Direct and indirect estimates of Peregrine Falcon population size in northern Eurasia

We used two different methods to estimate the density of nesting Peregrine Falcons (Falco peregrinus) across different parts of northern Eurasia. In t he "territory-density" method, we extrapolated our density estimate of 406 km(2) per territory (95% CI = 295 to 650 km(2) per territory) in a high-den sity area, the Pyasina basin on the Taymyr Peninsula, to other similar area s across the range defined by published estimates. To estimate numbers in l ow-density areas, we used published data that suggested that Peregrine Falc on territories occur every 1,000 km(2). Based on the nesting association be tween Peregrine Falcons and Red-breasted Geese (Branta ruficollis), we used a second, post hoc method to provide a comparative estimate where the rang es of the two species overlay. This model was based primarily on the popula tion ecology of the Red-breasted Goose and included parameters such as the proportion of the goose population nesting with peregrines, the proportion of peregrine pairs associated with geese, goose population size, and three other variables. Some of these variables were already known, whereas others had been estimated as part of another study. The territory-density and nes ting-association methods led to estimates of 1,586 (95% CI = 991 to 2,179) and 2,417 (95% CI = 1,306 to 3,528) falcon territories, respectively, acros s the common range of Peregrine Falcons and Red-breasted Geese; the first m ethod suggested a population of 3,652 falcon territories (95% CI = 2,282 to 5,018) across the entire range E p. calidus. Although both approaches enta iled several major assumptions, together they provide the only quantitative estimate of this remote population of Peregrine Falcons.