Gyrfalcon - Systematics and Evolution

Systematics and Evolution

The Gyrfalcon is a member of the hierofalcon complex. In this group, there is ample evidence for rampant 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 (where the radiation probably originated) 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.

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 (although their habitus is different). Comprehensive phylogeographic studies to determine the proper status of the Icelandic population have yet to be determined.

A recent molecular 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 northern Greenland. Further work is required to determine the ecological factors contributing to these distributions relative to plumage differences, and whether renewed subspecies designations are warranted.

A paleosubspecies, Falco rusticolus swarthi, existed during the Late Pleistocene (125,000 to 13,000 years ago). Fossils found in Little Box Elder Cave (Converse County, Wyoming), Dark Canyon Cave (Eddy County, New Mexico), and McKittrick, California were initially described as Falco swarthi ("Swarth Falcon" or more properly "Swarth's Gyrfalcon") 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 (Miller 1935). 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 (which are generally composed of smaller birds) 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.