At wintering sites of the migratory Short-eared Owl (Asio flammeus) in subtropical Asia, behavioral interactions between males and females are limited, and the body mass and plumage features between the sexes overlap significantly. Thus, sexing the species morphometrically for research and conservation activities is difficult. We aimed to develop a quantitative method for sexing Short-eared Owls, and we here present a new formula to do so using plumage features. We used a total of 198 Short-eared Owls (163 from bird rescue efforts at 15 airports and 35 preserved specimens from a museum in Taiwan) and carefully examined their morphological data and plumage images, then determined their sex using polymerase chain reactions of chromodomain helicase DNA-binding protein genes on sex chromosomes or gonad inspection. Eight of the ten traits we evaluated differed significantly between males and females. Females had a greater number of cross bars on primaries, secondaries, and outermost tail feathers; a higher proportion of yellowish-brown underwing coverage; and larger measurements in head length, bill length, tarsus length, and body mass. The best-fit model suggested simplifying the formula to just the proportion of the yellowish-brown underwing coverage, which provided sexing accuracy exceeding 95.9% for the 49 live individuals in the test data set and 94.3% for the 35 preserved specimens. This formula addresses the challenges posed by ambiguous individuals in the wintering region and offers an efficient and accurate means for sexing Short-eared Owls when DNA or gonad inspection is unavailable.

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