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In most species of birds of prey (Falconiformes) females are larger than males. This is unusual in higher vertebrates where males are generally larger than females. We present a model for the evolution of reversed sexual dimorphism (RSD) in raptors which may also be applicable to other groups of birds and mammals: owls (Strigiformes), skuas (Stercoraridae), frigate birds (Fregatidae), boobies (Sulidae) and bats (Chiroptera). We propose that the key factor which has resulted in the evolution of RSD is a constraint on female flight performance prior to egg laying. Two predictions of the model are consistent with the correlations between RSD and the degree of reliance on high flight performance for hunting and between RSD and an index of breeding increment.
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