclear.
Several aspects of this species' life history have been relatively well studied, primarily in the eastern United States: migration behavior and physiology (WY and others), physical and ecological separations from closely related thrushes (Dilger Dilger 1956a, Dilger 1956b, Noon 1981), and diet and foraging ecology (Beal 1915a, Holmes and Robinson 1988). Habitat selection has been described for this species in northeastern montane forests (e.g., Sabo 1980) and Alaska taiga (Kessel 1998), but similar information is lacking for other habitats. Other than observations of a few nests in Maine in the early 1900s (Stanwood 1913), the breeding ecology of this species is not known, perhaps because nests can be so difficult to find. Recent studies in western coniferous forests (DEM and others), northeastern coniferous forests (K. McFarland and others), and riparian systems in California (T. Gardali, S. Small et al.) have focused on how habitat (especially nest sites) affects reproductive success. Combined with additional research at wintering sites, this information may help explain why this thrush has declined.