Abundance and Multi-year Occupancy of Gyrfalcons (Falco rusticolus) on the Seward Peninsula, Alaska.

Abstract

—The purpose of this work is to determine interannual variability in abundance, occupancy, distribution, and nesting success of Gyrfalcons (Falco rusticolus) and other cliff-nesting raptors within 14,150 km2 of the Seward Peninsula in western Alaska. From 2005 through 2010, comprehensive helicopter surveys were used for annual inspections of 679 discrete rock cliffs typically occupied by raptors within the study area. Based on 1,372 cumulative nesting events at 454 unique locations, Gyrfalcons comprised 18% of total raptor occupancy, ranging from 40 – 43 locations annually. Individual Gyrfalcon observations (n = 250) show use of 154 separate cliff locations by single birds (16%; n=39), unsuccessful pairs (11%; n=28), and successful pairs (73%; n=183). Nest types and usage included: cliff stick nests constructed by other raptors and ravens, 73%; rock ledges, 16%; man-made structures, 7%; and tree stick nests, 2%. Mean nearest neighbor distances varied from 8.8 to 10.0 km with distances ranging from 0.8 to 37.6 km indicating that both clustered and dispersed nesting are characteristics of this area, each strongly influenced by local topography, dissected rock cliffs and broad lowland areas with no cliffs. Clustering of closely spaced alternate nesting locations yields 96 unique nesting areas with minimal interannual overlap and maximum distance separation among occupied sites, suggesting that pairs have relatively high rates of movement to ‘new’ nesting areas in successive years. Based on iterative counts of pair occupancy at the same location (n=248 nesting events), the majority were occupied 1 year (n=79 nesting events) before becoming vacant. Longer periods of pair occupancy were observed at 30 locations with 2-year tenure (n=60), 14 locations with 3-year (n=42), 8 locations with 4-year (n=32), 4 locations with 5-year (n=20) and 3 locations with 6-year occupancy (n=18). Since abundance of pairs is stable through time, the high frequency of 1-year nesting events suggests variable site selection and low nest site tenacity among pairs. Gyrfalcon occupancy ranged from 31–39 pairs, yielding a maximum nesting density of 2.7 pairs per 1,000 km2. Successful pairs varied annually from 72–93% of total occupied locations; brood size (n=183 locations with nestlings) ranged from 2.35–2.70 young/successful pair (1.50–2.23 young/total pair). Hatch dates ranged from 7 May to 27 June with low intraor interannual nesting synchrony and little evidence of recycling due to nesting failure. Brood size rates are probably minimum values due to errors associated with finding or counting nestlings from an aerial survey platform. Even if brood rates assessed in June decline as fledging approaches, the observed values remain high enough to explain the stability of numbers and pairs in the Seward Peninsula population. Received 28 February 2011, accepted 22 July 2011. GYRFALCONS, Falco rusticolus, largest of falcons, are characterized by their Arctic distribution, large size, and wide color variation ranging from white to very dark plumages. They have a broad circumpolar nesting distribution and are typically found nesting above the timberline in remote tundra environs punctuated with rock cliff habitats. Interest in this species is keen due to its Arctic associations, remote nesting locations, inaccessibility, predatory skills and relationships, and color variation. Aspects of general biology, distribution, ecology, behavior, and species characteristics in North America are summarized by Clum and Cade (1994) and revised by Booms and Cade (2008). Additionally, Potapov and Sale (2005) present worldwide information and give detailed descriptions for a wide variety of biological and historical topics. Within Alaska, distributional and ecological studies of Gyrfalcons have been completed along the Colville River (Cade 1960), Alaska Range (Bente 1981), and Seward Peninsula (Roseneau 1972, Walker 1977, Kessel 1989). A statewide summary of abundance in Alaska has estimated the population at 375–635 pairs (Swem et al. 1994). Satellite telemetry of juvenile Gyrfalcons from the Seward Peninsula, Alaska, has been reported by McIntyre et al. (1994). Of the known occurrence of Gyrfalcons in Alaska, the Seward Peninsula represents an important area with high densities and improved access allowing for additional study of this species. This work reports a 6-year study of nesting Gyrfalcons and other raptors in a portion of the southern Seward Peninsula in western Alaska. Based on survey observations, estimates are developed for abundance, density, occupancy, nearest neighbor distance, kernel density distribution, nest site tenacity, hatch dates, and brood sizes. Consecutive, multi-year surveys reveal interesting characteristics for this localized, relatively stable Gyrfalcon population.