Editing Swainson's thrush (section)

==Subspecies==

Four subspecies are recognised, ''Cathartus ustulatus alame'', ''C.&nbsp;u. swainsoni'', ''C.&nbsp;u. ustulatus'' and ''C.&nbsp;u. oedicus''. Subspecies ''Cathartus ustulatus alame'' and ''C.&nbsp;u. swainsoni'' summer east of the [[British Columbia]]n [[Coast Mountains]], the [[Cascade Range|Cascades]] and the [[Sierra Nevada]], and ''C.&nbsp;u. ustulatus'' and ''C.&nbsp;u. oedicus'' summer west of these ranges. There is a small area of overlap in the Coast Mountains. Recent [[molecular systematics]] work<ref>Ruegg & Smith (2002)</ref> confirms that these two pairs of subspecies form two genetically distinct [[clade]]s, referred to as the continental and coastal clades, which diverged during the Late [[Pleistocene]] era, probably about 10,000 years ago as the [[Last glacial period|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. 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 notwithstanding, analysis of [[mtDNA]] [[cytochrome b|cytochrome ''b'']] and [[NADH dehydrogenase]] [[Protein subunit|subunit]] 2 as well as [[nuclear DNA|nuclear]] [[fibrinogen|β-fibrinogen]] [[intron]] 7 [[DNA sequence|sequence]] data<ref>Winker & Pruett (2006)</ref> 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]].