Pygmy-owls (Glaucidium) are very small owls, with a large rounded head, a pair of prominent black marks (false "eye spots") on the nape, and a relatively long tail. Ferruginous Pygmy-Owl is the largest species of Glaucidium. The upperparts often are rufous ("ferruginous") but also may be duller grayish brown; the underparts are white, marked with broad streaks that are the same color as the upperparts. The crown also has fine white streaks.
All species of pygmy-owls are superficially similar in appearance, and often are distinguished most easily by voice (see Vocalizations), elevational preferences, and habitat. Due to its wide geographic range, the Ferruginous Pygmy-Owl overlaps with many other species of pygmy-owl. Ferruginous Pygmy-Owl can be distinguished from all members of the Least Pygmy-Owl group (Tamaulipas Pygmy-Owl G. sanchezi; Colima Pygmy-Owl G. palmarum; Central American Pygmy-Owl G. griseiceps; Subtropical Pygmy-Owl G. parkeri; Amazonian Pygmy-Owl G. hardyi; Pernambuco Pygmy-Owl G. mooreorum; and Least Pygmy-Owl G. minutissimum) by its larger size, longer tail with a greater number of transverse pale buffy bars (5-7 bars on the upper surface of the tail, or 3-5 bars on the under surface of the tail, as opposed to 2-4 whitish bars on the tails of species in the Least Pygmy-Owl group), and by the presence of narrow pale streaks (not small pale spots) on the crown.
Ferruginous Pygmy-Owl also is similar to members of the Northern Pygmy-Owl group (Northern Pygmy-Owl G. gnoma; Costa Rican Pygmy-Owl G. costaricanum; and Cloud-forest Pygmy-Owl G. nubicola), members of which are similar in size and shape to Ferruginous Pygmy-Owl, and may have a similar number of pale bars on the tail. As is the case with many members of the Least Pgymy-Owl group, all species in the Northern Pygmy-Owl group typically occur at higher elevations than Ferruginous Pygmy-Owl. These species also have small pale spots on the crown, not the narrow pale crown streaks of Ferruginous.
In the Andes, Ferruginous Pygmy-Owl also overlaps geographically with Andean Pygmy-Owl (G. jardinii) and Yungas Pygmy-Owl (G. bolivianum), both of which also are similar in size and shape to Ferruginous, and both of which have a rufous morph. Both species typically occur at higher elevations than Ferruginous Pygmy-Owl, and in more humid and more closed forest. Adults of both species have small pale spots on the crown, but the crown of juveniles may be streaked, thus replicating the crown pattern of Ferruginous Pygmy-Owl. These juveniles are best distinguished from Ferruginous by voice, habitat, and elevation.
In southern South America, Ferruginous Pygmy-Owl is parapatric with Austral Pygmy-Owl (G. nana), which is very similar to (and perhaps only a subspecies of) Ferruginous. Austral Pygmy-Owl does not have a rufous morph, and the tail bars are pale rufous, not white.
Ferruginous Pygmy-Owl is very similar as well to Peruvian Pygmy-Owl (G. peruanum) of the lowlands of western Ecuador and Peru, and to semiarid intermontane Andean valleys. Ferruginous and Peruvian pygmy-owls are exceedingly similar to one another, although brown and gray morphs are much more frequent in Peruvian than in Ferruginous. These two species are allopatric; otherwise they best are distinguished by voice.
Adult: Sexes similar, but occurs in several plumage morphs. In the rufous morph, upperparts generally are cinnamon rufous to rufous. The crown and nape have narrow longitudinal buff streaks. There is a broken white collar across the nape; the tips of the feathers on either side of the midline are tipped black, forming an oval black spot ("false eye"), enclosed by white, on each side of the nape. Tail rufous with 5-7 transverse paler bars. Lores and supercilium whitish. Throat white. Sides of breast rufous, almost meeting across the center. Belly white or whitish, broadly streaked with rufous. In other morphs, the general tone of the upperparts is duller brown or grayish brown; there are whitish or cinnamon buff spots on the scapulars and wing coverts; the outer webs of the primaries are spotted with buff and white; and the bars across the tail are white or whitish.
Juvenile: Similar to the adult, but crown lacks streaks, or streaks are greatly reduced.
Information very limited. In adults in Texas, tail molt occurs between mid July and late August when fledglings have mostly dispersed. Tail molts in Guatemala begins in late September or early August. Molting of primaries probably begins in July and ends in October, but no conclusive data exists. In fledglings in Texas, remiges and retrices achieve adult length by week 8 (Proudfoot and Johnson 2000).
Iris: bright yellow
Bill and cere: light greenish yellow, sides of bill brighter green
Tarsi and toes: honey yellow
Bart parts color data from Wetmore (1968).
Total length: 16-16.5 cm (Ridgely and Greenfield 2001), 16.5 cm (Hilty 2003), 16.5-19 cm (Howell and Webb 1995)
Data from Proudfoot and Johnson (2000); note that female are slightly larger than males.
bill length, males: mean 11.0 mm± 0.8; n = 291
females: mean 11.4 mm ±0.8; n = 186
wing length, males: mean 94.4 mm ± 4.4; n = 304
females: mean 98.7 mm ± 3.7; n = 194
tail length, males: mean 60.6 mm ± 4.0; n = 299
females: mean 63.6 mm ± 3.4; n = 188
Mass: male, mean 66.3 g ± 6.3; n = 20
female. mean 73.0 g ± 7.9; n = 7
In Texas, females may lose 15-20% of their mass during nesting, and males may lose 10-15% (Proudfoot and Johnson 2000).