This species account is dedicated in honor of Robert Berry, member of the Cornell Lab of Ornithology's Administrative Board.
Largest of all falcons, and the most northern diurnal raptor, the Gyrfalcon inhabits circumpolar arctic and subarctic regions, with some individuals moving south into northern temperate zones during fall and winter. “Only then do most birdwatchers have a chance for a rare glimpse of this great falcon, which the Emperor Frederick II of Hohenstaufen, in his thirteenth century treatise on falconry (De Arte Venandi cum Avibus), extolled above all others as a hunter of cranes and similar large quarry. The Emperor wrote that the Gyrfalcon ‘holds pride of place over even the Peregrine [Falco peregrinus] in strength, speed, courage, and indifference to stormy weather'” ( Cade 1982b ).
Gyrfalcons exhibit pronounced reversed sexual size dimorphism (on average, adult males weigh 1,100-1,300 g, females 1,700-1,800 g), meaning males typically weigh about 65% as much as females. Gyrfalcon coloration is not conspicuously sexually dimorphic, because the species' coloration is extremely variable and ranges from nearly pure white to an almost uniform dark gray-brown. Intermediate (“gray”) plumages are most commonly seen in North America. The Gyrfalcon is therefore considered a monotypic, but highly variable species ( American Ornithologists' Union 1998a ) and previous subspecies designations based primarily on plumage variation are no longer recognized.
Most Gyrfalcons nest on cliffs above treeline, either in scrapes or in stick nests of other birds. Some individuals do not breed every year; both reproduction and winter movements are strongly influenced by food availability. Gyrfalcons respond functionally, and in some areas numerically, to changes in the availability of a variety of prey, but especially ptarmigan (Lapogus spp.), their principal food in most areas. The Gyrfalcon is a ptarmigan specialist and its breeding distribution is strikingly similar to that of the Rock Ptarmigan (Lagopus muta) ( Holder and Montgomerie 1993 ). Gyrfalcon numbers appear to be cyclic in some regions but not in others, for reasons that are still not fully understood but likely related to ptarmigan population cycles ( Cade et al. 1998 , Nielsen 1999 ).
Although an uncommon species, the Gyrfalcon is not rare, as frequently stated. Remoteness of habitat, fluctuations in breeding populations and in migratory movements, variability in plumage and behavior, and rumors of rarity have all combined to make this species frequently misidentified or overlooked. Some of these same characteristics have enabled North America's Gyrfalcons to thus far escape the population declines that other raptors have suffered from persecution, chemical contamination, and habitat degradation. However, these traits do not protect the species from the potential effects of global warming, which is an emerging conservation concern because of the Gyrfalcon's northern breeding distribution, narrow ecological niche as a specialist predator, and reliance on Arctic habitats and prey.