The rusty-breasted nunlet is 14 to 16 cm (5.5 to 6.3 in) long and weighs 17 to 20 g (0.60 to 0.71 oz). The subspecies vary in structure and color from each other. In general, the more northerly subspecies have a longer and narrower bill and a shorter tail than the more southerly ones.
The nominate subspecies has what amounts to two or three color morphs. Some members of it have plain dark gray-brown upperparts and wings. Their lores are rufous and they have a whitish band from the lores that joins a white ring around the eye. Their tail is dark brown. Their throat, breast, and flanks are brown with a reddish tinge and the abdomen is whitish. Other members of the nominate are paler and grayer, especially on the crown, but in the southeast are a darker brown with a warmer brown tail. Some have a bright orange-rufous (instead of brown) throat, breast, and flanks.
N. r. tapanahoniensis has a very gray crown and a cinnamon tinge to the vent. N. r. duidae has a rufous chin. N. r. interfluvialis has a grayer tail and N. r. simulatrix a darker tail than the others. N. r. simplex is drab and dark while N. r. cineracea has warmer brown underparts.
The rusty-breasted nunlet is found in two disjunct areas. The subspecies are distributed thus:
The rusty-breasted nunlet primarily inhabits humid terra firme and várzea forest, and is also found in secondary forest and forest edges. It can be found at any level within the forest but is most often in the midstory. In Venezuela it is usually found in areas of sandy soil; in southeastern Brazil's Paraná state it occurs in gallery forest, and in upper Amazonia it shuns igapó forest.
The rusty-breasted nunlet's diet is reported to include large arthropods, both adult and larval. The nunlet occasionally briefly joins mixed-species foraging flocks.
The rusty-breasted nunlet was thought to nest in a burrow, but a 2020 publication revealed that it nests in a shallow scrape on sloping ground that it roofs with twigs and leaves. Over a seven year period the authors studied 17 nests in Agentina's Misiones Province; they were in various stages of construction and use when discovered. Most clutches were of four eggs though one had three. Both sexes incubated the eggs and brooded and fed the nestlings.
The IUCN has assessed the rusty-breasted nunlet as being of Least Concern. It has an extremely large range but its population is unknown and believed to be decreasing. Its apparent density varies from rare to rather common in various parts of the range, though it might be undercounted due to its unobtrusive behavior. It occurs in several protected areas.
