Nombre en español: Gavilán Variable

Nombre en ingles: Variable Hawk

Nombre científico: Geranoaetus polyosoma

Familia: Accipitridae

Foto: Nick Athanas/Fabian Barrueto

Canto: John V. Moore

El busardo dorsirrojo, aguilucho común, águila parda, águila de pecho blanco, ñanco o pihuel(Geranoaetus polyosoma), es una especie de ave accipitriforme de la familia Accipitridae.

Distribución y hábitat

Su área de distribución cubre desde Colombia y Ecuador por el norte, a través de Perú, Bolivia, Paraguay y Uruguay, gran parte de Argentina y todo Chile, llegando hasta el extremo austral del continente, así como en islas oceánicas próximas al continente sudamericano, como las islas Malvinas y el archipiélago Juan Fernández. Accidentalmente también se ha reportado en Brasil. Vive en diversas elevaciones, incluso altiplánicas, aunque es más frecuente en las zonas costeras y en zonas bajas de la Cordillera de los Andes. Prefiere ambientes no selváticos, bosques abiertos, arbustales y estepas.

Descripción

De tamaño aproximado a 50 cm, con las partes inferiores claras y las superiores de color gris. La hembra se distingue por tener los hombros y parte del dorso de color canela, aunque a veces se ve ese color en los inmaduros. Se le distingue por la cola blanca con rayas delgadas negras horizontales y una banda negra subterminal en la cola.

Pasa por varias fases en su desarrollo variando en los colores entre grises, rufos y marrones.

Historia natural

Es un muy buen cazador, vive cerca a matorrales, bosques, accidentes de altura y llanuras arboladas.

Se alimenta de pequeños animales mamíferos, reptiles, aves pequeñas, invertebrados y anfibios no mayores que un conejo. Para cazar vuela circularmente sobre su objetivo y se lanza en diagonal sobre la misma para golpearla con sus garras.

Al volar despliega las alas en toda su longitud, similar a los gallinazos.

Ilustración de un Geranoaetus polyosoma poecilochrous en Figuras i descripciones de aves chilenas, Anales del Museo Nacional de Chile, 1902

Variable hawk

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.^[2][3] 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.

Description

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 morphometricvariables and only minor geographic variation.

Size

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). The Puna hawksubspecies generally considered to be at the larger end of the size spectrum and the red-backed hawk at the smaller (Juan Hernandez hawk being intermediate), but these distinctions are difficult at best in the field. Overall, however, this species rates as a large Buteo.

Habitat

Variable hawks occupy open habitats at all elevations. The red-backed race inhabits the widest range of areas of the variable hawk races, including above tree line in mountains, Pacific coastal foothills, Patagonian steppes, agricultural areas and edges of river galleries, beech woods and humid premontane and lowland forests. Though often the most likely race to be found in lowlands, even the red-backed is less than common below an elevation of 500 m (up to 3,000 m). The Juan Fernandezrace is found on the islands’ volcanic slopes and barren grazed grasslands at all elevations. The Puna race are often a common element above tree line in páramo and puna habitat, at higher elevations (5,000+ m) than almost any other raptor. Smaller numbers of the latter race may visit mountain scrub and stunted Polylepis woodland at as low 900 m (but rarely below 2,900 m).

Behavior

They are most often seen soaring on warm thermals but may be seen on almost any type of raised perch (from sign posts to large trees). They prey on almost any small to medium-sized animals that can be caught, but smallish mammals comprise more than 90% of prey in some studies. The most commonly recorded prey includes cavies, tuco-tucos, rabbits, mice and páramo rats. Earthworms, weevils, orthopterans and other invertebrates are often taken. Birds are sometimes taken, including tired petrels around Juan Fernández Islands. Other prey include other rodents & lagomorphs, frogs, lizards, snakes, and fish. The variable hawk hunts with prey being spotted while soaring from the air and pinned on the ground.

Breeding is at various seasons and may be variable for all races. They build large stick nests on any elevated structure available, and sometimes breed cooperatively. One to three eggs are laid. The incubation period is 26 to 36 days. The nestlings fledge anywhere from 40 to 74 days. The larger-bodied, high-elevation hawks take longer to incubate and much longer to fledge than lower elevation hawks.