Introduction

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 1982).

Gyrfalcons exhibit pronounced sexual dimorphism in size (males weigh only about 65% as much as females) but not in plumage, which ranges from nearly pure white to an almost uniform dark gray-brown, referred to as “black.” Intermediate (“gray”) plumages, with various degrees of barring, spotting, and edging on the feathers, are most commonly seen in North America. Subspecies based primarily on plumage variation are no longer recognized, making the Gyrfalcon a monotypic but highly polymorphic species (Am. Ornithol. Union 1983). There continues to be speculation about the genetic and evolutionary basis of these plumages, but no recent systematic study has been done.

Most Gyrfalcons nest on cliffs above treeline, either in scrapes or in the 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 to changes in the availability of a variety of prey, but especially ptarmigan (Lapogus spp.), their principal food in most areas. Indeed, the breeding distribution of this falcon 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 well understood but may be related to the population cycles of ptarmigan.

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 misidentified or overlooked frequently. Some of these same characteristics, however, have enabled North America’s Gyrfalcons to escape the population declines that other raptors have suffered from persecution, chemical contamination, and habitat degradation.