Xenoglaux is a monotypic genus, and is highly distinctive. This is a very small owl, with bare legs, no ear tufts, and very long, delicate facial plumes. The plumage is mostly brown with a whitish belly, finely vermiculated with dusky. John O'Neill, the first ornithologist to view a Long-whiskered Owlet, described his first impression of this bird that it was "shaped like an Otus [Megascops], the size of a Glaucidium, and colored like a Lophostrix" (O'Neill and Graves 1977).
Long-whiskered Owlet is very distinctive and would be difficult to confuse with any other species. It is smaller than screech-owls (Megascops), lacks ear tufts, and does not have the "herring bone" pattern on the underparts that is typical of most species of Megascops. Pygmy-owls (Glaucidium) also are larger and are longer tailed, have broadly streaked underparts, and have a distinctive black nape patch. No other small owl has the namesake long facial plumes ("whiskers") of Long-whiskered Owlet.
The following description is based on O'Neill and Graves (1977):
Adult: Crown, nape, back, rump, upper tail coverts, and scapulars brown, the feathers with numerous distinct but minute dull black bars. Lower nape with collar of large, distinct white spots. Sides of neck brown with small whitish spots that merge with the pale throat. Scapulars each with a distinct subterminal white spot on the outer web. Ttertials and secondaries brown, with 4-5 large white spots on the inner web and 1-2 small white spots on the outer web of each feather. Wing coverts brown; proximal greater secondary coverts with an indistinct medial and a large, distinct subterminal spot of white; lesser secondary coverts similar in color, but with white spots confined to outer edge of distal 2-3 feathers; alula, greater primary coverts, and primaries dull black; inner primaries with a large, irregular patch of white at the base of the inner web of each feather, this patch decreasing in size distally and present on outer primary only as a small spot; outer primaries with outer edge of outer web pale brown and with 1-3 small white spots along this edge. Tail dull brown. Facial ruff dull cinnamon brown. Outer feathers of facial ruff long with distal barbs exceedingly long and decomposed to form a whiskery ruff that extends out beyond the main plumage of the head; bristles at base of bill long, projecting over base of bill to cover cere, and extending upward between the eyes so as to form a vertical, fanlike "crest". Throat and conspicuous eyebrows whitish infused with light buff; upper breast brown minutely barred with dull black, but beginning on the lower breast the feathers having some whitish coloration mixed in with the brown, this paleness increasing posteriorly to give the belly and undertail coverts a "salt-and-pepper" effect, which is enhanced posteriorly by the widening of the spaces between the minute black bars. Tarsi with only a few scattered bristles; toes bare. Tail short, slightly rounded, composed of 12 rectrices.
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Iris: amber orange
Bill: greenish gray with yellowish tip; cere pinkish gray
Tarsi and toes: flesh pink
Bare parts color data from O'Neill and Graves (1977).
Total length: 13–14 cm (Schulenberg et al. 2010)
Linear measurements (from O'Neill and Graves 1977):
male (n = 1, holotype): wing unflattened 105.2 mm, tail 50.3 mm, tarsus 17.7 mm,
middle toe without claw 17.2 mm, culmen from anterior edge of cere 9.6 mm.
female (n = 2): wing unflattened 100.0 mm, 104.6 mm; tail 51.7 mm, 55.4 mm; tarsus 17.3 mm, 17.8 mm; middle toe without claw 17.2 mm, 17.8 mm; culmen from anterior edge of cere 9.6 mm, 9.8 mm.
Mass: male, 47 g (n = 1; Holt et al. 1999, König and Weick 2008); female, 46 g, 51 g (n = 2; Holt et al. 1999, König and Weick 2008)