Like other antwrens in the genera Epinecrophylla and Myrmotherula, Brown-backed Antwren is a small bird with a thin, warbler-like bill and short tail. This species is a typical member of the “stipple-throated” antwren assemblage, being overall brownish above with prominent spotted wingbars and a streaked throat patch. Both sexes have a light gray-brown iris, the underparts are grayish in the male and buffy-brown in the female.
Brown-backed Antwren is most similar to the allopatric Stipple-throated Antwren (Epinecrophylla haematonota); males are distinguished from this species by brown (vs. rufous) back and overall darker plumage with paler flanks and tail. The female Brown-backed Antwren is nearly identical to the female Stipple-throated Antwren; the white throat and yellowish brown upperparts of female Brown-backed Antwren distinguish it from females of the subspecies pyrrhonota, but these characteristics partially overlap those of the subspecies haematonota. Male Brown-backed Antwren is distinguished from the allopatric Foothill Antwren (E. spodionota) by its brown rather than gray back and its lighter gray underparts. Female Brown-backed Antwren is paler than Foothill Antwren, with a whiter throat and heavier throat streaking, and lacks the flammulations on the breast and belly seen in many females of that species (Krabbe et al. 1999). Range may overlap locally with White-eyed Antwren (E. leucophthalma phaeonota); male Brown-backed Antwren is distinguished by its brown back, grayish edges of inner webs of remiges, and overall darker plumage with paler spotting on the wings, and the female is distinguished by its whitish throat with black streaks (vs. buffy unstreaked throat; Krabbe et al. 1999, Schulenberg et al. 2007)
Brown-backed Antwren is sympatric with Rufous-tailed Antwren (E. erythrura) and is often found in close proximity to that species; Brown-backed Antwren is identified by the brown (vs. rufous) back, pale (vs. dark) iris, and heavier throat streaking in both sexes (throat streaking lacking in female Rufous-tailed Antwren).
The following descriptions are based on Krabbe et al. (1999), Ridgley and Greenfield (2001), Isler et al. (2006), and Restall et al. (2006):
Adult Male: Dark brown upperparts, with darker, rufous-tinged flight feathers and tail. Wing coverts blackish tipped with reddish yellow forming three rows of spots. Covert spots paler distally, with tips of smallest lesser wing coverts whitish. Alula feathers tipped whitish. Wing covert spots may tend toward lighter buff to white. Face gray, with grizzled white and brown in the cheeks. Chest gray, flanks and crissum brownish buff. Chin and throat black with white feather tips forming triangular spots.
Adult Female: Upperparts similar to that of male. Underparts pale buffy brown to light yellowish brown, with chin and throat white with blackish streaks, sides of throat tinged yellow brown.
Juvenile: An immature female described by Krabbe et al. (1999) was lighter and yellower than adult, with brown lateral edges to tail feathers, sides of head with buff mottling. Breast with faint pale flammulations, throat white with pale yellow tinged, irregularly streaked with black edges. Immature male and nestlings apparently undescribed.
No information for Brown-backed Antwren. This species likely follows a Complex Basic Strategy like other Thamnophilidae; Myrmotherula antwrens have a partial preformative molt and delayed plumage maturation in males (Ryder and Wolfe 2009, Johnson and Wolfe 2014).
Iris pale gray-brown, sometimes more brownish in female. Bill blackish with thin gray blue line on the cutting edges of mandible and maxilla. Legs slaty gray, feet blue gray.
Total length: 11 cm (Ridgely and Greenfield 2001a)
Linear Measurements (from Krabbe et al. 1999, sexes combined):
Bill Length (from anterior nostril): 8.97 mm (SD = 0.32, n = 10, range = 8.4-9.5)
Bill Depth: 3.72 mm (SD = 0.15, n = 7, range = 3.6-6.9)
Bill Width: 3.79 mm (SD = 0.14, n = 7, range = 3.6-4.0)
Tarsus Length: 15.7 mm (SD = 0.5, n = 8, range = 15-16)
Tail: 35.8 mm (SD = 1.5, n = 10, range = 33-39)
Wing Chord: 49.8 mm (SD = 1.9, n = 10, range = 37-54)
Mass: 9.0-10.7 g (Krabbe et al. 1999)