Appearance

Overall, Slaty-backed Nightingale-Thrush is a small dark thrush with conspicuous orange bill, white iris, and orange orbital ring. More often heard than seen.

In Central America, Slaty-backed Nightingale-Thrush is highly distinctive and is unlikely to be confused with other species. In South America, male Pale-eyed Thrush (Turdus leucops) is completely black with white eye, but is larger, lacks the orange orbital ring, and is usually not found in dense undergrowth (Clement 2000, Hilty 2003). Slaty-backed Nightingale-Thrush also may be confused with Spotted Nightingale-Thrush (C. dryas), which can be distinguished by its yellowish, spotted underparts, and Black-billed Nightingale-Thrush (C. mexicanus), which has a dark brown dorsum and lacks the white iris. Each of these species can also be distinguished based on song.

Definitive Basic Plumage—Polytypic (see Geographic Variation). The dorsum is typically a dark slate color, with the chin and sides of head likewise dark, sometimes darker than the dorsum. Upper foreneck and throat white to gray. Center of breast and abdomen with varying amounts of white, sometimes infused with a yellowish tinge. There are some reports of sexual dimorphism in the intensity of dorsal coloration (Allen 1900, Todd and Carriker 1922), but this may vary among populations. More study is needed.

Basic I Plumage—No data on molt sequence in C. fuscater, but presumed to be similar to other members of Catharus. Basic I presumably obtained from partial Pre-basic I molt, which includes body feathers and lesser coverts in the migratory Catharus (Dwight 1900, Pyle 1997); flight feathers and scapulars are retained until the Definitive Prebasic Molt.

Juvenile Plumage—Best described by Wetmore et al. (1984): "A juvenile in the Museum of Comparative Zoology from Coliblanco de Sarapiqui, on Irazu, Costa Rica, taken August 23, 1899, has the entire crown fuscous-black; hindneck natal brown; back, rump, scapulars, and wing coverts between natal brown and bone brown, wings and tail fuscous; throat and upper foreneck buffy brown, with the bases of the feathers grayish white; flanks and undertail coverts olive-brown. The culmen (in the dried skin) is fuscous; rest of the bill, tarsi, and feet light brownish yellow, indicating that these areas were light-colored in life, as they are in the adult bird".

Hatchlings—Young emerge from the egg naked with wisps of light brown down in the capital, humeral, and spinal feather tracts (Beltrán and Kattan 2001).

As in other Cathari, each individual seems to molt once per year. In Colombia, molting individuals can be found throughout the year but the proportion of molting birds seems to be greatest from August–December (Beltrán and Kattan 2001). A banded individual that was captured 9 times over 29 consecutive months was twice found to be molting in October and once in December.

Color of the iris, which is typically white but sometimes cinnamon brown (see Beltran and Kattan 2001), may exhibit age or sex differences, or may vary by population. The skin around the eye is bright orange to yellowish orange in life. The bill is also bright orange, but fading to yellow in museum specimens. In Eurasian Blackbird (Turdus merula), a closely related species with a similar phenotype to C. fuscater (i.e., dark dorsum, bright orange orbital skin and bill), the coloration of the bill is correlated with individual condition and exhibits strong seasonal variation in hue and reflectance (e.g., Delhay 2010). Coloration may also vary with sex and age. More study needed.

Mass—Mean (± SD) weight of 5 unsexed specimens from southern Ecuador was 34.6 ± 1.1g, range 33.5–36 (Rahbek et al. 1993). In Colombia, 33.7 ± 2.2g, range 30.5–38 (n = 22), and individuals showed considerable variation in body mass throughout the year (Beltrán and Kattan 2001).

Wing length (mm)—From Wetmore et al. (1984): mirabilis male (n = 6), range 83.3–88.4 (mean 86.6); mirabilis female (n = 4), range 78.8–82.9 (mean 80.7); hellmayri male (n = 10), range 88.0–94.4 (mean 89.7); hellmayri female (n = 4), range 78.6–81.9 (mean 79.7¹).

Tarsometatarsus length (mm)—From Wetmore et al. (1984): mirabilis male (n = 6), range 33.2–37.5 (mean 34.6); mirabilis female (n = 4), range 32.7–34.7 (mean 33.5); hellmayri male (n = 10), range 34.1–36.8 (mean 35.4); hellmayri female (n = 4), range 31.3–35.0 (mean 33.4).

Tail length (mm)—From Wetmore et al. (1984): mirabilis male (n = 6), range 68.0–70.9 (mean 69.5); mirabilis female (n = 4), range 57.2–64.0 (mean 60.9); hellmayri male (n = 10), range 64.6–73.6 (mean 68.5); hellmayri female (n = 4), range 54.0–60.4 (mean 58.2).

Notes: ¹This mean was erroneously reported as 89.7 by Wetmore et al. (1984)

See Beltran and Kattan (2001) for some measurements for populations in Colombia. Generally, morphometric variation is not well known and would benefit from a multi-variate analysis.