Structural complexity of hunting habitat and territoriality increase the reversed sexual size dimorphism in diurnal raptors

Despite numerous efforts and many hypotheses to explain the selective pressures that may have favoured reversed sexual dimorphism (RSD) in raptors – i.e. that the female is larger than the male – some drivers of RSD are still unknown. Here we analyse how much variation in RSD is explained by hunting habitat structure, territoriality or territory size. We do so using data on diurnal raptors from the New World and the Western Palearctic – i.e. Cathartidae, Pandionidae, Accipitridae and Falconidae, the largest bird group showing RSD – taking into account the phylogenetic relationships among species. Our results identify the type of the main prey as a major factor explaining RSD in raptors. We also found RSD to increase with increasing structural complexity in the hunting habitat from open or semi‐open habitats to forest interior. RSD also increased with increasing degree of territoriality of the species (non‐territorial < facultative < territorial). Finally, for territorial species RSD increased with increasing size of nesting territory. A model comprising only three predictor variables (prey type, structural complexity of hunting habitat and territoriality) explained up to 50% of the variation in RSD of European and American diurnal raptor species, and up to 40% of the variation in RSD when only territorial species were considered. Our results highlight the relevance of spatial facets of the niche – e.g. hunting habitat, territoriality and territory size – in exerting selective pressures on the body size of diurnal raptors. These selective pressures, joint with already known trophic factors – e.g. diet – are decisive for the evolution of the RSD, a key trait in the functional ecology of raptors. Our findings open up new perspectives in the study of sexual size divergence in birds.