Swainson's Thrush (Catharus ustulatus), also called Olive-backed Thrush, is a medium-sized thrush. This species is 16-18 cm in length, and has the white-dark-white underwing patterm characteristic of Catharus thrushes. Swainson's Thrush was named after William Swainson, an English ornithologist.
The breeding habitat of Swainson's Thrush is coniferous woods with dense undergrowth across Canada, Alaska and the northern United States, also deciduous wooded areas on the Pacific coast of North America.
These birds migrate to southern Mexico and as far south as Argentina. The coastal subspecies migrate down the Pacific coast of North America and winter from Mexico to Costa Rica, whereas the continental birds migrate eastwards within North America (a substantial detour) and then travel southwards via Florida to winter from Panama to Bolivia. Swainson's Thrush is a very rare vagrant to western Europe.
This species may be displaced by the Hermit Thrush where their ranges overlap. Possibly, the latter species adapts more readily to human encroachment upon its habitat. At least in the winter quarters, Swainson's Thrush tends to keep away from areas of human construction and other activity.
The genetic differences between the subspecies, and the circuitous migratory route of the continental birds, strongly suggest that these species underwent a rapid range expansion following the end of the last ice age, with populations originally summering in the south-east of North America expanding their ranges northwards and westwards as the ice retreated. Details of the molecular genetic analysis support the hypothesis of rapid expansion of both coastal and continental populations. The current migratory routes of the continental birds, especially the western populations, are not optimal in ecological terms, and presumably represent an inherited, historical route pattern that has not yet adapted to the birds' modern population locations.
These results nonwithstanding, analysis of mtDNA cytochrome b and NADH dehydrogenase subunit 2 as well as nuclear β-fibrinogen intron 7 sequence data shows that Swainson's Thrush is the most ancient North American species of its genus; it is not closely related to other Catharus and the outward similarities with the other North American species are due to convergent evolution.