Species in the genus Zimmerius are small tyrannids with short, stubby bills; Zimmerius tyrannulets lack wing bars, but the wing coverts and most remiges are edged with yellow. Mishana Tyrannulet's small size and reddish bill place it in a group of just three species within the genus. It differs from the other species in the group in measurements (smaller than Red-billed Tyrannulet Zimmerius cinereicapilla) and voice (higher-pitched than Chico's Tyrannulet Zimmerius chicomendesi, and substantially different from Red-billed).
Within its Amazonian range, Mishana Tyrannulet co-occurs with the related Slender-footed Tyrannulet (Zimmerius gracilipes) as well as with Yellow-crowned Tyrannulet (Tyrannulus elatus). It is distinguished from both of these species by the plainer plumage on the head and face and by its red bill (Alvarez Alonso and Whitney 2001).
The disjunct population of Mishana Tyrannulet in San Martín co-occurs with the similar Red-billed Tyrannulet (Zimmerius cinereicapilla). It differs from that species in its plain (rather than gray) head and its smaller size, as well as voice (Alvarez Alonso and Whitney 2001).
By far the most similar species to Mishana Tyrannulet is the recently described, and allopatric, Chico's Tyrannulet (Zimmerius chicomendesi) of south central Amazonian Brazil. Chico's Tyrannulet differs from Mishana Tyrannulet in its marginally smaller size and diagnostically lower-pitched vocalizations (Whitney et al. 2013).
The following description is based on Alvarez Alonso and Whitney (2001):
Adult: Sexes similar. Upperparts generally olive, slightly paler and more greenish on the sides of the face. Wings dull blackish; wing coverts narrowly edged and tipped with sulphur yellow; secondaries and outer primaries narrowly edged with sulphur yellow. Rectrices dusky, narrowly edged with olive green. Throat pale greenish olive, shading to olive yellow on the breast; belly and undertail coverts sulphur yellow.
Molts of Mishana Tyrannulet are undescribed.
The maxilla is pale brown to dull reddish; the mandible is dull pink. The tarsi are bluish-black, gray, or very dull reddish. The iris is pale creamy white (Alvarez Alonso and Whitney 2001).
Alvarez Alonso and Whitney (2001) provide the following measurements of the Nanay and Tigre river basins population:
wing length (chord): male, mean 45.8 mm (range 45.2-46.2 mm, n = 2); female, mean 43.5 mm (range 42.4-45.3 mm, n = 3)
tail length: male, mean 41.7 mm (range 40.9-42.5 mm, n = 2); female, mean 37.4 mm (range 36.9-37.8 mm, n = 2)
bill length (culmen from base of skull): male, mean 8.7 mm (range 8.4-9.0 mm, n = 2); female mean, 8.8 mm (range 8.7-9.0 mm, n = 2)
bill width (at anterior edge of nares): male, mean 2.9 (range 2.6-3.2 mm, n = 2); female, mean 3.0 (range 2.7-3.1 mm, n = 3)
body mass: male, 6.2 g (n = 1); female, mean 6.8 g (range 6.6-7.0 g, n = 2)