Zimmerius tyrannulets are very small tyrant flycatchers, with short stubby bills. Species of Zimmerius do not have wing bars, but the wing coverts and most remiges are edged with yellow green. Spectacled Tyrannulet is a typical member of the genus, with a black bill, dark gray crown, a short white supercilium, greenish back, and a very lightly striped white breast. The outer primaries are edged with yellow (particularly at the base), while the inner primaries lack this pattern.
The wing pattern is unique throughout the range of the species: the bright yellow edges spanning more than half of the length of each secondary creates a patch of color that distinguishes Spectacled Tyrannulet from any similar species. Spectacled Tyrannulet overlaps geographically with its congener Golden-faced Tyrannulet Zimmerius chrysops); Golden-faced Tyrannulet is similar but has a yellowish face and a green (not gray) crown. Spectacled Tyrannulet otherwise is most likely confused with some Tolmomyias flycatchers (especially Yellow-olive Flycatcher Tolmomyias sulphurescens), but Tolmomyias species are larger bodied, with a much larger and broader bill, and tend to have pale lores and pale mandibles. Some elaenias (Elaenia spp.) may show somewhat similar wing patterns, but they are larger with an upright stance, and typically have a bicolored bill as well as a crest. Olivaceous Flatbill (Rhynchocyclus olivaceus) has similar overall body and wing patterns, but is much larger, has an eye ring, and a distinctive larger, broad bill.
Adult: Sexes similar (but females are slightly smaller, in wing and tail length; see Measurements). Crown and nape dark grayish green, becoming green on the back and rump. Forecrown, loral streak, and arcs above and below eyes white, imparting a spectacled appearance. The wing coverts and remiges are dusky. Wing coverts narrowly edged with yellow (but not forming wing bars); secondaries and outer primaries also narrowly edged with yellow. The tail is narrow and long (as compared to other small tyrannids), and is dark gray edged with yellow. The breast, belly and undertail coverts are weakly yellow and may contrast with a more white throat in the field; there are indistinct light streaks on the breast.
Immature: Similar to adult but with duller colors, including darker lores, eyes, and back, and gray legs and feet. Restall et al. (2006) indicates that the tips of the greater coverts are paler tipped, creating the effect of a wing bar that is absent in the definitive plumage.
Molts of Spectacled Tyrannulet apparently are undescribed.
Iris: dark brown (Restall et al. 2006).
Bill: the short, narrow, rounded black bill is distinctive among flycatchers.
Tarsi and toes: black; the tarsi are relatively long for a flycatcher.
Total length: 11.5-12 cm (Restall et al. 2006), 12.7 cm (Hilty 2003); note that in both references, these data refer both to Spectacled Tyrannulet and to Venezuelan Tyrannulet Zimmerius petersi.
Linear measurements (all data from Phelps and Phelps 1954):
subspecies tamae, male
wing length: mean 61.2 mm (range 60-62 mm, n = 5); range 58-62 mm (n = 9)
tail length: mean 50.6 mm (range 49-52 mm, n = 5); range 48-52 mm (n = 9)
bill length (culmen from base): mean 11.9 mm (range 11.5-12 mm, n = 8)
subspecies tamae, female
wing length: mean 55 mm (range 53-57 mm, n = 2)
tail length: mean 44 mm (range 42-46 mm, n = 2)
bill length (culmen from base): mean 11.5 mm (range 11-12 mm, n = 2)
subspecies improbus, male
wing length: mean 59.6 mm (range 59-62 mm, n = 5)
tail length: mean 50.2 mm (range 50-51 mm, n = 5)
bill length (culmen from base): mean 11.5 mm (range 11-12 mm, n = 5)
subspecies improbus, female
wing length: mean 54.2 mm (range 53-55 mm, n = 5)
tail length: mean 45 mm (range 44-46 mm, n& ;= 5)
bill length (culmen from base):;mean 11.3 mm (range 11-12 mm, n = 5)
improbus, 10.9 g (n = ?, range?; Hilty 2003; and note that is refers both to Spectacled Tyrannulet and to Venezuelan Tyrannulet Zimmerius petersi.