Reversed sexual size dimorphism: body size patterns in sexes of lesser kestrels (Falco naumanni) in the Ikh Nart Nature Reserve, Mongolia

In normal sexual size dimorphism, males generally possess larger body sizes than females in vertebrates, especially birds, mammals, and some reptiles. However, most birds of prey from the orders Falconiformes and Strigiformes have reversed sexual dimorphism (RSD) in body size (i.e., females larger than males). We investigated the degree of RSD in lesser kestrels (Falco naumanni), a small falcon breeding in Mongolia, which may contribute to a better understanding of body size variation based on gender for other birds of prey that do not have plumage dimorphism. We explored the degree of RSD between male and female lesser kestrels in a semidesert region of Mongolia by capturing and measuring 79 males and 83 females during the breeding seasons of 2007 through 2016. Our 10 years of data showed that females were significantly larger than males in five of nine measurements, with an especially high degree of RSD in body weight and bill measurements. For males, we found significant relationships between body weight and both tail length and wingspan, but no significant relationships for females. Our results support previous hypotheses raised to explain RSD in birds of prey.