Buteo fuscescens

Black-chested Eagle-buzzard

The Black-chested Eagle-buzzard (Geranoaetus melanoleucus) is a bird of prey of the hawk and eagle family (Accipitridae). It lives in South America. This species is also known as the Black Buzzard-eagle, Grey Buzzard-eagle or analogously with "eagle" or "eagle-buzzard" replacing "buzzard-eagle", or as the Chilean Blue Eagle. It is sometimes placed in the genus Buteo..

Description

With a total length of c.25-30 in (about 62-80 cm) and weighing around 70 oz (2 kg), it is a huge eagle-like "buzzard" ("hawk" in American terminology). It is rather long- and broad-winged, with a wingspan of about 70-80 in (175-200 cm), and the slightly tapering tail is short by comparison and colored black, with grey tips in fresh plumage. 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..

The immature plumage is reminiscent of that of the Great Black-hawk (Buteogallus urubitinga). Its upperparts are deep brown, sometimes almost black, and it has no ligth 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.

The Black-chested Eagle-buzzard 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. But as this bird is usually encountered in the wild when it soars, you are less likely to see its grey upperparts.

Taxonomy and systematics

Its scientific name is Latinized Ancient Greek and means "black-and-white crane-eagle" or "black-and-white buzzard": Geranoaetus comes from Ancient Greek géranos (γέρᾶνoς), "crane" + aetós (ἆετός), "eagle". The "crane" reference is due to its grey upper wings and its loud cries. The alternative genus name Buteo is simply the Latin term used for these hawks in Ancient Rome. melanoleucus is from Ancient Greek mélan- (μέλαν-), "black-" + leukós (λευκός), "white". This refers to the contrasting coloration when seen from below.

When the Black-chested Eagle-buzzard was first described by Louis Jean Pierre Vieillot in 1819, it was placed in the genus Spizaetus, as Spizaetus melanoleucus. Today however, the monotypic genus Spizastur is merged in Spizaetus, and the Black-and-white Hawk-eagle, originally described as by Vieillot three years earlier as Buteo melanoleucus, is now known as Spizaetus melanoleucus. The earlier use of the specific name melanoleucus for the Black-and-white Hawk-eagle precludes its use for the Black-chested Eagle-buzzard, except when it is placed in Geranoaetus. The correct name to use when placing this species in Buteo would be Buteo fuscescens. This specific name was established – as Spizaetus fuscescens – by Vieillot for the immature of the Black-chested Eagle-buzzard at the very same time as he described the adult, because he could not believe that such differently-colored birds were conspecific. In fact, in the mid-20th century Buteo fuscescens was the prevailing name for the Black-chested Eagle-buzzard for some years, but it was eventually dismissed as erroneous – which ironically was an error in itself. Unfortunately, the researchers who presented new data advocating a merger of Geranoaetus into Buteo were unaware of the convoluted taxonomic history of Buteo melanoleucus and formally recommended using this name for the Black-chested Eagle-buzzard. It has since already been adopted by other scientists.

Systematics

There are two subspecies:

Larger, Plain white below.

Smaller. White with fine dark barring below.

The Black-chested Eagle-buzzard is allied to the Buteo hawks, and it is sometimes included with these. Other authors place it in the monotypic genus Geranoaetus. Though the former seems to be more appropriate from a phylogenetic standpoint, the latter is still used here, as much more research into phylogeny and hybridization has to take place before the correct taxonomy of the buteonines can be resolved. It stands to note that the taxonomic and systematic dispute goes back to the early-mid 20th century already. But as it seems, there is no real reason to suppose that the lineage of the Black-chested Eagle-buzzard is North American in origin; fossils that might have been its ancestors at first sight differ in details and are more likely to belong to other buteonine lineages.

This species could be close to the White-tailed Hawk (Buteo albicaudatus), and perhaps to the Grey-backed (Leucopternis occidentalis), White (L. albicollis) and Mantled Hawks (L. polionotus) which it resembles in habitus except for being larger. But its closest living relatives may well be the Red-backed (B. polyosoma) and Puna Hawks (B. poecilochrous). Particularly some populations of the former look like small Black-chested Buzzard-eagles. The Barred Hawk (L. princeps) looks similar to the Black-chested Eagle-buzzard in general color pattern, though the tail differs much in shape, size, and the bright white central band stands out.

