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Variable Hawk
Geranoaetus polyosoma

Status: Lower risk

Population Trend: Unknown.

Other Names: Buteo polyosoma, Buteo erythronotus, Buteo exsul, Buteo poecilochrous, Cordilleran Buzzard (poecilochrous), Gurney's Buzzard (poecilochrous), Gurney's Hawk (poecilochrous), Juan Fernández Hawk (exsul), Másafuera Hawk (exsul), Puna Hawk (poecilochrous, Red-backed Hawk (polyosoma).


Geranoaetus polyosoma
click to enlarge
Distribution: Neotropical. Central Andes of COLOMBIA and ECUADOR south through the Andes through PERU, BOLIVIA, and ARGENTINA (Patagonia), CHILE, and TIERRA DEL FUEGO; JUAN FERNÁNDEZ ISLAND; FALKLAND ISLANDS. more....

Subspecies: 3 races. B.p. exsul: Endemic to the Juan Fernández Islands, off Chile; B. p. poecilochrous: High montane Andes from southwestern COLOMBIA and northern ECUADOR south through PERU and southwestern BOLIVIA (La Paz, Cochabamba) to northern CHILE (Tarapacá) and northwestern ARGENTINA (Jujuy); B.p. polyosoma: Andean and Pacific lowlands from northern ECUADOR south to PERU and southwestern BOLIVIA to CHILE and lower elevations of the Andes in ARGENTINA south through Patagonia to TIERRA DEL FUEGO; FALKLAND ISLANDS. more....

Taxonomy: Relationships of this group have been difficult to determine, owing to a myriad of confusing plumage variations, some originally described as different species. Throughout most of the 20th century, authorities recognized two separate species, the "Red-backed Hawk" (B. polyosoma), of the lowlands and foothills of the Pacific side of South America and Patagonia, and the "Puna Hawk" (B. poecilochrous), occurring in montane regions of the Andes. An insular population off Chile, the "Juan Fernández Hawk" (exsul), has generally been regarded as a race of B. polyosoma, but Ferguson-Lees and Christie (2001) treated it as a full species because the sexes are monomorphic, unlike those of mainland B. polyosoma, and they also lack rufous in the plumage. Stresemann and Amadon (1979) suggested that B. poecilochrous might merely be an altitudinal race of B. polyosoma, rather than a full species. An extensive analysis of plumage and mensural characters by Farquhar (1998) led him to recommend treating polyosoma and poecilochrous as a single species, B. polyosoma. Subsequent molecular studies of Riesing et al. (2003) and Farquhar (unpublished) showed little genetic variability within this group, confirming that it represents only a single species, including exsul. The DNA studies of Lerner et al. (2008) indicated that B. polyosoma is in a clade with B. albicaudatus and B. melanoleucus.

Movements: Partial migrant (Bildstein 2006), moving eastward from the Cuyan, Patagonian, and Fueguian Zones northward along the Andes into northern Argentina, Bolivia, and Peru in the non-breeding season (Ortiz and Capllonch 2007). The montane birds formerly separated as B. poecilochrous were regarded as an irruptive or local migrant (Bildstein 2006). Birds breeding on Tierra del Fuego migrate north to the South American mainland in the austral winter (Humphrey et al. 2004), but the Falkland Islands population is sedentary (Woods and Woods 1997). There are movements in some areas in response to the rainfall regime and altitudinal migration also occurs.

Habitat and Habits: Mostly middle and montane elevations, but also in lowlands in some regions, e.g., Santa Elena Peninsula, Ecuador (Ridgely and Greenfield 2001). Inhabits dry open country with scattered trees or shrubs, grasslands, or even agricultural lands. At higher elevations in the Andes, it inhabits rugged montains, montane grasslands, and Polylepis woodland. Soars high and frequently above mountain ridges and valleys, often perching on rock ledges or on the ground. Occurs singly or in pairs. more....

Food and Feeding Behavior: Preys mostly on small mammals (rats, rabbits), but also takes birds and insects. Generally hunts from a perch (Baladrón et al. 2006), but sometimes hunts actively and may hover in the wind like Red-tailed and White-tailed Hawks, suspended by updrafts (Fjeldså and Krabbe (1990), or swoop low over ridges to surprise prey. more....

