The Olivaceous Woodcreeper is a small, distinctive woodcreeper with a slim body and straight, warbler-like bill that is relatively wide basally. The plumage lacks streaks and spots. At any age, color does not differ between the sexes, but females average smaller in all dimensions (see Measurements).
The combination of small size—approached only by the even smaller Wedge-billed Woodcreeper (Glyphorynchus spirurus) and the slightly larger Lesser Woodcreeper (Xiphorhynchus fuscus)—and unmarked plumage is unique among woodcreepers and, indeed, in the whole of the broad family Furnariidae. This species is a small, distinctive woodcreeper with a slim body and straight, warbler-like bill that is relatively wide basally. Unlike nearly all other species of woodcreepers, the adult lacks streaks or spots of any kind. Instead, the uniform washes of color cover the underparts and upperparts, with the latter generally darker in tone than the former.
All populations share a single, simple plumage pattern, although the coloration of the plumage may differ; see Geographic Variation.
Adult: Generally the forehead, crown, auriculars, and nape are olive, olive-yellow, or brown, with the lores and supercilia slightly paler than the crown and auriculars and with a thin, complete white or whitish eyering. In some populations the mantle more-or-less is concolorous with the head, but in others it is a contrasting russet-brown or flat brown. The rump, wing feathers (remiges), and tail generally are rufous or chestnut (they are pale tawny in one population), yet the wing coverts are similar but are washed with olive. Except on the outermost primaries, a patch of ochraceous-buff to whitish at the base of the inner web of individual remiges forms a striking band across the spread wing. The chin and throat are grayish white, contrasting somewhat with the breast, belly, and flanks, which tend to be grayish olive, olive-yellow, or buffy olive; the belly is often slightly paler than the breast, whereas the flanks are often slightly richer in color. The undertail coverts are rufous or chestnut, and the underwing coverts and axillaries are ochraceous-buff in most population but may be yellowish or whitish.
Juvenile: Similar to adults but are slightly paler ventrally, more brightly colored dorsally (especially on the rump), and the wing coverts are edged rufous.
There are no comprehensive data on the specific timing or cycle of this species’s molt. Still, as expected for a passerine, the molt cycle does not overlap the breeding cycle (Mallet-Rodrigues 2005). In Middle America and northern South America, active molt has been reported in July and August (Eaton and Edwards 1948, ffrench 1973, Ibáñez-Hernández 2000). Molt appears to be more protracted in Amazonia and the Atlantic Forest, with molt of body (contour) feathers reported in July–September (Oniki and Willis 1999, Piratelli et al. 2000, Oniki and Willis 2001) and December–March (Oniki 1981, Willard et al. 1991, Piratelli et al. 2000, Oniki and Willis 2001) and molt of flight feathers in July (Oniki and Willis 1999), September–March (Novaes 1976, Oniki 1981,
Piratelli et al. 2000, Oniki and Willis 2001).
Plumage may be degraded by chewing lice (Phthiraptera) and feather mites (Analgoidea) , both of which infested ~4% of Olivaceous Woodcreepers in a study in southeastern Brazil (Marini et al. 1996). A newly described species of mite (Hernandes et al. 2007) may be specialized on this woodcreeper.
The maxilla is dark gray to black, whereas the mandible is black to light gray, often with its tip dark (Ridgway 1911, Wetmore 1972, Belton 1984, Willard et al. 1991). Cutting edges to the bill may look paler than the remainder of the bill (Wetmore 1972).
The irides are dark brown to reddish brown (Ridgway 1911, Wetmore 1972, Belton 1984). Under most field conditions, the eyes look simply “dark.”
Tarsi and Feet
The tarsi and feet may be anywhere from gray or bluish-gray to black (Ridgway 1911, Wetmore 1972, Novaes 1976, Belton 1984). As with the eyes and bill, they often appear to be “dark” under most field conditions.
Overall length 13.1–19.3 cm (Taczanowski 1884, Ridgway 1911), with the female averaging slightly smaller (see Mass, below). Size varies geographically (Figure 2), the smallest birds being on the Yucatan Peninsula, the Pacific Coast of South America, and in the Atlantic Forest (male wing chord 70–74 mm, tail 68–78 mm, bill 12–13 mm; Hellmayr 1925), the largest being in the Amazon Basin (male wing chord 76–87 mm, tail 73–82 mm, bill 16.5–18.5 mm; Zimmer 1934).
As with body measurements, body mass (weight) varies between the sexes, with the male (9.0–18.0 g) averaging slightly heavier than the female (8.6–16.0 g). Body mass also varies geographically (Figure 2).