The geographically most widespread species of Nonnula, the Rusty-breasted Nunlet is, nonetheless, a rather infrequently seen bird over most of its range, in common with its congenerics. However, like other nunlets, it is generally unobtrusive species that does not sing frequently, and this undoubtedly affects our perceptions of its rarity. The Rusty-breasted Nunlet ranges across much of the northern half of Amazonia, and thereafter reappears in parts of the Atlantic Forest and adjacent biomes in the southeast of the continent. Its plumage is typical of the genus, being largely dull brown above and pale rufous below, with a pale supraloral perhaps the most striking feature, although as many as seven subspecies have been tentatively recognized; these generally differ in minor color changes.