Black-and-chestnut Eagle Spizaetus isidori Scientific name definitions

Tomás Rivas-Fuenzalida, Juan M. Grande, Sebastián Kohn, Felix Hernán Vargas, and Santiago Zuluaga Castañeda
Version: 2.0 — Published November 23, 2022

Plumages, Molts, and Structure


Black-and-chestnut Eagles have 10 full-length primaries (numbered distally, p1 to p10), 13 secondaries (numbered proximally, s1 to s10, and including 3 tertials, numbered distally, t1 to t3), and 12 rectrices (numbered distally, r1 to r6, on each side of the tail). Little or no geographic variation in appearance reported (see Systematics). See Molts for molt and plumage terminology. The following is based primarily on detailed plumage descriptions in Blake (1), Thiollay (2), and Ferguson-Lees and Christie (3); see Pyle (4) for age/sex-related criteria in other large raptors. Sexes are similar at all ages. Definitive plumage assumed at Third Basic Plumage or Fourth Basic Plumage.

Natal Down

Composed of a thick white down.

Juvenile (First Basic) Plumage

Darker above; upperwing coverts gray-brownish with white edges; scapulars and mantle feathers similarly colored, but may be paler overall, with dark centers. Flight feathers are white with profuse blackish barring, the remiges and rectrices grayish to sooty barred black, and the tail with a subterminal band that is wider than the other bands on the tail. Rectrices are narrower and more rounded than those of later generations. Remiges are uniform in wear and not showing molt clines or contrast between the inner primary and outer secondary (see Definitive Basic Plumage, below). The underparts are largely white, with some browner stains on the neck, chest sides, and wing coverts.

Formative Plumage

Similar to Juvenile Plumage but some to many body feathers replaced with formative feathers (see Molts). Formative back feathers more blackish; formative head and underpart feathers buff, some with black streaks. The sides of the neck, chest, belly, and tarsi have light chestnut coloration (with some blackish feathers); juvenile feathers are retained on the throat, ventral part of the neck, chest, and belly. A long black-feathered mustache extends from the corner of the bill. Barred juvenile remiges and rectrices are retained and become increasingly worn.

Second and Third Basic Plumages

Similar to Definitive Basic Plumage but with a white throat with thick black middle and side bands, mottled black and brown head, and lighter brown on the supercilium and cheeks. Some juvenile white feathers on the center of the chest and belly, and among underwing coverts, can be retained through Second Basic Plumage. Pale chestnut belly feathers are finely edged with white. Replacement of all juvenile remiges and rectrices can take three years, as in other large raptors. Barred juvenile outer primaries and secondaries among s3–s4 and s7–s10 are the last feathers replaced. Barred rectrices among r2 and r5 may be the last replaced. Third Basic Plumage resembles Definitive Basic Plumage more but underparts often streaked white, juvenile primaries (p10) and secondaires (s4, s8–s9) can be retained through two years.

Definitive Basic Plumage

Head black with a large black crest; back and upperparts black. Rectrices are broad and truncate at the tips, gray with wide black tips forming terminal tail band. The underside of the rectrices are light-gray to gray at the base with a broad, black terminal band. The upperside of the primaries are largely white to pale brown with black tips, creating a bold pale patch; the upperside of the secondaries are dark chestnut to blackish with a broad terminal black band; in addition, the primaries have two thin blackish bands across the middle of the feather, while secondaries have two thicker black bands across the middle of the feather. Underparts are largely chestnut in color with small black longitudinal streaks; underwing coverts and axillaries also largely chestnut in color with narrow black streaks, the greater primary and secondary underwing coverts black with a bold white band near the base. The femoral feathers along the upper side of the tarsi are black, while those of the lower portion of the tarsi are chestnut.

