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ABSTRACT.-We studied two breeding populations of Gyrfalcons (Falco rusticolus) approximately 1,200 km apart in Greenland. During 1998-2006, we collected data from a population in the central-west (Kangerlussuaq), and during 1994-2006 and 2008-2010 from a population in the northwest (Thule). Gyrfalcons in Kangerlussuaq bred approximately 18 days earlier than those in Thule, with egg laying, on average, starting on 17 April and 5 May, respectively. Reproduction was similar between areas, with no significant difference between the number of young produced at occupied or successful nests. Years in which the Kangerlussuaq population bred later showed fewer successful nests, while the lay-date advanced significantly for the Thule population during this study. Diet varied between areas, with the more coastal population in Thule feeding primarily on Dovekies (Alle alle) and the more inland Kangerlussuaq population feeding primarily upon Rock Ptarmigan (Lagopus muta) and passerines. Plumage color in Thule was > 99% white, while nearly equal numbers of white and grey adults were observed in Kangerlussuaq. No changes in plumage color were observed in Kangerlussuaq from the 1950s onward; however, an unexplained difference in plumage color between sexes was documented during this study. The Gyrfalcon population in Thule appeared relatively stable, whereas the Kangerlussuaq breeding population was likely cyclical, and possibly declining since the early 1990s. Based on the relatively long-term data sets available, these populations are ideal for continued monitoring to determine what effects future climate change may have on Gyrfalcon populations.

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