Vireos (Vireo) are small songbirds with a relatively stout, hooked bill. Dwarf Vireo is a small vireo with olive upperparts, pale lores and a broken eyering creating a slightly spectacled appearance, white tips to the coverts forming two wingbars and white fringes to the remiges, and pale grayish white underparts. The sexes differ slightly, the male having a bluish gray. Immatures are similar to adults, but have dull irides.
Male Dwarf Vireo can be very similar to female Black-capped Vireo (Vireo atricapilla), and the two perhaps are not always distinguishable (Howell and Webb 1995); typically Black-capped Vireo has a more constrasting gray crown, a whiter loral streak, and yellower wingbars. Ruby-crowned Kinglet (Regulus calendula) is smaller, has a slimmer bill and more slender tarsi, and less contrast between the throat and the sides of the head. Hutton´s Vireo (Vireo huttoni) is bulkier, with duller underparts, and different voice. Cassin´s Vireo (Vireo cassinii) is larger, with a larger bill, and dark lores.
The following description is based on Ridgway (1904) and Howell and Webb (1995):
Adult male: Upperparts grayish olive to olive, crown grayer or (at least in fresh plumage) bluish gray. Lores grayish white; eyering grayish white, and incomplete, being broken above the eye. Rectrices dusky with pale yellowish olive green edgings. Remiges dusky with pale olive gray edgings. Median and greater wing coverts tipped with grayish white, producing two narrow but distinct wingbars. Sides of head grayish olive to olive, slightly paler than crown. Underparts white, faintly washed with grayish on breast and sides of breast.
Adult female: Very similar to adult male, but crown more olive, essentially the same color as the back.
Little information. Adults reportedly have a single annual (prebasic) molt (Phillips 1968).
Iris: reddish (duller in juvenile)
Bill: black or blackish
Tarsi and toes: blue gray
Bare parts color data from Ridgway (1904) and Howell and Webb (1995).
Total length: 9.8 cm (Ridgway 1904), 10-11 cm (Howell and Webb 1995)
Linear measurements (from Ridgway 1904):
male (n = 1; holotype)
wing length: 53 mm
tail length: 43 mm
bill length (exposed culmen): 10 mm
tarsus: 17 mm
Mass: male, 9.4 g (n = 1; Dunning 2008, from a specimen in the Louisiana State University Museum of Natural Science); sex undeterminced, 9.2 (n = 1, immature; specimen in the Museum of Vertebrate Zoology, University of California, Berkeley)