Arremon are largely terrestrial sparrows of forested or wooded habitats. In size, they vary from relatively small to large, but typically have long tails and olive upperparts. Many species also have a prominent white throat and have a prominent white or gray supercilium. The two species of Arremon that formerly were classified in the genus Lysurus, Olive Finch and Sooty-faced Finch (Arremon crassirostris), are somewhat different, however. Both have dark throats and have no pale supercilium (although Sooty-faced Finch has a prominent white submoustachial stripe). Olive Finch has a dull chestnut crown and nape; otherwise the head and throat are dark gray. The rest of the plumage is primarily dull olive green.
The closely related Sooty-faced Finch (Arremon crassirostris) of Costa Rica, Panama, and northwestern Colombia is similar to Olive Finch, but is allopatric; it is further distinguished by its yellow belly and broad white submoustachial stripe (Ridgway 1901, Hilty and Brown 1986, Jaramillo 2011). White-rimmed Brushfinch (Atlapetes leucopis), which occurs in Colombia and Ecuador, is similar in plumage to Olive Finch but occurs at higher elevations, is larger, is black rather than olive above, and has a conspicuous white eye ring (Hilty and Brown 1986). Chestnut-capped Brushfinch (Arremon brunneinucha) is broadly sympatric with Olive Finch; juveniles of the two species are similar, but the juvenile of the brushfinch has dark rather than pinkish feet and a strongly bicolored bill, with the maxilla blackish and the mandible yellowish (Restall et al. 2007). In general, Olive Finches have darker and more uniform plumage than most brushfinches (Ridgely and Greenfield 2001b), and the brushfinches that have chestnut on the crown can be told from the Olive Finch by their various distinctive white markings (Restall et al. 2007).
Adult: Sexes alike. The forecrown, crown, and nape are rusty chestnut, the throat and rest of the head are sooty dark gray, and the rest of the body feathers are unstreaked olive green (Jaramillo 2011). The flight feathers are dusky with olive fringes (Jaramillo 2011). The olive green color on the underparts is slightly brighter than that of the upperparts (Sclater 1886). The wings are more rounded when compared to other species of Arremon (with the exception of its sister species, Sooty-faced Finch Arremon crassirostris), with its ninth primary being significantly shorter than the secondaries and the eighth primary being of equal length or slightly longer than the secondaries (Ridgway 1901). The tail is also more rounded with broader retrices compared to other Arremon species (Ridgway 1901).
Juvenile: Similar to adults but with a duller crown, olive throat and face, and more uneven overall plumage coloration (Jaramillo 2011).
Although a juvenile plumage is described for Olive Finch (Jaramillo 2011), more specific information on molt and its timing is not available for this species. For a description of the nestling, see Reproduction.
Iris: brown, dark brown
Bill: maxilla black or dark gray; mandible pale gray, tip black
Tarsi and toes: black, dusky
Bare parts color data from Schulenberg et al. (1984) and Jaramillo (2011).
Total length: 15 cm (Hilty and Brown 1986, Ridgely and Greenfield 2001b), 15.5-16.5 cm (Schulenberg et al. 2010)
No reported differences between males and females. All measurements are of adults.
wing length: 78.7 mm (n = ?; Sclater 1886); male, 82-83 mm (n = 2; Traylor 1951); female, 76 mm (range 76-76 mm, n = 3; Traylor 1951)
tail length: 66 mm (n = ?; Sclater 1886)
ratio of ulna length to femur length: 0.8-1.0 (Webster and Webster 1999)
male, mean 36.9 g (range 34.5-39 g, n = 3; Dunning 2008)
male, 37.5 g (n = 1; Schulenberg et al. 1984)
female, mean 33 g (range 32-35 g, n = 3; Schulenberg et al. 1984)