Gyrfalcon or gerfalcon

Appearance

The gyrfalcon is a very large falcon, being about the same size as the largest buteos . Males are 48 to 61 cm long, weigh 805 to 1350 g and have a wingspan from 110 to 130 cm . Females are bulkier and larger, at 51 to 65 cm long, 124 to 160 cm wingspan, and of 1180 to 2100 g weight. Among standard measurements, the wing chord is 34.5 to 41 cm , the tail is 19.5 to 29 cm , the culmen is 2 to 2.8 cm and the tarsus is 4.9 to 7.5 cm . The gyrfalcon is larger than the peregrine falcon, which it is known to hunt, and differs from the buzzard in general structure, being unmistakably a falcon with pointed wings, and broader-winged and longer-tailed than the Peregrine.

The gyrfalcon is a very polymorphic species, so its plumage varies greatly. The archetypal morphs are called "white", "silver", "brown", and "black", though they can be coloured on a spectrum that begins with all-white and ends with very dark. The brown form of the gyrfalcon is distinguished from the Peregrine by the cream streaking on the nape and crown and by the absence of a well-defined malar stripe and cap. The black morph has its underside strongly spotted black, rather than finely barred as in the Peregrine. White form gyrfalcons are unmistakable, as they are the only predominantly white falcons. Silver gyrfalcons resemble a light grey lanner falcon of larger size. There is no difference in colouring between males and females; and juveniles are darker and browner than the corresponding adults.

The black color seems to be sex-linked and to occur mostly in females; it proved difficult for breeders to get males darker than the dark side of slate grey. A color variety that arose in captive breeding is "black chick".

Habitat

The gyrfalcon was originally thought to be a bird of tundra and mountains only; however, in June 2011 it was revealed that it spends considerable periods during the winter on sea ice far from land. It feeds only on birds and mammals, the latter of which it takes more regularly than many other ''Falco'' species. Like other hierofalcons, it usually hunts in a horizontal pursuit, rather than with the peregrine's speedy stoop from a height. Most prey is killed on the ground, whether they are captured there or, if the victim is a flying bird, forced to the ground. The diet is to some extent opportunistic, but a majority of breeding birds mostly rely on ''Lagopus'' grouse and avian marine species on coastal habitats. Avian prey can range in size from redpolls to geese and can include gulls, corvids, smaller passerines, waders, and other raptors . Mammalian prey can range in size from shrews to marmots , and often includes lemmings, voles, ground squirrels, and hares. They are rarely observed eating carrion.

Reproduction

The gyrfalcon almost invariably nests on cliff faces. Breeding pairs do not build their own nests, and often use a bare cliff ledge or the abandoned nest of other birds, particularly golden eagles and common ravens. The clutch can range from 1 to 5 eggs, but is usually 2 to 4. The average size of an egg is 58.46×45 mm ; the average weight is 62 g . The incubation period averages 35 days, with the chicks hatching at a weight of around 52 g . The nestlings are brooded usually for 10 to 15 days and leave the nest at 7 to 8 weeks. At 3 to 4 months of age, the immature gyrfalcons become independent of their parents, though they may associate with their siblings through the following winter.

The only natural predator of gyrfalcons are golden eagles and even they rarely engage with these formidable falcons. Gyrfalcons have been recorded as aggressively harassing animals that come near their nests, although common ravens are the only predators known to successfully pick off Gyrfalcon eggs and hatchlings. Even brown bears have been reportedly dive-bombed. Humans, whether accidentally or intentionally , are the leading cause of death for gyrfalcons. Gyrfalcons that survive into adulthood can live up to 20 years of age.

As ''F. rusticolus'' has such a wide range, it is not considered a threatened species by the IUCN. It is not much affected by habitat destruction, but pollution, for instance by pesticides, depressed its numbers in the mid-20th century, and until 1994 it was considered "Near Threatened". Improving environmental standards in developed countries have allowed the birds to make a comeback, and today they are not considered rare or endangered.

Evolution

The gyrfalcon is a member of the hierofalcon complex. In this group, there is ample evidence for hybridisation and incomplete lineage sorting which confounds analyses of DNA sequence data to a massive extent. The radiation of the entire living diversity of hierofalcons took place in approximately the Eemian Stage at the start of the Late Pleistocene. It represents lineages that expanded into the Holarctic and adapted to local conditions; this is in contrast to less northerly populations of northeastern Africa which evolved into the saker falcon. Gyrfalcons hybridize not infrequently with sakers in the Altai Mountains and this gene flow seems to be the origin of the Altai falcon.

There is some correlation between locality and colour morph. Greenland gyrfalcons are lightest, with white plumage flecked with grey on the back and wings being most common. Other subpopulations have varying amounts of the darker morphs: the Icelandic birds tend towards pale, whereas the Eurasian populations are considerably darker and typically incorporate no white birds. Natural separation into regional subspecies is prevented by gyrfalcons' habit of flying long distances whilst exchanging alleles between subpopulations; thus, the allele distributions for the color polymorphism form clines and in darker birds of unknown origin, theoretically any allele combination might be present. For instance, a mating of a pair of captive gyrfalcons is documented to have produced a clutch of 4 young: one white, one silver, one brown, and one black. Molecular work suggests that plumage color is associated with the melanocortin 1 receptor gene , where a nonsynonymous point substitution was perfectly associated with the white/melanic polymorphism.


In general, geographic variation follows Bergmann's Rule for size and the demands of crypsis for plumage coloration. Several subspecies have been named according to perceived differences between populations but none of these are consistent and thus no living subspecies are accepted today. Perhaps the Icelandic population described as ''Falco rusticolus islandus'' is the most distinct. The predominantly white Arctic forms are parapatric and seamlessly grade into the subarctic populations, whereas the birds of Iceland have presumably less gene flow with their neighbors and indeed show less variation in plumage colors and often look quite similar to a large, washed-out peregrine falcon . Comprehensive phylogeographic studies to determine the proper status of the Icelandic population have yet to be determined.

A population genetic study, however, identified the Iceland population as genetically unique relative to other sampled populations in both eastern and western Greenland, Canada, Alaska and Norway. Further, within Greenland, differing levels of gene flow between western and eastern sampling locations was identified with apparent asymmetric dispersal in western Greenland from north to south. This dispersal bias is in agreement with the distribution of plumage colour variants with white gyrfalcons in much higher proportion in north Greenland. Although further work is required to determine the ecological factors contributing to these distributions relative to plumage differences, a study using demographic data suggested that plumage color distribution in Greenland may be influenced by nesting chronology with white individuals and pairs laying eggs earlier in the breeding season and producing more offspring.

A paleosubspecies, ''Falco rusticolus swarthi'', existed during the Late Pleistocene . Fossils found in Little Box Elder Cave , Dark Canyon Cave , and McKittrick, California were initially described as ''Falco swarthi'' on account of their distinct size. They have meanwhile proven to be largely inseparable from those of living gyrfalcons, except for being somewhat larger.

Swarth's gyrfalcon was on the upper end of the present gyrfalcon's size range, strong females even surpassing it. It seems to have had some adaptations to the temperate semiarid climate that predominated in its range during the last ice age. Ecologically more similar to the Siberian populations of today or to the prairie falcon, this population of temperate steppe habitat must have preyed on landbirds and mammals rather than the water—and on the seabirds which make up much of the American gyrfalcon's diet today.