Zimmerius tyrannulets are very small tyrant flycatchers, with short stubby bills. They lack true wing bars, as the wing coverts are not tipped with white or yellow; instead, the wing coversts and most remiges are narrowly edged with yellow. Mistletoe Tyrannulet is a typical member of the genus, with a black bill, gray crown, white supercilium, greenish back, and a lightly striped white breast. Within its range, Mistletoe Tyrannuelt is best identified by its small size and by the Zimmerius pattern of its wing feathers.
The wing pattern is unique throughout most of the range of the species: Mistletoe Tyrannulet lacks wing bars, and instead has pale edges to the wing coverts and all flight feathers. The yellow leading edge of each feather is wider than the trailing edge. Mistletoe Tyrannulet is most likely confused with Yellow-olive Flycatcher (T. sulphurescens), but Tolmomyias species are larger bodied, with a much larger and broader bill, and tend to have pale lores and pale mandibles. Slate-headed Tody-Flycatcher (Poecilotriccus sylvia) also is very small, but among other differences, it has wing bars and a much larger bill. Some elaenias (Elaenia spp.) may show similar wing patterns, but they are larger birds with an upright stance, and typically have a bicolored bill as well as a crest. Olivaceous Flatbill (Rhynchocyclus olivaceus) has similar overall colors and wing pattern, but is much larger, has an eye ring, and distinctive larger, broad bill.
Mistletoe Tyrannulet is very similar to the allopatric or parapatric Guatemalan Tyrannulet (Zimmerius vilissimus), and additional study is needed to clarify field marks separating these two taxa. Mistletoe Tyrannulet is reported to be "decidedly smaller and with the sides and flanks less strongly olive and yellow" (Ridgway 1907), as compared to Guatemalan Tyrannulet. The supercilium appears more substantial in Guatemalan Tyrannulet, with more pale color extending over the base of the bill (personal observations). Other potential differences include tail length (relatively longer in Guatemalan) and the definition of the yellow feather margins on the wing coverts and remiges (narrower and less conspicuous in Guatemalan, broader in Mistletoe). These two taxa may not overlap in range, though additional work is needed to define their ranges, particularly in Honduras. Mistletoe Tyrannulet also primarily occurs at lower elevations, and its mournful, more inflected piping call is very different from the simpler, more finch-like call of Guatemalan.
Adult: Sexes similar. The upperparts are olive green, except that the crown is grayish, and there is a moderately wide, short whitish supercilium. Primaries, secondaries, tertials, and upperwing coverts are edged in yellow, and the leading margin of each of these feathers is more substantial than the tip and trailing edge. Significantly, there are no wing bars. Underparts are off white, with indistinct light streaks on the breast. A weak yellow wash may be visible on the belly and undertail coverts. The tail is narrow and long (as compared to other small tyrannids), and is dark gray. Bill short and rounded. See also Ridgway (1907), Stiles and Skutch (1989), and Wetmore (1972).
Immature: Similar to adult, but with brown irides, an olive gray crown, broader but indistinct wing edgings, and yellow tinge in the supercilium (Stiles and Skutch 1989).
Nestling: Skutch (1960) described the appearance of freshly hatched nestlings in Costa Rica as having pink skin covered with long, gray down, and bright yellow bare parts.
Molts of Mistletoe Tyrannulet apparently are undescribed. Howell (1957) reported a specimen "in fresh plumage with enlarged testes" collected in late August, suggesting a postbreeding molt (see Breeding).
Iris: grayish white, pale brownish white, pale yellow, or grayish brown (Wetmore 1972, Stiles and Skutch 1989).
Bill: maxilla dusky gray or black; mandible gray or black, base dusky or brownish (Wetmore 1972, Stiles and Skutch 1989).
Tarsi and toes: dusky gray or blackish (Wetmore 1972, Stiles and Skutch 1989).
Total length: 9.5 cm (Stiles and Skutch 1989)
Linear measurements (from Wetmore 1972):
male (n = 10)
wing length: mean 51.4 mm (range 47.7-55.2 mm)
tail length: mean 42.2 mm (range 39.0-46.7 mm)
bill length (culmen from base): mean 9.4 mm (range 9.0-9.9 mm)
tarsus length: mean 16.1 mm (range 15.6-16.7 mm)
female (n = 10)
wing length: mean 45.2 mm (range 42.5-50.8 mm)
tail length: mean 34.2 mm (range 31.7-37.0 mm)
bill length (culmen from base): mean 9.0 mm (range 8.6-9.7 mm)
tarsus length: mean 14.5 mm (range 13.9-15.0 mm)
parvus, mean 9.5 g ± 0.7 g (n = 4, sexes unknown, September; Leck 1975); mean 9.8 g ± 0.9 g (n = 11, , sexes unknown, March; Leck 1975); 8.5 g (n = ?, range?; Stiles and Skutch 1989)