Juncos are a closely related group of small sparrows. Juveniles are streaked, but in most species, the plumage of the adult lacks streaking, and the outer rectrices are white, contrasting with the darker central tail feathers. Volcano Juno is the exception to these rules: its upperparts are streaked black and brown, and the tail is entirely dark. The head and underparts are pale gray. Volcano Junco also has a black mask, covering the area from the eyes to the bill; this black mask contrasts with the bright orange yellow irides, and the pinkish bill.
Volcano Junco easily is identified by its bright orange yellow irides; other species of sparrow, or sparrow-like bird, in Costa Rica and Panama have dark irides. Within Junco, Yellow-eyed Junco (Junco phaeonotus) and Baird's Junco (Junco bairdi) also have yellow irides, but these species do not overlap geographically with Volcano Junco; they also have unstreaked, rufous or cinnamon brown upperparts and white outer rectrices.
The following description is based on Ridgway (1901) and Wetmore et al. (1984):
Adult: Sexes similar. Crown and nape grayish olive, sometimes obscurely streaked with dusky. Lores dull black and area surrounding the eye black or slate black, forming a distinct mask. Rest of head and neck pale gray, tinged with olive on the auriculars. Back, scapulars, rump, and uppertail coverts buffy brown, streaked with black. Wings dusky, feathers narrowly edged with buffy brown, edgings somewhat broader on the greater wing coverts and the tertials. Rectrices dusky, narrowly edged with gray; outer rectrices with pale gray tips. Underparts pale gray, the throat slightly paler than the breast, and with a buffy brown wash on the flanks.;
Juvenile: Similar to adult, but the upperparts are more heavily streaked, and the brown of the upperparts is a more reddish shade of brown; the underparts are dingy buff, streaked with dusky (except on the center of the belly).
Very little is known about the molts of Volcano Junco, apart from a brief summary by Byers et al. (1995). Adults have a complete molt after breeding; no alternate molt has been described. Juveniles have a preformative molt, involving all of the body feathers and the tertials; but this molt may be complete, "probably [replacing] the rectrices and perhaps the other remiges as well" (Byers et al. 1995). The plumage acquired after the preformative molt is similar to that of the adult, but possibly birds in this plumage can be aged by darker irides (see Bare Parts).
Iris: yellow, bright yellow, or bright orange-yellow in adults (Boucard 1878, Wetmore et al. 1984, Stiles and Skutch 1989); dark brown in juveniles, but gradually changes to yellow
Bill: pink (Boucard 1878); or maxilla sometimes darker, pale brown or tinged with dusky (Wetmore et al. 1984, Stiles and Skutch 1989)
Tarsi and toes: pink, pale brownish white, or flesh (Boucard 1878, Wetmore et al. 1984, Stiles and Skutch 1989)
Total length: 16 cm (Stiles and Skutch 1989, Byers et al. 1995)
Linear measurements: (from Wetmore et al. 1984)
male (n = 10)
wing length: mean 78.6 mm (range 76.0-81.0 mm)
tail length: mean 72.0 mm (range 68.1-75.0 mm)
bill length (culmen from base): mean 14.3 mm (range 13.9-14.9 mm)
tarsus length: mean 26.3 mm (range 25.6-27.1 mm)
female (n = 10)
wing length: mean 74.7 mm (range 70.8-77.6 mm)
tail length: mean 68.3 mm (range 64.9-71.8 mm)
bill length (culmen from base): mean 14.6 mm (range 13.8-15.7 mm)
tarsus length: mean 26.9 mm (range 25.2-27.8 mm)
Mass: 28 g (Stiles and Skutch 1989)