AppearanceThis is a huge eagle-like "buzzard". It has a total length of 62 to 76 cm and a wingspan of 149 to 200 cm. In the nominate race, males averaged 2.13 kg and females averaged 2.75 kg, with a range between both of 1.67 to 3.17 kg. In the linearly slightly smaller ''G. m. australis'' subspecies, a male weighed 1.7 kg and a female weighed 3.2 kg, indicating the bulk of the two races is roughly the same. It is noted for its bulky, powerful-looking build. It is rather long- and broad-winged and the slightly tapering tail is short by comparison and colored black, with grey tips in fresh plumage. When perched, the wings cover the tail almost completely, giving the bird a unique near-tailess appearance when perched. The adult has a white underside, sometimes with fine blackish stripes; its upperparts are dark grey with a blackish, brownish or bluish hue. The feathers of the neck and the lowest dark feathers of the breast are somewhat elongated. Adults have an ash-grey-and-white zone on the wings, the silvery white seen clearly from afar. The female is distinguished by a reddish-cinnamon hue to the upper- and underwing secondaries and is considerably larger than the male. Among standard measurements, males possess a wing chord of 463 to 555 mm , a tail of 203 to 300 mm , a culmen of 29 to 45 mm and a tarsus of 95 to 102 mm . In comparison, females have a wing chord of 490 to 566 mm , a tail of 220 to 310 mm , a culmen of 30 to 45 mm and a tarsus of 89 to 110 mm .
The immature plumage is reminiscent of that of the great black hawk . Its upperparts are deep brown, sometimes almost black, and it has no light wing patch. The underside is white or light buff with heavy dark streaks on the breast and dark bars on the belly and thighs. It does not acquire the full adult plumage until 4–5 years old.
It is not very vocal, calling usually in flight and when close to the nest. Some calls resemble a wild human laugh, others are a curlew-like whistle. Occasionally flying birds give a high-pitched vocalization "kukukukuku".
The black-chested buzzard-eagle is readily identified in flight by its short wedge-shaped tail scarcely protruding from its long, broad wings. It is usually easy to make out the generally white underparts with the dark chest-band and tail if the birds are adult. Yet as this bird is usually encountered in the wild when it soars, you are less likely to see its grey upperparts.
HabitatThe black-chested buzzard-eagle is found in mountainous or hilly terrain with sparse vegetation, shrubland or ''Nothofagus'' forest, where it spends a lot of time soaring in thermals and vertical drafts while looking for prey. It requires large territories with suitable habitat, the páramos at the north of its range, for example, while providing the latter, fail to provide the former, and thus it has only been recorded in the largest patches of such habitat, such as Páramo de Frontino. Most common between about 5,000 feet and 15,000 feet ASL, it rarely ventures into the lowlands.
It is most conspicuous in the mid-morning and afternoon, when individuals will seek out places which provide the best soaring conditions, such as north and west facing slopes and ridges. Apparently, their main interest at these times is aerial play and display; they tend to ignore places where food is more plentiful or easily hunted in favor of simply soaring alone or in pairs in strong air currents.
The food of this carnivore consists mainly of mid-sized mammals; the introduced European rabbit seems to have become a key prey item. The black-chested buzzard-eagle is helpful to farmers by keeping down the numbers of the rabbits, which can be serious agricultural pests. Among the native fauna degus and hog-nosed skunks are important prey, but mammals as formidable as a gray fox – two to over three times as heavy as the birds and certainly not defenceless – are occasionally hunted and killed by this hawk. Its diet is rounded off with an occasional bird – including carnivorous species like the burrowing owl and sizeable prey such as ''Penelope'' guans or the Chilean tinamou –, large squamates, and if need be also arthropods and carrion. While not aggressive under normal circumstances, the black-chested eagle-buzzard will fiercely attack humans if it considers itself or its offspring threatened. It is near the top of the avian food chain in its range, in part since it is the only Andean eagle found outside forest habitats. It may compete for carrion with the much larger Andean condor.
It nests in high trees or on rocky cliffs, or if these are not available on high trees or even cacti. If no appropriate high place is available this species will nest in bushes or even on the ground. In Ecuador, nesting can be observed all year round; elsewhere it might have a more restricted breeding season but information is scant and somewhat contradictory. The nest is a huge mass of sticks about 85 centimetres in diameter; the black-chested buzzard-eagle is just as likely to reuse an existing nest as to build a new one, and several abandoned nests are often found in the vicinity of an active one. The male and female engage in courtship flights, and copulate over a prolonged time of several weeks as the pairs bond. Little is known of the actual nesting; the clutch contains usually but sometimes 1 or 3 eggs, which are incubated for about a month. The nestlings presumably are covered in white down like in its relatives.
Owing the wide overall range ''Geranoaetus melanoleucus'' is considered a Species of Least Concern by the IUCN. While it is rare and declining in places – e.g. in Rio Grande do Sul and Santa Catarina states in Brazil, or in parts of Argentina – its habitat requirements mean that it will to some degree benefit from deforestation and it has for example colonized regions of the former Mata Atlântica forest in Alagoas. The declines in Argentina have been attributed to poisoning by strychnine baits deployed by sheep farmers trying to eradicate pests.
EvolutionSome fossils have been placed in ''Geranoaetus'', but those from North America have since been moved elsewhere:
⤷ ''"Geranoaetus" ales'', ''"G." contortus'' and ''"G." conterminus'' are now in ''Buteo''.
⤷ ''"Geranoaetus" fragilis'' and ''"G." milleri'' are now in ''Buteogallus''. The type specimens of the latter were at first erroneously believed to be of the black-chested eagle-buzzard.
⤷ ''"Geranoaetus" grinnelli'' is now in ''Spizaetus''.
⤷ ''"Geranoaetus" dananus'', originally described as ''"Aquila" danana'', is of rather unclear affiliations but probably belongs to the same lineage as the above.
Bones indistinguishable from those of living black-chested buzzard-eagles were found in a spring deposit at the Baños de Ciego Montero in Cienfuegos Province, Cuba. A partial left carpometacarpus – Specimen AMNH FR 6190 – as well as a fingerbone probably date from some time in the Pleistocene, during the last ice age. Its contemporary close relatives on Cuba, as far as it is known, consisted of the gigantic eagle-like buteonine hawks which were clearly distinct by size alone, while the Pleistocene record of similar-sized birds from continental North America is from the far west.
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