The Bicknell's thrush (Catharus bicknelli) is a medium-sized thrush, at 17.5 cm (6.9 in) and 28 g (0.99 oz). It was named after Eugene Bicknell, an American amateur ornithologist, who discovered the species on Slide Mountain in the Catskills in the late 19th century.
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 mean weight of 27.8 g (0.98 oz), with a range from 21 to 36.8 g (0.74 to 1.30 oz). The sexes are roughly the same size, although males average slightly larger in wing length. Adults are olive-brown on the upperparts, slightly redder on the tail. The underparts are white with gray on the flanks; the breast is greyish brown with dark spots. They have pink legs, a faint grey eye ring, and gray cheeks. They average slightly smaller than the very similar gray-cheeked thrush but are all but indistinguishable in outward appearance. Together, gray-cheeked and Bicknell's thrush form a cryptic species pair, and were indeed formerly considered conspecific. The song is a jumbled series of flute-like tones ending on a higher note.