Reversed sexual size dimorphism in raptors: evaluation of the hypotheses in kestrels breeding in a temporally changing environment

Reversed sexual size dimorphism (RSD, females larger than males) is commonl y found in birds of prey. We used kestrels (Falco tinnumculus), breeding in western Finland in a temporally varying environment of 3-year vole cycles, to assess current hypotheses for the evolution and maintenance of RSD. Our 12-year data showed only weak correlations between parental size and breed ing parameters (laying date, clutch size and the number of fledglings produ ced). The degree of RSD per se was unrelated to breeding success, contrary to the prediction of the female dominance hypothesis. Females with small ma les produced larger clutches in low-vole years. independently of laying dat e, which supports the small male (or its equivalent inter-sexual selection) hypothesis. Small females tended to have more fledglings, particularly in low-vole years, which is inconsistent with the hypotheses for an advantage of large female size (the starvation. intra-sexual selection, reproductive effort, and supplementary feeding hypotheses). As for males, smaller female s may be more efficient hunters, the importance of which should be most pro nounced under harsh breeding conditions. Our results suggest that the direc tional selection on a particular size in kestrels may be under contrasting selection pressures by the environment, and, at least in breeding females, the advantages of large size can actually be counterbalanced during harsh e nvironmental conditions.