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—The Yukon-Kuskokwim Delta in western Alaska is dominated by tundra, wetland, and riparian communities in a subarctic landscape characterized by minimal topographic relief. Extensive uplands north of the Yukon River and east of the Kuskokwim River, however, support breeding Gyrfalcons (Falco rusticolus). There are also several isolated mountain ranges surrounded by deltaic lowlands that provide habitat oases for cliff-nesting raptors including Gyrfalcons. A few eyries were documented on the Delta prior to the late 20th century, but we now know of 79 nesting territories within this 130,000 km2 region. Breeding Gyrfalcons occur across the Delta wherever suitable cliff-nesting habitat occurs in, or adjacent to, tundra habitats for foraging. In addition, tree-nesting Gyrfalcons occur regularly at low frequency in at least three geographically separate regions. Since 1987, we have worked in three primary study areas: the Askinuk Mountains, the Ingakslugwat Hills, and the Kilbuck Mountains. Mean nearest neighbor distances between occupied territories were 5.34 km (N = 10 occupied territories), 3.70 km (N = 11), and 5.49 km (N = 13) in those areas, respectively. Densities (in pairs per 1000 km2) in years of highest territory occupancy were 24, 16, and 7, in the same three areas, respectively. A fourth area, the Andreafsky Wilderness, was searched less frequently and less comprehensively, but may support even higher local densities than our primary study areas. At the regional level (N = 35 territories monitored), minimum estimates of annual territory occupancy between 2000 and 2004 ranged from 71% to 80%. Among individual study areas, annual variation in territory occupancy was lower among Gyrfalcons than among co-occurring Rough-legged Hawks (Buteo lagopus) or Golden Eagles (Aquila chrysaetos). Variation in density among study areas was also lower in Gyrfalcons than in the other two species of cliff-nesting raptors. THE COMBINED DELTAS of the Yukon and Kuskokwim rivers cover an area of nearly 130,000 km2 adjacent to the Bering Sea in western Alaska (Thorsteinson et al. 1989). This vast region is dominated by riparian, wetland, and tundra communities in a subarctic landscape characterized by minimal topographic relief. At this regional scale, there is relatively little suitable habitat for nesting Gyrfalcons (Falco rusticolus; Swem et al. 1994, Booms et al. 2010a). Potential Gyrfalcon nesting habitat does occur, however, along the periphery of the Yukon-Kuskokwim Delta (henceforth, the Delta) in the southern Nulato Hills (north and east of the Yukon River) and in the Kilbuck Mountains (east of the Kuskokwim River; Figure 1). In addition, there are several isolated ranges, including the Askinuk Mountains, the Ingakslugwat Hills, Nelson Island, and Nunivak Island, that also provide habitat oases for cliff-nesting raptors. Prior to the late 1970s, very few Gyrfalcon nest sites had been documented on the Delta. Nelson (1887, p. 146) reported that Gyrfalcons were rare breeders on cliffs “along the seacoast in the vicinity of St. Michael” at the northern limit of the Delta, but he did not specify the exact locations. The first confirmed nest in the region was discovered in 1900 at Crater Mountain, 10 km south and inland of St. Michael (McGregor 1902). In 1947 and 1948, H. Kyllingstad and his colleagues spent time in the Askinuk Mountains (Kyllingstad 1948), where they discovered three Gyrfalcon eyries (Gabrielson and Lincoln 1959, Kessel et al. 1964). Cade (1960, Figure 2, p. 159) depicts these four sites (i.e., at Crater Mountain and in the Askinuks), as well as a fifth on Nunivak Island reported by F. Glaser. In the 1960s, Gyrfalcons were reported nesting in the Askinuks in 1963, but not from 1966 through 1969 (Kessel et al. 1964, Holmes and Black 1973). In 1977, the first formal searches for cliff-nesting raptors on the Delta were conducted (White and Boyce 1978). Gyrfalcons were found along the upper East Fork of the Andreafsky River in the southern Nulato Hills, as well as along the Fog, Kisaralik, and Eek rivers in the Kilbuck Mountains (Figures 2, 3).. Between 1978 and 1984, the Kisaralik and Tuluksak rivers were searched intermittently by helicopter and river-rafting (Weir 1982, 192 – MCCAFFERY ET AL. – Gyrfalcon concentrations on the Delta exhibited a wide range of ecologies, varying in breeding landscape, potential and realized diets, the relative abundance of other cliff-nesting raptors, nesting substrate, and reproductive metrics. This variation in ecology within a single subarctic region suggests that a) the Delta may be a natural laboratory for more intensively studying the factors that regulate Gyrfalcon populations, and b) the Delta’s Gyrfalcons may be more resilient to the effects of environmental change than is commonly supposed. Received 18 March 2011, accepted 28 June

The Ecology of Gyrfalcons (Falco rusticolus ) on the Yukon-Kuskokwim Delta, Alaska.