We found an adult specimen of a melanistic Black-chested Buzzard-Eagle (Geranoaetus melanoleucus; also called grey eagle-buzzard and black-chested eagle-buzzard) in the bird collection of The Natural History Museum (NHM), Tring, United Kingdom (BMNH reg. no. 1955.6.N.20.507). It was an adult female and was collected in Ecuador by M. Verreaux, with no date of collection listed on the tags.
This overall dark raptor differed from normal adults of this species by having completely dark underparts (Fig. 1). Note that immatures of this species have dark plumage (Fjeldså and Krabbe 1990, Birds of the high Andes. Zool. Mus. Univ. and Appollo Books, Copenhagen and Sverdborg, Denmark). Leg feathers of the melanistic specimen were similar to those of normal adults except that the base color was a darker gray. Normal adults have whitish spotting on their dark breasts and whitish throats, bellies, and undertail coverts, with narrow dark barring on the latter two in the race australis that occurs in Ecuador. The head and upperparts of this dark specimen are also dark slate or sooty-gray, lacking the whitish spots on the nape and gray tones of normal adults (Fig. 2). Its upperwing coverts are similar to those of normal adults, but are a darker gray with still darker and coarser barring. Its underwing coverts and axillars are sooty-gray with darker coarse barring, compared to white with fine gray barring on normal adults.
Most of the standard bird or raptor references or field guides do not mention any records of melanistic adults nor describe a dark morph (e.g., Brown and Amadon 1968, Eagles, Hawks, and Falcons of the World, Vol. 2., McGraw-Hill, New York, NY U.S.A.; Meyer de Schauensee 1982, A guide to the birds of South America, (Reprinted), Acad. Nat. Sci., Philadelphia, PA U.S.A.; Grossman and Hamlet 1984, Birds of Prey of the World, Bonanza, New York, NY U.S.A.; Hilty and Brown 1986. A guide to the birds of Colombia, Princeton Univ. Press, Princeton, NJ U.S.A.; and del Hoyo et al. [Eds.] 1994, Handbook of the Birds of the World. Vol. 2. Lynx Edicions, Barcelona, Spain). However, Fjeldså and Krabbe (1990) make brief mention of a specimen of a rare dark morph, presumably referring to this bird, and Ferguson-Lees and Christie (2001, Raptors of the World, Christopher Helm, London, United Kingdom) refer to this specimen as a possible rare dark morph.
Gurney (1876, Ibis 1876:66–67) made reference to this specimen when it was part of the Norwich Castle Museum raptor collection. He considered it a previously undescribed, older immature plumage found only in females.
The term “melanistic” (“abnormal melanism”) is most often used for an unusual plumage, as this plumage is usually not passed on to future generations; as distinguished from “dark morph” (“normal melanism”) used when the genetic signal for dark coloration is passed on to offspring (see Clark [1998, Wilson Bull. 110:289–290] for a case of a melanistic Osprey (Pandion haliaetus) in which the dark coloration was apparently not passed on). Nevertheless, the genetic signal for both types of melanism is most likely the same.
Reisling et al. (2003, Mol. Phyl. and Evol. 27:328–342) recently reported on a phylogeny for the genus Buteo based on mtdna analyses that placed Geranoaetus within that genus. We agree with them and with Wetmore (1933, Auk 50:212), who had earlier placed this species in Buteo. It is very similar in shape in flight to two African buzzards, Jackal (B. rufofuscus) and Augur Buzzards (B. augur), showing the same broad-based, long pointed wings and short tails; all three hold their wings in a strong dihedral when soaring. This species had been placed in its own monotypic genus primarily because of its large size (see Amadon [1963, Condor 65:407–409] for other reasons), although some females of the Upland Buzzard (B. hemilasius) are larger than males of this species. Thus, we suggest that B. melanoleucus be called the black-breasted buzzard.
Although we feel that it is unlikely, it is also possible that this specimen is a dark morph rather than a melanistic adult and that there are other dark adults in the breeding population. Observers could have seen similar dark adults in the field but assumed that they were immatures, as some immatures are dark overall.
We thank R. Prys-Jones and M. Adams of the MNH for permission to look at specimens and to publish the photos of this specimen and for assistance during our visits to the collection. R. Prys-Jones, K. Bildstein, and an anonymous reviewer made helpful comments on a previous draft.