But the relationship of the Black-chested Eagle-buzzard to the prehistoric genera Titanohierax from the Caribbean and the pan-American Amplibuteo also warrant more study. The crab-hawks (Buteogallus) and the solitary "eagles" (Harpyhaliaetus) seem to be allied with the latter, to the extent that all these three genera might be united in Buteogallus. That genus in the present restricted sense contains species also quite similar in habitus and size to the Black-chested Eagle-buzzard.

Fossil record

Some 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 (Fragile "Eagle") and "G." milleri (Miller's "Eagle") 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 (Grinnell's "Eagle") 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 Eagle-buzzards 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 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.

Ecology

The Black-chested Eagle-buzzards is found in mountainous or hilly terrain with sparse vegetation, shrubland or (in the south of its range) Nothofagus forest, where it spends a lot of time soaring in thermals and vertical drafts while looking for prey. Most common between about and 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 (Oryctolagus cuniculus) seems to have become a key prey item. The Black-chested Eagle-buzzard is helpful to farmers by keeping down the numbers of the rabbits, which can be serious agricultural pests. Among the native fauna degus (Octodon) and hog-nosed skunks (Conepatus) are important prey, but mammals as formidable as a Grey Fox (Urocyon cinereoargenteus) – 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 (Athene cunicularia) and sizeable prey such as Penelope guans or the Chilean Tinamou (Nothoprocta perdicaria) –, 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 nests in high trees or on rocky cliffs, or if these are not available on high trees or even cacti, though 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 in diameter; the Black-chested Eagle-buzzard is just as likely to build new nests as to build new ones, 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 fgor about a month. The nestlings presumably are covered in white down like in its relatives.

Due to its wide overall range it 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.

Footnotes

References

  • (1963): Comparison of Fossil and Recent Species: Some Difficulties. Condor 65: 407-409. DjVu fulltext PDF fulltext
  • (2007): AMNH FR 6190 specimen record Retrieved 2008-SEP-24.
  • (2006): Novos registros de aves raras em Santa Catarina, Sul do Brasil, incluindo os primeiros registros documentados de algumas espécies para o Estado [New records of rare birds, and fi rst reports of some species, in the state of Santa Catarina, southern Brazil]. Revista Brasileira de Ornitologia 14(2): 145-149 [Portuguese with English abstract]. PDF fulltext Electronic supplement
  • (2007): Forty-eighth Supplement to the American Ornithologists’ Union Check-List of North American Birds. Auk 124(3): 1109-1115. DOI:10.1642/0004-8038(2007)124[1109:FSTTAO]2.0.CO;2 PDF fulltext
  • (1994a): 170. Black-chested Buzzard-eagle. In: : Handbook of Birds of the World (Vol.2: New World Vultures to Guineafowl): 175, plate 16. Lynx Edicions, Barcelona. ISBN 84-87334-15-6
  • (1994b): 224. Black-and-white Hawk-eagle. In: : Handbook of Birds of the World (Vol.2: New World Vultures to Guineafowl): 201, plate 21. Lynx Edicions, Barcelona. ISBN 84-87334-15-6
  • (1942): An Oligocene hawk from Colorado. Condor 44: 39-40. DjVu fulltext PDF fulltext
  • (2003): Black-chested Eagle-buzzard Retrieved 2008-SEP-23.
  • (2004): The Identity of the Fossil Raptor of the Genus Amplibuteo (Aves: Accipitridae) from the Quaternary of Cuba. Caribb. J. Sci. 40(1): 120-125. PDF fulltext
  • (1928): Bones of birds from the Ciego Montero deposit of Cuba. American Museum Novitates 301: 1-5. PDF fulltext
  • (1933): Status of the Genus Geranoaëtus. Auk 50(2): 212. DjVu fulltext PDF fulltext

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