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

Abstract

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.

Spatial relationships and mechanisms of coexistence between dominant and subordinate top predators

“…A1). The study area has a relatively high breeding pair density of goshawk, buzzard and sparrowhawk (Rebollo et al , Pérez‐Camacho et al ). Other diurnal forest raptors such as European honey buzzard Pernis apivorus are rare in this area (0–5 breeding pairs yr –1 ) and were not considered in this study.…”

Spatial size dimorphism in New Zealand's last endemic raptor, the Kārearea Falco novaeseelandiae, coincides with a narrow sea strait

“…, Pérez‐Camacho et al . ). It has long been recognized that ecological and reproductive demands on male and female animals differ (Darwin ).…”

Plasma carotenoid concentrations of incubating American kestrels (Falco sparverius) show annual, seasonal, and individual variation and explain reproductive outcome

“…However, maintenance of exogenous energy reserves during the breeding season may not be the best strategy for male kestrels. For several raptor species with reverse sexual size dimorphism, smaller males are selected more often by females, have superior hunting skills, and produce more young compared with larger males (Hakkarainen et al ., ; Pérez‐Camacho et al ., ). Male kestrels may benefit from maintaining lower body mass, higher carotenoid concentrations, and provisioning the female.…”