What Determines the Population Density and Reproductive Success of Rough-Legged Buzzards, Buteo lagopus, in the Siberian Tundra?

The annual variability of the breeding performance of rodentivorous birds of prey has traditionally been explained in terms of annual fluctuations in rodent abundance. This paper presents data from the Siberian tundra showing that this relationship may not be so simple as generally thought. The data consist of parameters of small mammal population fluctuations (small mammal density (SMD), average body weight, species diversity and rate of change of species diversity), inter-specific competition and meteorological factors (mean temperature and snow melt date) during every stage of the breeding cycle of the rough-legged buzzard (Buteo lagopus). The density of breeding pairs was found to be correlated with the SMD, whereas the total number of territorial pairs including non-breeding ones could be most accurately predicted by a model incorporating both the SMD and the population density of a larger competitor, the snowy owl (Nyctea scandiaca). Clutch size showed no correlation with the SMD. The best-fit multiple regression model to predict clutch size includes only small mammal diversity and its rate of change as predictive factors. Number of nestlings and egg losses did not show any correlation with any of the prey-related or weather parameters. Although the total number of rough-legged buzzard fledglings produced per unit area showed a significant positive correlation with the SMD in a single-factor regression, the most accurate multiple regression model to predict overall reproductive output included only the rate of change of the small mammal species diversity. The number of offspring produced by the buzzards in a given area fluctuated synchronously with the SMD, but the cycle was significantly more consistent in amplitude in the predator as compared to the prey.