Also known as the Brazilian Pygmy-Owl, on account of it being almost endemic to that country, the Least Pygmy-Owl occurs from northern Bahia state south to at least Paraná state, and has been recorded in neighbouring Paraguay, as well as being speculated to be present in adjacent northeastern Argentina. This species formerly comprised part of a much wider species complex that also includes Tamaulipas Pygmy-Owl (Glaucidium sanchezi), Colima Pygmy Owl (Glaucidium palmarum), and Central American Pygmy-Owl (Glaucidium griseiceps), as well as the recently described Subtropical Pygmy Owl (Glaucidium parkeri). However, in terms of identification, it overlaps geographically only with the near-ubiquitous Ferruginous Pygmy-Owl (Glaucidium brasilianum), which species has a quite different voice, is larger, with a streaked (not spotted) crown, and a more barred tail. Least Pygmy-Owl is also never found in second growth or the more open country frequently favored by the Ferruginous Pygmy-Owl. Instead, it favours evergreen Atlantic Forest and its edges, and is found at elevations up to about 1000 m. Like other pygmy-owls, this species is partially diurnal and is easily attracted using a whistled imitation of hollow-sounding, double- or treble-noted song. Despite being relatively abundant, currently very little is known of the Least Pygmy-Owl’s ecology.