Higher reproductive success of small males and greater recruitment of large females may explain strong reversed sexual dimorphism (RSD) in the northern goshawk.

Reversed sexual dimorphism (RSD), which occurs when the female of a species is larger than the male, is the rule for most birds of prey but the exception among other bird and mammal species. The selective pressures that favour RSD are an intriguing issue in animal ecology. Despite the large number of hypotheses proposed to explain the evolution of RSD, there is still no consensus about the mechanisms involved and whether they act on one or both sexes, mainly because few intrapopulation studies have been undertaken and few raptor species have been investigated. Using the strongly size-dimorphic northern goshawk (Accipiter gentilis L.) as a model, we studied a population with one of the highest densities of breeding pairs reported in the literature in order to understand selective pressures that may favour RSD. We evaluated life-history processes, including recruitment of adult breeders and reproductive success, and we explored the mechanisms thought to act on each sex, including hunting efficiency, diet, body condition and mate choice. We found that smaller males produced more fledglings than larger ones, but there was no relationship between size and reproductive success for females. The mean body size of female breeders was larger than that of female fledglings, but male fledglings and breeders did not differ in size. Male body size was related to the type but not to the amount of prey captured during the nestling stage. We conclude that RSD may be favoured in this goshawk population because small males tend to enjoy higher reproductive success and large females greater recruitment. Our results do not support the hypotheses that evolutionary reduction in male size is driven by hunting efficiency, at least during the nestling stage, or the hypotheses that it is driven by greater recruitment. Our findings also suggest that increase in female size is driven by recruitment, rather than by reproductive success as previously postulated.