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Concept
Variable hawk
Summary
The variable hawk (Geranoaetus polyosoma) is a polymorphic species of bird of prey in the family Accipitridae.
It is widespread and often common in open habitats in western and southern South America, including the Falkland Islands. Its taxonomy is disputed, with some splitting it into the widespread red-backed hawk (G. polyosoma) and the Puna hawk or Gurney's hawk (G. poecilochrous) of the central and north Andean highlands, but the differences between the two are unclear. Most recent authorities have supported the lumping together of the two hawks although the issue still is controversial. On the contrary, the rare taxon from the Juan Fernández Islands is relatively distinctive, and possibly worthy of species recognition as the Juan Fernández hawk (B. exsul). In this article and most current accounts, the three races are regarded as subspecies.
The name variable hawk is fully deserved, as both sexes occur in several morphs. Adults of all have a white tail with a contrasting black subterminal band and grey wings barred dark (in flight from below, the remiges appear whitish with fine barring and a broad black tip). The remaining plumage varies from very dark grey to whitish, and some individuals have reddish-brown to the underparts. Females usually have a reddish-brown back, which males usually lack, although at least some males also have this. The taxon exsul from the Juan Fernández Islands is far less variable, being whitish below and grey above in adults of both sexes. At least 27 distinct adult plumages are known in this species, possibly the most of any raptor (although the widespread red-tailed hawk & Eurasian buzzard have also been noted for a widespread but more uniform spectrum of plumage variations) with no relationship to morphometric variables and only minor geographic variation.
Size is also variable in this confusing species. Length can range from 45 to 62 cm (18–25 in) and wingspan ranging from 113 to 151 cm (45–60 in). Weight can range from 800 to at least 1,800 grams (1.8-4 lbs).
Official source
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