Factors limiting reproductive performance and nestling sex ratio in the Lesser Spotted Eagle Aquila pomarina at the northern limit of its range: the impact of weather and prey abundance

Weather conditions and prey abundance impact many birds, but the influence differs between species and regions. I used 18-year data from Estonia, north-eastern Europe, in order to analyse the comparative effects of these factors on the breeding performance and nestling sex ratio in the Lesser Spotted Eagle. All studied reproductive parameters showed strong inter-annual fluctuations but no significant trends. Breeding frequency (proportion of pairs laying) was positively influenced by a warm pre-laying period (temperature in April) and a wet preceding season. These factors are probably associated with the higher occurrence and activity of amphibians that constituted important prey of the Lesser Spotted Eagle in spring. Breeding success (proportion of successful breeders) was determined mostly by the abundance of voles. Productivity (number of fledglings per nest) depended additively both on the spring temperature and abundance of voles in the breeding season; fledging of two nestlings was recorded regularly, mostly in years with high vole abundance. The total offspring sex ratio was slightly, though non-significantly, female-biased. Annual proportion of female nestlings was positively correlated with reproductive parameters and associated with rain in the preceding season, suggesting determination of fledgling sex ratio prior egg-laying. The results help to explain fluctuations in the reproductive performance of the Lesser Spotted Eagle and should be taken into account when monitoring and managing for conservation of this species.