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A BSTRACT.—Gyrfalcons (Falco rusticolus) breeding in the High Arctic Canadian Islands and in northern Greenland have available to them widely scattered nesting sites on cliffs throughout much of this region, but their food resources are limited to four principal species, Rock Ptarmigan (Lagopus muta), Snow Bunting (Plectrophenax nivalis), Collared Lemming (Dicrostonyx torquatus), and Arctic Hare (Lepus arctica), with seabirds, waterfowl, and shorebirds playing a role in some localities. The distribution and abundance of these principal prey are highly variable in time and space, and consequently the serviceability of eyries is unpredictable and largely determined by the abundance of food in a particular year. Most Gyrfalcon populations depend on an abundance of ptarmigan (Lagopus spp.) for their food during the critical period of establishing a pairbond and laying eggs in late winter (mid-April and May in the High Arctic). Ptarmigan are now rather sparse in the High Arctic and apparently seldom build up to numbers that can support Gyrfalcon reproduction without supplementation by other prey. Where the Collared Lemming occurs and reaches “peak” numbers in some years, it apparently serves as a substitute, abetted by whatever ptarmigan are available. Early spring flocks of male Snow Buntings may also help. Whether or not Gyrfalcons can exploit adult hares effectively at this time remains a question that needs study. At the time female gyrs are incubating eggs in late May and June, the leverets of Arctic Hare become available, and Gyrfalcons utilize them heavily where they are common. Lemmings, if they survive into the summer, and leverets can support falcon reproduction through the nestling period into August. During migration in late August and on into September and October, the juvenile falcons feed heavily on the large flocks of young Snow Buntings. In special situations, large breeding colonies of Dovekies (Alle alle) provide the bulk of food during the nestling period of the falcons. The principal herbivores included in these trophic relationships—ptarmigan, lemming, and hare—fluctuate drastically in numbers from year to year and may have “cyclic” peaks ranging in interval from four to 11 years depending on species and locale. The correspondence of peak years in two or more of these species may strongly influence successful reproduction by Gyrfalcons and may account for the spectacular fall movements of falcons formerly reported in NE Greenland. Evidence is growing that the amplitude of these “cycles” has been decreasing and their periodicity increasing or breaking down under the influence of climate change. Gyrfalcon populations might well decline under these circumstances. The strong dependence of the High Arctic gyrs on maritime and marine birds for their winter food could also be jeopardized if the arctic sea

Seasonal Changes in Diet of Gyrfalcons Nesting at Ellesmere Island and other High Arctic Locations.