Zimmerius tyrannulets are very small tyrant flycatchers, with short stubby bills. Zimmerius do not have wing bars, but the wing coverts and most remiges are edged with yellow green. Red-billed Tyrannulet is mostly olive above, with a pale gray crown. The throat is whitish, the breast is olive yellow, and the lower underparts are bright 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.
Zimmerius tyrannulets readily can be identified to genus by their small size, stubby bill, and wing pattern (yellow edgings to the coverts and remiges, but lacking wing bars). Within the genus, however, these tyrannulets are similar to one another. Red-billed Tyrannulet overlaps geographically with several other species of Zimmerius. Slender-footed Tyrannulet (Zimmerius gracilipes) is widespread in Amazonia; it primarily occurs at lower elevations than Red-billed Tyrannulet, but its distribution extends into the Andean foothills. Slender-billed Tyrannulet has a darker gray crown, with an indistinct white or whitish supercilium, and its bill is entirely black. Slender-footed also is slightly smaller than Red-billed, although this difference is not readily apparent in the field. The rising whistle call of Slender-footed is very different from the emphatic, two note call of Red-billed. Bolivian Tyrannulet (Zimmerius bolivianus) overlaps geographically with Red-billed from south central Peru south to Bolivia, and is the same size as Red-billed. Bolivian Tyrannulet usually occurs at higher elevations than Red-billed, however; its crown is olive (not gray), and its bill is entirely dark. The descending whistle call of Bolivian Tyrannulet also is very different from the call of Red-billed. Very locally, in the Mayo Valley in northern Peru, Red-billed Tyrannulet might overlap with Mishana Tyrannulet (Zimmerius villlarejoi). Mishana Tyrannulet also has a reddish bill, but has an olive crown, a yellower throat, and is much smaller than Red-billed. The thin rising whistle call of Mishana Tyrannulet also distinguishes this species from Red-billed.
Adult: Sexes similar. A nondescript tyrannulet with a notably pale mandible and pale iris and with prominent pale feather margins on the wings. Crown pale gray or dusky olive. Sides of the face, back and rump olive green. Tail dusky, rectrices narrowly edged pale olive green. Lesser wing coverts dusky, tipped with olive green; median and greater wing coverts dusky, margined pale greenish white. Remiges blackish, bases of outer webs of primaries 6-9 very narrowly edged pale greenish white (but the outermost and innermost primaries entirely blackish); outer webs of secondaries narrowly edged pale greenish white, tertials margined whitish. Upper throat whitish. Breast light olive gray, clouded with pale yellow; belly and vent pale yellow, upper belly and flanks with a few blurry olive streaks.
Iris: pale straw yellow, cream, creamy white
Bill: maxilla dark gray, blackish, or brownish black; mandible pale purplish gray or dull pink
Tarsi and toes: dark purple gray, dark gray
Bare parts color data from specimens in the Field Museum of Natural History.
Total length: 11.5-12 cm, 12 cm (Ridgely and Greenfield 2001b)
Linear measurements (from Alvarez Alonso and Whitney 2001):
wing length (chord): male, mean 58.6 mm (range 56.1-61.6 mm, n = 5); female, mean 51.7 mm (range 50.8-52.7 mm, n = 3)
tail length: male, mean 52.6 mm (range 51.9-53.4 mm, n = 4); female, 50.2 mm (n = 1)
bill length (from base of skull): male, mean 10.3 mm (range 9.7-10.7 mm, n = 5); female, mean 10.2 mm (range 10.2-10.2 mm, n = 2)
Mass: male, mean 12.2 g (range 11.3-13.8 g, n = 4); female, mean 10.9 g (range 10.7-11.2 g, n = 2) (Alvarez Alonso and Whitney 2001)