Antpittas (Grallaria) have a distinctive morphology: they are plump-bodied, with short tails and notably long tarsi. Moustached Antpitta is a medium sized antpitta with a broad white malar stripe, slate-gray crown and nape, a russet-colored throat bordered by a white chest-band, and cinnamon-washed flanks and undertail coverts. Moustached Antpitta is similar in size and appearance to Scaled Antpitta (Grallaria guatimalensis); see Similar Species.
Moustached Antpitta is "confusingly similar" to the sympatric population of Scaled Antpitta (Grallaria guatimalensis regulus) (Krabbe and Coopmans 2000). These two species can be distinguished most reliably by differences in the pattern of the breast, in the width of the submoustachial streak, and in the shape and color of the bill (Krabbe and Coopmans 2000). In both species, the breast is mostly buff or olive brown, with feathers in the centre of the breast have a contrasting pattern. In regulus, these feathers are pale buff (or, rarely, white), "with a noticeable difference in the distribution of white or buff between the inner and outer web of individual feathers, and with black on each side of the basal half of the shafts"; in alleni, in contrast, these feathers are white, "with equal distribution of white on the outer and inner webs of each feather", and with the white forming "large, symmetrically elongated, narrowly black-edged spots extending onto the lower breast as thin, white to buff streaks surrounding the shafts" (Krabbe and Coopmans 2000).
The submoustachial streak of alleni is much larger than that of regulus, and always is white; the submoustachial streak of regulus is only half the width of that of alleni, and only rarely is white (Krabbe and Coopmans 2000).
The bill of alleni is thinner and straighter than the bill of regulus, and is entirely black; the outer half of the maxilla of regulus is pale (Krabbe and Coopmans 2000).
The following description is based on Krabbe and Schulenberg (2003), and refers to nominate alleni; see also Geographic Variation:
Adult: Sexes similar. Forecrown light olive brown; crown and nape slate gray, feathers narrowly edged blackish. Upperparts dark rufescent brown to olive brown, feathers edged blackish, giving a scaled effect. Rectrices rufous chestnut. Lores white, feathers narrowly dark-tipped. Broad white submoustachial stripe. Auriculars uniform olive brown, shafts pale only at their bases. Chin and throat rufescent brown, lower throat white forming a crescent across chest. Breast olive brown, with narrow white streaks. Belly buffy white. Flanks and crissum washed cinnamon.
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Iris: dark brown
Bill: black; mandible paler on some individuals (younger birds?)
Tarsi and toes: vinaceous gray
Bare parts color data from Krabbe and Schulenberg (2003).
Total length: 16.5 cm (Ridgely and Greenfield 2001b, Krabbe and Schulenberg 2003)
Linear measurements from Krabbe and Coopmans (2000: 85; their Table 2).
The three bill measurements are (from top to bottom) bill from skull; bill from anterior edge of nostril; and mandible from fork.
Mass: 64 g, 77 g (n = 2, males; Krabbe and Schulenberg 2003)