Tyrannulets in the genus Zimmerius are readily identified by short, stubby bills, unusual among the small tyrant flycatchers. Species in this genus lack wing bars, but the wing coverts and most remiges are edged with yellow. Guatemalan Tyrannulet is a typical member of the genus, with a black bill, gray crown, white short supercilium, greenish back, and a lightly striped white breast, closely matching Mistletoe Tyrannulet (Zimmerius parvus). Within its range, this species is best identified by the Zimmerius pattern of its wing feathers.
The wing pattern is unique throughout the range of the species: Guatemalan 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. Guatemalan Tyrannulet may be confused with Yellow-olive Flycatcher (Tolmomyias sulphurescens), but that species is larger bodied, with a much larger and broader bill, and pale irides. Slate-headed Tody-Flycatcher (Poecilotriccus sylvia) has a similar wing pattern, but with additional yellow wing bars, and a larger bill. 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.
Guatemalan Tyrannulet is very similar to Mistletoe Tyrannulet (Zimmerius parvus), 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), the definition of the yellow feather margins on the wing coverts and remiges (narrower and less conspicuous in Guatemalan, broader in Mistletoe), and the pattern of the underparts (Guatemalan has more contrasting yellow flanks) (M.J. Iliff, personal communication). 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: Upperparts olive green, except for a grayish crown, and there is a moderate, 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 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), with yellow edges. The sexes are identical. See also Ridgway (1907).
Immature: Similar to adult, but with dark irides and more green in the crown (Howell and Webb 1995).
Molts of Guatemalan Tyrannulet apparently are undescribed.
Iris: light brown, brownish black (Dickey and van Rossem 1938).
Bill: brownish black (Dickey and van Rossem 1938). The short, narrow, rounded black bill is unusual among flycatchers.
Tarsi and toes: brownish black (Dickey and van Rossem 1938); as in Mistletoe Tyrannulet, the tarsi are relatively long for a flycatcher.
Total length: 11-12 cm (Howell and Webb 1995)
Linear measurements (Ridgway 1907):
male (n = 3; sex not determined, but presumed to be male)
wing length: mean 59.3 mm (range 58-60 mm)
tail length: mean 47.5 mm (range 46.5-48 mm)
bill length (exposed culmen): mean 9 mm (range 8.5-9.5 mm)
tarsus length: mean 18.5 mm (range 18-19 mm)
female (n = 1)
wing length: 52 mm
tail length: 39 mm
bill length (exposed culmen): 8.5 mm
tarsus length: 17 mm