| Swainson's thrush | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Passeriformes |
| Family: | Turdidae |
| Genus: | Catharus |
| Species: | C. ustulatus |
| Binomial name | |
| Catharus ustulatus (Nuttall, 1840) | |
 | |
Breeding Migration Nonbreeding | |
| Synonyms | |
Hylocichla swainsonii | |
Swainson's thrush (Catharus ustulatus), also calledolive-backed thrush,russet-backed thrush, andsalmonberry bird is a medium-sizedthrush. It is a member of genusCatharus and is typical of it in terms of its subdued coloration and beautiful, ascending flute-like voice. Swainson's thrush was named afterWilliam Swainson, anEnglishornithologist.
The genus nameCatharus comes from theAncient Greekkatharos, "pure or clean" and refers to the plumage of theorange-billed nightingale-thrushC. aurantiirostris. The specificustulatus isLatin for "burnt", fromurere, "to burn".[2]
The breedinghabitat of Swainson's thrush isconiferous woods with dense undergrowth acrossCanada,Alaska, and the northern United States; also,deciduous wooded areas on thePacific coast ofNorth America.
These birdsmigrate to southernMexico and as far south asArgentina. The coastalsubspecies migrate down the Pacific coast of North America and winter fromMexico toCosta Rica, whereas the continental birds migrate eastwards within North America (a substantial detour) and then travel southwards viaFlorida to winter fromPanama toBolivia. Swainson's thrush is a very rare vagrant to westernEurope. It has also occurred as a vagrant in northeast Asia.[3]
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This species may be displaced by thehermit 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.
This species is 16–20 cm (6.3–7.9 in) in length. The wingspan averages at 30 cm (12 in) and the wing chord is 8.7–10.5 cm (3.4–4.1 in). The bill measures 1.5–1.9 cm (0.59–0.75 in) in length and the tarsus is 2.5 to 3.1 cm (0.98 to 1.22 in) long. This species' body mass can range from 23 to 45 g (0.81 to 1.59 oz).[4][5][6] This thrush has the white-dark-white underwing pattern characteristic ofCatharus thrushes. Adults are brown on the upperparts. The underparts are white with brown on the flanks; the breast is lighter brown with darker spots. They have pink legs and a light brown eye ring. Birds in the east are more olive-brown on the upperparts; western birds are more reddish brown. This bird's song is a hurried series of flute-like tones spiralling upwards.
They forage on the forest floor, also in trees.[7] Swainson's thrushes mainly eat insects, fruits and berries.[8] They make a cup nest on a horizontal tree branch.
Its song is said by local tribes to makesalmonberries ripen,[9] including theSaanich people who claim the bird says, "xwexwelexwelexwelexwesh!" meaning "ripen, ripen, ripen, ripen!" A similar belief appears among theTlingit,Haida,Haisla,Oweekeno,Kwakwaka'wakw, Nuu-Chah-Nulth,Ditidaht,Sḵwx̱wú7mesh, and theStraits Salish people.[citation needed]
Four subspecies are recognised,Cathartus ustulatus alame,C. u. swainsoni,C. u. ustulatus andC. u. oedicus. SubspeciesCathartus ustulatus alame andC. u. swainsoni summer east of theBritish ColumbianCoast Mountains, theCascades and theSierra Nevada, andC. u. ustulatus andC. u. oedicus summer west of these ranges. There is a small area of overlap in the Coast Mountains. Recentmolecular systematics work[10] confirms that these two pairs of subspecies form two genetically distinctclades, referred to as the continental and coastal clades, which diverged during the LatePleistocene era, probably about 10,000 years ago as thelast 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 lastice 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 ofmtDNAcytochromeb andNADH dehydrogenasesubunit 2 as well asnuclearβ-fibrinogenintron 7sequence data[11] shows that Swainson's thrush is the most ancient North American species of its genus; it is not closely related to otherCatharus and the outward similarities with the other North American species are due toconvergent evolution.
