Lifetime reproductive success in common buzzard, Buteo buteo:: from individual variation to population demography

We studied annual and lifetime reproductive success (LRS) of both sexes of common buzzard Buteo buteo in eastern Westphalia, Germany. We followed a bottom-up approach starting from individual breeding attempts, over lifetime reproductive success to derive population demography. Annual breeding performance and survival followed a quadratic relationship with breeding experience; individuals starting their breeding career were less likely to survive and breed successfully than birds of intermediate breeding experience. According to an analysis of selection gradients, both the opportunity and intensity of selection peaked in the early stages of the breeding career. The distribution of both LRS and another fitness measure, lambda, was highly skewed, with ca 17% of adult birds producing 50% of fledglings in both sexes. Besides breeding life span and number of breeding attempts, habitat quality and plumage morph were significant predictors of LRS. There were strong differences in LRS and lambda between the plumage morphs in both sexes: intermediate pigmented buzzards were much more successful than either dark or light ones. There was no significant difference between buzzard cohorts either in LRS or lambda, nor did these fitness measures differ between individuals starting their breeding career at different conditions of food availability. Based on individual life histories, we formed a transition matrix and analysed its properties to study the population as a whole. This analysis showed that the population growth rate was close to unity (0.906, bootstrapped 95% confidence limits: 0.834 and 0.962). Analysis of reproductive values and elasticities further emphasised colour morph differences: the contribution of intermediate individuals to population growth greatly exceeded that of dark or light individuals. Thus most phenomena on all levels from individual breeding attempts over lifetime reproductive success to population demography can be explained by the fitness differences between the colour morphs with the intermediate morph maintaining the current population renewal potential.