Breeding: Builds a large, bulky stick nest placed in a tree, large cactus, or on a rock ledge. Nests are refurbished and used in successive years (Woods and Woods 1997). Clutch size is 1-3 eggs, which are greenish-white or white, with variable reddish-brown markings. more....

Conservation: Fairly common or common throughout most of its extensive range. Categorized as a species of "Least Concern" by BirdLife International (2007). more.....

Important References: 
Bierregaard, R.O. 1994. Red-backed Hawk. P. 183 in del Hoyo, J., A.
  Elliott, and J. Saragatal (eds). Handbook of birds of the world. Vol. 2. New
  World vultures to guineafowl. Lynx Edicions, Barcelona, Spain.
Bierregaard, R.O. 1994. Puna Hawk. P. 183 in del Hoyo, J., A. Elliott, and
  J. Sargatal (eds). Handbook of birds of the world. Vol. 2. New World
  vultures to guineafowl. Lynx Edicions, Barcelona, Spain.
Cabot, J. 1991. Distribution and habitat selection of Buteo polyosoma and
  B. poecilochrous in Bolivia and neighbouring countries. Bulletin of the
  British Ornithologists' Club 114:199-209.
Cabot, J., and T. de Vries. 2003. Buteo polyosoma and Buteo poecilochrous
  are two distinct species. Bulletin of the British Ornithologists' Club
  123:190-207.
Cabot, J., T. de Vries, and F.G. Stiles. 2006. Aberrant distributional
  records of Cordilleran Buzzard (Hawk Buteo poecilochrous in Colombia reflect
  confusion with White-tailed Buzzard (Hawk) B. albicaudatus. Bulletin of the
  British Ornithologists' Club 126:65-68.
De Lucca, E.R. 2011. [Observations on the Red-backed Hawk (Buteo
  polyosoma
) in central and southern Argentina]. Nótulas Faunísticas
  (Segunda Serie) 77:1-15. (In Spanish with English summary)
Farquhar, C.C. 1998. Buteo polyosoma and Buteo poecilochrous of South
  America are conspecific. Condor 100:27-43.
Figueroa-Rojas, R.A., E.S. Corales-Stappung, and O.S. Alvarado 2003. Diet
  of the Red-backed Hawk (Buteo polyosoma) in a forested area of the Chilean
  Patagonia and its relation to the abundance of rodent prey. Hornero
  18:43-52.
Jaksic, F.M., S. Silva, P.A. Marquet, and L.C. Contreras. Food habits of
  Gurney's Buzzard in pre-Andean ranges and the high Andean plateau of
  northermost Chile. Journal of Raptor Research 25:116-119.
Jiménez, J.E. 1995. [Natural history of the Red-backed Hawk Buteo
  polyosoma
: a review]. Hornero 14:1-9. (In Spanish)
Jiménez, J.E., and F.M. Jaksic. 1990. Diet of Gurney's Buzzard in the puna
  of northernmost Chile. Wilson Bulletin 102:344-346.
Jiménez, J.E., and F.M. Jaksic. 1991. Behavioral ecology of Red-backed
  Hawks in central Chile. Wilson Bulletin 103:132-137.
more....

Sites of Interest:
South American Classification Committee
Details supporting the recent (27 June 2008) decision by the Committee to merge Buteo poecilochrous into B. polyosoma
Xeno-canto
Vocalizations.
VIREO
Variable Hawk photos.
Aves de Rapina do Brasil
Species account with emphasis on Brazil.

Researchers:
Alvarado Orellana, Sergio Alfredo
Azevedo, Marcos Antônio Guimarães
Barbar, Facundo
De Lucca, Eduardo Raul
Escobar Gimpel, Víctor Mauricio
Jaksic, Fabián
Lambertucci, Sergio
Marin, Manuel
Norambuena, Heraldo V.
Olivo Quiroga, Cristian E.
Pereyra Lobos, Roberto
Raimilla Almonacid, Victor
Salvador Jr, Luiz

Last modified: 6/6/2013

Recommended Citation: Global Raptor Information Network. 2017. Species account: Variable Hawk Geranoaetus polyosoma. Downloaded from http://www.globalraptors.org on 18 Aug. 2017








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