Megascops are small owls with large heads and relatively short tails; many species also have a tuft of feathers ("ear" tufts) on either side of the head. Balsas Screech-Owl is a large screech-owl. The plumage is typical for the genus: it primarily is soft brown and gray on the upperparts, with dusky streaking, and the scapulars are tipped with white, forming a contrasting line of pale spots. The facial disk is grayish brown, with a narrow black rim. Elongated feathers above the eyes form a pair of "ear" tufts. The underparts are whitish with dark shaft streaks and narrow dark cross bars. A distinctive feature of Balsas Screech-Owl is that the irides are brown, rather than yellow as in many other species of Megascops.
Balsas Screech-Owl overlaps with very few similar species. Whiskered Screech-Owl (Megascops trichopsis) has yellow irides, differs vocally, and occurs in oak and pine-oak forest (not in thorn forest). Vermiculated Screech-Owl (Megascops guatemalae) is much smaller, has yellow irides, differs vocally, and occurs in more humid forest. Flammulated Owl (Psiloscops flammeolus) also has brown irides, but is much smaller, differs vocally, and occurs on pine and pine-oak forest. Mottled Owl (Ciccaba virgata) is much larger, has prominent whitish "brows", lacks the "ear" tufts, and differs vocally.
The following description is based on König and Weick (2008); see also Moore (1941):
Adult: Sexes similar. Facial disc is grayish brown, mottled and vermiculated with brownish; the disk is bordered with a blackish brown rim, edged with pale. Pale supraorbital lines ("eyebrows") are not very prominent. "Ear" tufts are relatively short. Crown is grayish brown with blackish brown shaft streaks, and with some whitish spots and brown vermiculations. Upperparts are grayish brown overlaid with vinaceous pink and with dark streaks and vermiculations. Outer webs of scapulars whitish, forming a band of whitish spots. Wing coverts are tipped whitish, creating a pale second band on the closed wing. Remiges and rectrices are barred light and dark. Underparts are paler than the upperparts with rather narrow dark shaft streaks and faint vermiculations. The upper breast has several broad dark brown or deep chestnut shaft streaks, which give the neck and upper breast an irregularly spotted appearance. The tarsi are feathered to the base of the toes. No red morph is known.
Juvenile: Plumage is not described, but probably similar to other screech-owls: generally whitish or buffy, barred with dusky throughout (König and Weick 2008).
Undescribed; probably follows the complex basic molt strategy.
Iris: tobacco brown to (rarely) golden brown
Bill and cere: greenish
Tarsi and toes: tarsi feathered to base of toes; toes large, grayish brown; claws horn with darker tips.
Bare parts color data from König and Weick (2008).
Total length: 24-26.5 cm (Howell and Webb 1995), 24-27 cm (König and Weick 2008)
wing length, male: mean 175.3 mm (range 169.2-181.3, n = 2; Moore 1941)
wing length, sexes combined, mean 172.9 ± 4.7 mm (n = 48; Gehlbach 2003)
tail length, male: mean 92.5 mm (range 89.3-95.7 mm, n = 2; Moore 1941)
tail length, sexes combined: range 88-99 mm (n ?; König and Weick 2008)
bill length (culmen from cere), male: mean 14.9 mm (range 13.7-16.0 mm, n = 2; Moore 1941)
bill length (culmen from cere), sexes combined: mean 15.2 ± 0.6 mm (n = 48; Gehlbach 2003)
Mass: male, mean 160 g (range 158-161 g, n = 2; Dunning 2008); 150-174 g (n ?; König and Weick 2008); ca 155-165 g (n ?; Holt et al. 1999).