Responses of an avian predator to variations in prey density at a temperate latitude

Fluctuating prey populations and their effects on avian predator population dynamics have been studied particularly at high latitudes, where prey popu lations, especially microtines, are known to be cyclic; raptors show both n umerical and functional responses to variations in their prey. In this pape r, we investigate the response of a migratory raptor (Montagu's Harrier, Ci rcus pygargus) to variations in abundance of its main prey (common vole, Mi crotus arvalis) in France. We document multiannual fluctuation in the abund ance of common voles. The numerical response of Montagu's Harrier to this v ariable food supply was studied using breeding parameters (breeding density , breeding phenology, and various measures of breeding success). Breeding d ensity and mean clutch size were strongly correlated with spring vole abund ance, whereas mean brood size at fledging was correlated positively with su mmer vole abundance. The mechanism involved in the numerical response of Mo ntagu's Harrier indicates that dispersal and colonization by yearlings may be more important than natality per se. Pellets collected between 1986 and 1997 were used to determine the functional response of Montagu's Harrier to variations in vole abundance. In terms of biomass. Montagu's Harrier diet consisted mainly of voles (33.7-86.6%, between 1986 and 1997). Vole biomass in pellets was closely related to vole abundance estimated by trapping. A type II functional response was detected, with satiation at high prey densi ty, as the shapes of many relationships between breeding parameters and vol e abundance were more curvilinear than linear. Our results fit the pattern of relationships between predators and cyclic prey found in Fennoscandia, i n which specialist predators show a strong numerical response, although at such a temperate latitude, a more pronounced functional response might have been expected.