| Summary: | Peregrines Falco peregrinus and Gyrfalcons F. rusticolus were studied in Low and High Arctic Greenland, comparing populations within and between areas. Analyses of weather data from northwest Greenland (1979–2005) revealed a general warming trend, resulting in a lengthened breeding window for many bird species. Both falcon species depend on cliffs for nesting, and take a similar range of bird species as prey. However, Gyrfalcons lay six weeks earlier than Peregrines in Kangerlussuaq and one month earlier than Peregrines in Thule, and occupy more sheltered nest-sites. Being larger than Peregrines, Gyrfalcons also take some larger prey species. In addition, both species move to lower latitudes for the winter, but while most Gyrfalcons migrate relatively short distances, Peregrines are complete long-distance migrants to Central and South America. One Peregrine, satellite-tracked from its breeding site at 76.5° N, travelled ~12,500 km and >100° in latitude, one of the longest migrations ever documented for a raptor.Around Thule in northwest Greenland (at 76.5° N), scientists had historically documented no breeding Peregrines, but six occupied sites were discovered during this study, comprising what is probably the most northern nesting population in the world. Over the same period, breeding Gyrfalcons have apparently disappeared from southern Greenland, and may have declined in central-west Greenland (67° N). The ultimate cause of this retraction may be climate warming, but the proximate cause is probably competition from an increasing Peregrine population.Gyrfalcons tagged with satellite-received transmitters showed characteristics associated with both obligate and facultative migration. Their winter ranges varied greatly in size, with the largest, ~172,000 km2, being the biggest ever documented for a raptor. Many individuals made long movements within a winter, and some spent up to a month at sea. They may have rested on ice and fed upon seabirds. Carbon dating of stratified faecal accumulation from Gyrfalcon ...
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