Bicknell's thrush (Catharus bicknelli) is a medium-sized thrush, at 17.5 cm (6.9 in) and 28 g (0.99 oz). One of North America's rarest and most localized songbirds, it breeds on coniferous mountain tops and disturbed habitats of northeastern North America. While very similar in appearance and vocalization to the gray-cheeked thrush (Catharus minimus), the two species, with two completely different breeding ranges, differ slightly in their morphology and vocalizations. It was named after Eugene Bicknell, an American amateur ornithologist, who made the first scientific discovery of the species on Slide Mountain in the Catskills in the late 19th century. John Burroughs, in his essay, "The Heart of the Southern Catskills" (1886), writes effusively about the voice of Bicknell's thrush heard near the summit on his climbs of Slide Mountain, and how on his stays on Slide saw them nowhere else but "about the summit", and saw no other thrush but Bicknell's.

Bicknell's thrush is just slightly smaller than the other northern migratory Catharus thrushes, with an average length of approximately 17 cm (6.7 in) and a weight ranging generally from 26 to 30g. Both sexes are identical in the field and are roughly the same size, although males average slightly larger in wing length. Adults are olive-brown to brownish on the upperparts (head, nape, back) contrasting with chestnut-tinged tail. The contrast is, however, less evident in worn plumage. The underparts are off-white with gray on the flanks; the breast is off-white with buffy wash, showing dusky spots that becomes more diffuse toward the sides and the lower breast. They have pink legs, a faint grey eye ring, and gray cheeks. Two-thirds of the lower mandible is yellow colored, while the tip of the lower mandible and upper mandible is blackish. They average slightly smaller than the very similar gray-cheeked thrush but are all but indistinguishable in outward appearance. The song is a jumbled series of flute-like tones ending on a higher note.

A member of the family Turdidae, Bicknell's thrush belongs to the genus Catharus. This genus includes twelve species, five of which are found in North America. The closest relative of the Bicknell's thrush is its sister species, the gray-cheeked thrush. Together, the gray-cheeked and Bicknell's thrush form a cryptic species pair, and were indeed formerly considered conspecific. However, DNA analysis showed a divergence between the two species about 1 million years ago.

Bicknell's thrush scattered breeding range extends from southeastern Quebec to Nova Scotia in Canada and the sky islands of northern New England and the Adirondacks and Catskills in New York. It is the rarest and most secretive of the breeding thrushes in North America, and it is the only bird species whose breeding range is entirely restricted to the northeastern part of the continent. It is a habitat specialist in its breeding range. It is known to favor high altitude coniferous forests affected by strong winds and heavy ice conditions. They usually breed at higher elevations, normally nesting above 915 m (3,002 ft). However, they do not only live in this habitat, they also inhabit successional forests that have recently been affected by the forest industry. Its habitat is therefore best characterized by highly disturbed forest, where trees are small and stunted.

Conversely, Bicknell's thrush is more a habitat generalist in migration. The Bicknell's and gray-cheeked thrushes, along with the veery, make up a close-knit group of migrant species. The birds migrate to the Greater Antilles, with an estimated 90% of the individuals wintering in Hispaniola (mostly in the Dominican Republic, but also Haiti). During the winter, they live in broadleaf forests at various elevations but generally preferring higher elevations. Wintering individuals have been recorded from the mountain ranges of the Cordillera Central, Sierra de Baoruco, and Cordillera Septentrional in the Dominican Republic, the Massif de la Hotte in Haiti, the Cordillera Central in Puerto Rico, the Blue Mountains in Jamaica, and the Sierra Maestra in eastern Cuba.

Vocalizing mostly at dawn and dusk, the song is mostly performed by the male, but sometimes also by females. Like all North American thrushes, Bicknell's thrush song is flute-like. High pitched and vibrant, the song is composed of four phrases: "chook-chook, wee-o, wee-o, wee-o-ti-t-ter-ee". Unlike in the gray-cheeked thrush, the pitch of the last phrase is constant or rising.

The primary call is a downward whistle named the "Beer call" "beer". Other calls include growl call given in alarm situations "crr-rr-rr" and flight call "cree-e-e"

Bicknell's thrushes have an unusual mating system in which females mate with more than one male. Such a practice, known as polyandry, is not known to occur in other thrushes. As many as four males perform duties connected with one nest, including bringing food for the nestlings. It is possible that females decide to mate with more males when preys are less abundant. Pair formation is thought to occur after female arrival to breeding site in late May. The nest itself is typically a bulky cup made of twigs and moss, close to the trunk of a conifer at base of horizontal branches, usually 2 meters above ground. The female builds the nest alone and lay three or four eggs per clutch. While the incubation period is about two weeks, nestlings, fed by both parents, grow rapidly, developing in 12 days from peanut-sized hatchlings to completely feathered adult-sized birds.