It is found in Argentina, Brazil, Colombia, Ecuador, Guyana, Paraguay, Peru, Suriname, Venezuela, and possibly French Guiana.

In general, the more northerly subspecies have a longer and narrower bill and a shorter tail than the more southerly ones.

Their throat, breast, and flanks are brown with a reddish tinge and the abdomen is whitish.

[7] 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.

Over a seven year period the authors studied 17 nests in Argentina's Misiones Province; they were in various stages of construction and use when discovered.

[1] 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.