SPECIES

Chilean Mockingbird Mimus thenca Scientific name definitions

Natacha González, Vicente Pantoja, Maria Jesus S. Mallea, Matías Garrido, Antoine Touret, Angélica Almónacid, Heraldo V. Norambuena, and Fernando Medrano
Version: 2.0 — Published March 3, 2023

Plumages, Molts, and Structure

Plumages

Chilean Mockingbird has 10 functional primaries (numbered distally, from innermost p1 to outermost p10, the p10 reduced in length), 9 secondaries (numbered proximally, form outermost s1 to innermost s9 and including 3 tertials, s7–s9 in passerines), and 12 rectrices (numbered distally from innermost r1 to outermost r6 on each side of the tail). Little or no geographic variation in plumage occurs (see Systematics). See Molts for molt and plumage terminology. The following is based primarily on detailed plumage descriptions of Blake (3) and Jaramillo (4), along with examination of Macaulay Library images; see Pyle et al. (1, 5) for criteria used for age and sex determinations. Sexes are similar in all plumages; definitive appearance is assumed at the Second Basic Plumage.

Natal Down

Present primarily September–December, in the nest. Hatchlings have dark gray down feathers, mainly on the back and nape (6).

Juvenile (First Basic) Plumage

Present primarily November–March. Similar to Definitive Basic Plumage except browner overall (less gray); underparts with distinct streaks across the breast; juvenile upperwing median and greater coverts tipped buff to golden brown, rather whitish. Juvenile body feathers and wing coverts (especially undertail coverts) filamentous due to lower barb density than feathers of later plumages.

Formative Plumage

Present primarily April (fresh) to January (worn). Similar to Definitive Basic Plumage but differs by presence of juvenile wing and tail feathers. Most reliably distinguished from Definitive Basic Plumage by molt limits between worn juvenile and fresh formative feathers among upperwing and tail (1, 5): 3 to all 10 inner greater coverts replaced, contrasting with browner and more-abraded retained outer coverts; 1–2 tertials sometimes replaced, contrasting with older retained juvenile tertials and outer secondaries; primary coverts duller and browner with pale-buff fringing or tips when fresh that wears off when worn; retained juvenile outer primaries and rectrices thinner, more tapered or rounded at the tips, browner, and relatively more worn, 1–2 central rectrices occasionally replaced and contrastingly fresh. Molt clines among primaries and secondaries lacking (Figure 138 in7). See also iris color under Bare Parts.

Definitive Basic Plumage

Present primarily April (fresh) to January (worn). Crown and upperparts brownish gray to gray with diffuse dusky streaks, the rump browner and with few or no streaks. Rectrices dusky with white tips (except the central pair) becoming broader laterally; outer rectrices also with white edges (outer webs). Sides of head with broad whitish supercilium; a dark brown eyeline separates supercillium from paler graysih and brownish auriculars. Upperwing coverts dark gray to dusky, the median, greater and primary coverts with white tips forming wing bars; remiges blackish with white tips when fresh, especially to inner secondaries and tertials. Throat whitish with with thick brownish-black malar stripes; remainder of underparts whitish to lighter brownish gray, becoming dirty white on the belly,; flanks buffy brown with elongated blackish stripes; undertail coverts grayish with white tips. Proximal and marginal underwing coverts whitish; greater and primary coverts dusky with white tips.

Definitive Basic Plumage separated from Formative Plumage by having wing and tail feathers uniform in quality and freshness: upperwing coverts and tertials uniform in wear; primary coverts duskier, not contrasting in feather quality with replaced greater coverts, and with broad white tips; basic outer primaries and rectrices broader, more truncate, duskier, and relatively fresher (1, 5). Molt clines can also be visible among primaries and secondaries, feathers fresher from s1 to s6 and from p1 to p10, reflecting sequence of previous complete prebasic molt (Figure 138 in7). See also iris color under Bare Parts.

Molts

Molt and plumage terminology follows Humphrey and Parkes (8), as modified by Howell et al. (9). Under this nomenclature, terminology is based on evolution of molts along ancestral lineages of birds from ecdysis (molts) of reptiles, rather than on molts relative to breeding season, location, or time of the year, the latter generally referred to as “life-cycle” molt terminology (10; see also 11). Chilean Mockingbird exhibits a Complex Basic Strategy (cf. 9, 12), including complete prebasic molts and a partial preformative molt but no prealternate molts (1, 5).

Prejuvenile (First Prebasic) Molt

Complete, primarily September-February, in the nest. The first body feathers look as subcutaneous black dots at 2–3 days, emerging at 3–4 days and opening their sheaths at 6–7 days (6). Pins of wing feathers begin to emerge at 3 to 4 days and open their sheaths at 7 to 8 days. Rectrices begin to emerge at 5 to 6 days and open their sheaths at 8 to 9 days. Measurements of the tail are 31 mm at 12 days (26.6% of the adult size). Duration of Prejuvenile Molt among individuals probably ca. 12–15 d; completed (final growth of outer primaries and rectrices) following fledging at 14–19 days.

Preformative Molt

Partial, primarily January–May. Includes most or all body feathers, most to all upperwing proximal median coverts, 3 to all 10 inner greater coverts, sometimes 1–2 inner tertials, and occasionally 1–2 central rectrices, but no other wing or tail feathers (1, 5).

Definitive Prebasic Molt

Complete, primarily February–April. Primaries (and corresponding primary coverts) replaced distally (p1 to p10), secondaries replaced proximally from s1 and proximally and distally from the central tertial (s8) as typical of passerines, and rectrices replaced distally (r1 to r6) on each side of tail, with some variation in sequence possible.

Bare Parts

Bill and Gape

In adults, the bill is black. At hatching, the bill is orange with a yellowish tip. The oral ridges are pale yellow, while the palate and tongue are bright orange (6). Three days after hatching, the culmen turns grayish, and at 10 days, it possesses a blackish tone, except for the cutting edges (tomia), which are of a yellowish gray color, and the oral margins of a pale yellowish-white tone. In juveniles, the mouth lining and gape continue to be yellow for a month or more post fledging.

Bill malformations have been frequently noted in this species (13).

Iris

Iris color typically changes with age, from grayish to dull brownish olive in juveniles, to grayish green during the first year and green-olive or yellowish in adults (1). Other species of mockingbirds can show variation in iris color changes with age, with some adults keeping juvenile-like colors at older ages (7).

Tarsi and Toes

In juveniles and adults, legs and feet are black. At hatching, the legs and feet are orange, the nails are ivory; legs and feet turn grayish at 6–7 days and gray at 9–10 days (6).

Measurements

Linear Measurements

Female (n = 15): Wing chord 108–117 cm, tail 109–120 cm (1).

Male (n = 19): Wing chord 111–124 cm, tail 112–124 cm (1).

Mass

68 ± 2 g (n = 5, 14).

65.97 ± 6.25 g (n = unknown; 15).

It has a kidney mass of 0.92 ± 0.08g (n = 8; 16).

Wing Area, Wing Aspect Ratio, Wing Loading

Information needed.

Recommended Citation

González, N., V. Pantoja, M. J. S. Mallea, M. Garrido, A. Touret, A. Almónacid, H. V. Norambuena, and F. Medrano (2023). Chilean Mockingbird (Mimus thenca), version 2.0. In Birds of the World (N. D. Sly, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.chimoc1.02