Male birds of prey are usually smaller than females, known as “Reverse Size Dimorphism” (RSD). We quantified sexual size dimorphism in species that occur in Brazil, and suggest a bootstrap-based approach that allows an estimate of confidence intervals for dimorphism indices. We used body mass, total length, and wing length, measured from museum specimens and a few live birds, to quantify size. We gathered data from eight owl species, for which sample sizes were considered to be minimally adequate to provide reliable dimorphism estimates. We calculated a dimorphism index (DI) for each measurement and then these indices were reduced to a single latent variable using a principal component analysis. A clear trend toward positive DI values confirms RSD as a general pattern, except for negative values in Asio clamator (Striped Owl) for all measurements, for total length in Athene cunicularia (Burrowing Owl), and Ciccaba virgata (Mottled Owl), and for virtually equal to zero values in Pulsatrix koeniswaldiana (Tawny-browed Owl) for total length and A. cunicularia for wing length. Body mass was the most dimorphic trait in all species and DI values were consistent across all three traits. Wing length DI was the most precise index and total length DI was the least precise index. In descending order of overall RSD the species studied were Strix hylophila (Rusty-barred Owl), Megascops choliba (Tropical Screech-Owl), Glaucidium brasilianum (Ferruginous Pygmy-Owl), Tyto alba (Barn Owl), P. koeniswaldiana, C. virgata, A. cunicularia and A. clamator.
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