Variable Hawk
Encyclopedia
The Variable Hawk is a polymorphic
species of bird of prey
in the Accipitridae
family. 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 (B. polyosoma) and the Puna Hawk or Gurney's Hawk (B. 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 regarding as subspecies.
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.
s, 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 Fernandez race 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 paramo
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).
s comprise more than 90% of prey in some studies. The most commonly recorded prey includes cavies
, tuco-tuco
s, rabbit
s, mice
and paramo rats
. Earthworm
s, weevil
s, orthopterans and other invertebrate
s are often taken. Birds are sometimes taken, including tired petrel
s around Juan Fernández Islands
. Other prey include other rodent
s & lagomorphs, frog
s, lizard
s, snake
s, and fish
. The Variable Hawk hunts in the typical Buteo style: 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.
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
species of bird of prey
Bird of prey
Birds of prey are birds that hunt for food primarily on the wing, using their keen senses, especially vision. They are defined as birds that primarily hunt vertebrates, including other birds. Their talons and beaks tend to be relatively large, powerful and adapted for tearing and/or piercing flesh....
in the Accipitridae
Accipitridae
The Accipitridae, one of the two major families within the order Accipitriformes , are a family of small to large birds with strongly hooked bills and variable morphology based on diet. They feed on a range of prey items from insects to medium-sized mammals, with a number feeding on carrion and a...
family. It is widespread and often common in open habitats in western and southern South America, including the Falkland Islands
Falkland Islands
The Falkland Islands are an archipelago in the South Atlantic Ocean, located about from the coast of mainland South America. The archipelago consists of East Falkland, West Falkland and 776 lesser islands. The capital, Stanley, is on East Falkland...
. Its taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
is disputed, with some splitting it into the widespread Red-backed Hawk (B. polyosoma) and the Puna Hawk or Gurney's Hawk (B. 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
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
from the Juan Fernández Islands
Juan Fernández Islands
The Juan Fernández Islands are a sparsely inhabited island group reliant on tourism and fishing in the South Pacific Ocean, situated about off the coast of Chile, and is composed of three main volcanic islands; Robinson Crusoe Island, Alejandro Selkirk Island and Santa Clara Island, the first...
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 regarding 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 plumagePlumage
Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...
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
Bird of prey
Birds of prey are birds that hunt for food primarily on the wing, using their keen senses, especially vision. They are defined as birds that primarily hunt vertebrates, including other birds. Their talons and beaks tend to be relatively large, powerful and adapted for tearing and/or piercing flesh....
(although the widespread Red-tailed Hawk
Red-tailed Hawk
The Red-tailed Hawk is a bird of prey, one of three species colloquially known in the United States as the "chickenhawk," though it rarely preys on standard sized chickens. It breeds throughout most of North America, from western Alaska and northern Canada to as far south as Panama and the West...
& Eurasian Buzzard have also been noted for a widespread but more uniform spectrum of plumage variations) with no relationship to morphometric
Morphometrics
Morphometrics refers to the quantitative analysis of form, a concept that encompasses size and shape. Morphometric analyses are commonly performed on organisms, and are useful in analyzing their fossil record, the impact of mutations on shape, developmental changes in form, covariances between...
variables 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 Hawk subspecies 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 steppeSteppe
In physical geography, steppe is an ecoregion, in the montane grasslands and shrublands and temperate grasslands, savannas, and shrublands biomes, characterized by grassland plains without trees apart from those near rivers and lakes...
s, agricultural areas and edges of river galleries, beech
Beech
Beech is a genus of ten species of deciduous trees in the family Fagaceae, native to temperate Europe, Asia and North America.-Habit:...
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 Fernandez race 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 paramo
Páramo
The term páramo can refer to a variety of ecosystems. Some ecologists describe the páramo broadly as “all high, tropical, montane vegetation above the continuous timberline”. A more narrow term classifies the páramo according to its regional placement - specifically located in “the northern Andes...
and puna
Puna
Puna may refer to:* Puna grassland, a type of grassland in the central part of the high Andes* Puna , the king of Hiti-marama or of Vavau in the Tuamotu legend of Rata* Puna , a type of wind in the Andes...
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
Polylepis
Polylepis is a genus containing about twenty species of shrubs or trees native to the mid- and high-elevation regions of the tropical Andes. This group is unique in the rose family in that it is predominantly wind-pollinated. They are usually gnarled in shape, but in certain areas some trees are...
woodland at as low 900 m (but rarely below 2,900 m).
Behavior
In all behavioral aspects, the Variable Hawk is a typical Buteo. 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 mammalMammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s comprise more than 90% of prey in some studies. The most commonly recorded prey includes cavies
Cavia
Cavia is a genus in the subfamily Caviinae that contains the rodents commonly known as guinea pigs. The best-known species in this genus is the domestic guinea pig, Cavia porcellus, an important meat animal in South America and a common household pet in the West.-Taxonomic controversy:Cavia are...
, tuco-tuco
Tuco-tuco
The tuco-tucos are members of a group of rodents that belong to the family Ctenomyidae. The tuco-tucos belong to a single genus: Ctenomys, but they include some 60 different species. The relationships among the species are debated by taxonomists. Their closest relatives are degus and other...
s, rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
s, mice
MICE
-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...
and paramo rats
Thomasomys
Thomasomys is a genus of rodent in the family Cricetidae, named after British zoologist Oldfield Thomas. It contains the following species:* Anderson's Oldfield Mouse * Apeco Oldfield Mouse...
. Earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
s, weevil
Weevil
A weevil is any beetle from the Curculionoidea superfamily. They are usually small, less than , and herbivorous. There are over 60,000 species in several families, mostly in the family Curculionidae...
s, orthopterans and other invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s are often taken. Birds are sometimes taken, including tired petrel
Petrel
Petrels are tube-nosed seabirds in the bird order Procellariiformes. The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group...
s around Juan Fernández Islands
Juan Fernández Islands
The Juan Fernández Islands are a sparsely inhabited island group reliant on tourism and fishing in the South Pacific Ocean, situated about off the coast of Chile, and is composed of three main volcanic islands; Robinson Crusoe Island, Alejandro Selkirk Island and Santa Clara Island, the first...
. Other prey include other rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s & lagomorphs, frog
Frog
Frogs are amphibians in the order Anura , formerly referred to as Salientia . Most frogs are characterized by a short body, webbed digits , protruding eyes and the absence of a tail...
s, lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s, snake
Snake
Snakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s, and fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
. The Variable Hawk hunts in the typical Buteo style: 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.