It is a member of genus Catharus and is typical of it in terms of its subdued coloration and beautiful, ascending flute-like voice.

The genus name Catharus comes from the Ancient Greek katharos, "pure or clean" and refers to the plumage of the orange-billed nightingale-thrush C. aurantiirostris.

Recent molecular systematics work[9] confirms that these two pairs of subspecies form two genetically distinct clades, referred to as the continental and coastal clades, which diverged during the Late Pleistocene era, probably about 10,000 years ago as the last ice age came to its end and habitats shifted across North America.

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.

These results notwithstanding, analysis of mtDNA cytochrome b and NADH dehydrogenase subunit 2 as well as nuclear β-fibrinogen intron 7 sequence data[10] 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.