Red-billed Tyrannulet is a small, easily overlooked flycatcher of montane western South America. Found in foothills on the eastern slopes of the Andes of Ecuador, Peru, and northern Bolivia, this species occupies the canopy of humid forest between 750 and 1200 meters in elevation. This tyrannulet is mostly olive above, with a pale gray crown; it lacks wing bars, but the wing coverts and remiges are narrowly margined with yellow. The throat is whitish, the breast olive yellow, and the lower underparts are brighter yellow. The bill is not truly red; English name refers to the purplish red mandible, while the maxilla is dark gray or dusky. The iris is pale yellow or cream. Red-billed Tyrannulet is similar in appearance several other species of Zimmerius, such as Slender-footed Tyrannulet (Zimmerius gracilipes) of Amazonia or Bolivian Tyrannulet (Zimmerius bolivianus) of higher elevations in the Andes; Red-billed perhaps is most easily distinguished by its rapid, musical, twittering call. Otherwise very little is known about the natural history of Red-billed Tyrannulet.