Appearance

The only other species of nightingale-thrush that could be sympatric with the Black-billed Nightingale-Thrush is the Ruddy-capped Nightingale-Thrush (Catharus frantzii), which can be found at the same habitat up to about 3000 m. Overall the Black-billed is smaller and darker than Ruddy-capped, but the Black-billed Nightingale-thrush has a completely black bill, the face, crown are dark gray, the underparts are gray with a brownish band across its chest, in comparison to the Ruddy-capped whose underparts are light gray, the upperparts are more rusty, and a bright orange lower mandible.

The Black-billed Nightingale-Thrush has a chunky body, short rounded wings and long skinny legs. The plumage overall is dark in color. Upperparts, including the wings and the upper surtace of the tail, are olive-brown. The face and forecrown are slate-gray; the same color dominates most of chest and belly, except for a whitish throat, and a light gray to whitish area in the central lower part of the belly. There also is a olivaceus band across the chest, which is fainter at the center of the chest.
The young birds are duller above, and more tinged with olive; the wing coverts have pale fringes;  the forecrown, face and throat are sooty-gray, tinged with olive; the rest of underparts duller and darker than adult; and the chest and upper belly have washed spotting (Stiles and Skutch 1989)

No information.

Iris dark brown. Bill black. Tarsi and toes dark gray.

Catharus gracilirostris gracilirostris (n=51): culmen: 16.5 ± 1.51 mm; wing: 76.18 ± 3.19 mm; tail: 62.77 ± 4.06 mm; tarsus: 32.09 ± 1.87 mm. Mass 20.25 ± 1.51 g (C. Sánchez unpub. data).

Catharus gracilirostris accentor male (n=8): culmen 15.1-17.0 mm; wing: 71.2-78.4 mm; tail 56.7-66.7 mm; tarsus: 30.9-33.4 mm; female (n=6): culmen 14.3-18.3 mm; wing: 70.8-74.8 mm; tail 54.6-62.2 mm; tarsus: 30.0-31.1 mm (Wetmore et al. 1984).

Catharus gracilirostirs bensoni (n=3): male (n=1): culmen 15.3 mm; wing: 68.9 mm; tail 62.8 mm; tarsus: 30.5 mm; female (n=2): culmen 14.2-15.0 mm; wing: 68-69 mm; tail 59.7-62.7 mm; tarsus: 29.7 - 31.8 mm (Wetmore et al. 1984).