Piculets (Picumnus) are very small, short-tailed, short-billed woodpeckers. Plain-breasted Piculet is grayish olive above, and the underparts are dull white or pale yellowish white. The crown of the male is black, although most of the crown feathers have broad red tips; the crown of the female is entirely black.
Throughout most of its range, Plain-breasted Piculet is distinctive and is unlikely to be confused with any piculet species. It is broadly sympatric with Bar-breasted Piculet (Picumnus aurifrons), but Bar-breasted Piculet is more olive above and yellower below, is distinctly barred on the breast, is strongly striped on the belly, its crown has small white spots, and it is more associated with terra firme forest.
Plain-breasted Piculet also is similar to Fine-barred Piculet (Picumnus subtilis); these two species are not known to overlap, but their geographic ranges may meet, and in indeed one specimen of a suspected hybrid between the two has been reported. Fine-barred differs from Plain-breasted by having white spots on nape (and, in the female, on entire crown), yellower underparts, narrow gray barring on breast, and by its indistinct barring on the back.
The following description is based on Winkler et al. (1995) and on direct examination of specimens in the Louisiana State University Museum of Natural Science:
Adult male: Crown black, feathers tipped red. Sides of crown and nape whitish, faintly barred or speckled with black. Lores and nasal tufts white or buffy white. Auriculars light brown, vermiculated gray and whitish. Upperparts grayish olive. Wings brown, wing coverts and secondaries edged greenish yellow. Rectrices black, inner webs of inner pair and of three outer pairs white. Underparts dull yellowish white.
Adult female: Similar to male, but crown entirely black.
Juvenile: Similar to adult female, but "duller, with stronger hint of barring above, faint barring below" (Winkler and Christie 2002; see also Short 1982).
Bill: black; base of mandible blue-gray
Tarsi and toes: greenish yellow, bluish gray
Bare parts color data from specimens in the Louisiana State University Museum of Natural Science and Field Museum of Natural History.
Total length: 8-9 cm (Winkler et al. 1995, Winkler and Christie 2002), 9–10 cm (Schulenberg et al. 2010)
Linear measurements (from Short 1982 and Winkler et al. 1995):
wing length, 48.5-53.7 mm
tail length, 25 mm
bill length, 13 mm
Mass: female, 11.4 g (n = 1; Remsen 1977); 11-12 g (Winkler et al. 1995)