Following complete molts, primaries and secondaries can show "molt clines" from more-worn p1 to fresher p10, more worn s1 to fresher s4, and more worn s5 and tertials to fresher feathers among s8–s9 (5, 4). The outer secondary (s1) can also be replaced much later than the inner primary (p1), showing a contrast in freshness between these two feathers. Complete molts more often occur in large raptors, including Black-and-chestnut Eagle, during the Second Prebasic Molt, in birds that are not breeding. Following prebasic molts in older birds, molts are more often incomplete and remiges can show 2–4 sets of feathers in Staffelmauser patterns, the number of sets signifying minimum age (cf. 5, 6, 4). Rectrices can also show up to two generations of basic feathers following incomplete prebasic molts.


Molt and plumage terminology follows Humphrey and Parkes (7), as modified by Howell et al. (8). Black-and-chestnut Eagle exhibits a Modified Basic Strategy (c.f. 8, 9), including incomplete-to-complete prebasic molts and a limited-to-partial preformative molt (see 10), but no prealternate molts. The following is based on examination of Macaulay images.

Prejuvenile (First Prebasic) Molt

Complete, in the nest. Little information on the development of Juvenile Plumage.

Preformative Molt

Limited to partial, probably occurs from 2 to 8 months following fledging. Includes some to most, scattered body feathers but no rectrices or wing feathers. This is now considered a Preformative Molt in Accipitridae (10).

Second and Third Prebasic Molts

Incomplete to complete. Similar in replacement sequence to Definitive Prebasic Molt. It appears that the 3–6 juvenile outer primaries (among p4–p10), 4–7 juvenile secondaries (among s2–s4 and s6–s10), and 2–6 juvenile rectrices (among r2–r5) can be retained following the Second Prebasic Molt, and 1–2 juvenile primaries (among p1–p2) and 1–4 juvenile secondaries (among s3–s4 and s7–s9) can be retained during the third prebasic molt, as occurs in other large Accipitrid raptors (4), to set up Staffelmauser molting patterns. Juvenile upperwing coverts, underwing coverts, and body feathers (especially on the rump and center of the underparts) can be retained during the Second Prebasic Molt. Complete replacement of remiges and rectrices may occur more often during the Second Prebasic Molt in non breeding birds due to the lack of energetic costs associated with breeding.

Definitive Prebasic Molt

Incomplete, possibly to complete in some individuals. Primaries are replaced distally (p1 to p10), secondaries replaced proximally from s1 and s5 and distally from the tertials (often bilaterally from t2), and rectrices usually replaced in sequence r1–r3–r4–r6–r2–r5 on each side of tail, with some variation in this sequence possible. Staffelmauser molting patterns appear to occur in most individuals following incomplete molts, whereby remiges molt in sequence resuming from the point in which molt arrested the previous season and also commencing with new series of molt, resulting in up to three waves of replacement within both primary and secondary tracts (5, 6, 4). See images under Plumages. Staffelmauser occurs when large birds lack the time to complete flight-feather molt in a single year and it has the ultimate benefit of preventing larger gaps in the wing enabling flight and foraging during peak molting periods (5).

Bare Parts

Bill and Gape

Maxilla and tip of mandible are black, while the rest is bluish gray.

Iris and Facial Skin

Iris is deep yellow to orangish yellow in adults; it is green-gray in juveniles. The cere is pale yellow.

Tarsi and Toes

Toes are yellow. Claws are usually black, but are sometimes paler.


The male is 79% of the size of the female (3). All measurements presented are of a wild adult female captured and measured in northwestern Argentina by Tomás Rivas-Fuenzalida, Santiago Zuluaga, and Américo Vilte in 2019, and captive juvenile males (mass) from Ecuador, unless otherwise mentioned.

Linear Measurements

Total Length

60–80 cm (3); 77 cm.


144–166 cm (3); 172 cm.

Wing Chord

54.47 cm

Tail Length

42.4 cm

Tarsus Length

13.5 cm

Tarsus Width

1.97 cm

Toe III Length

11.9 cm

Hallux Claw Length

5.3 cm


Male, 2,500–2,700 g; female, 3,500 g.

Recommended Citation

Rivas-Fuenzalida, T., J. M. Grande, S. Kohn, F. H. Vargas, and S. Zuluaga Castañeda (2022). Black-and-chestnut Eagle (Spizaetus isidori), version 2.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.baceag2.02