A Visual Sexing Technique for American Oystercatchers (Haematopus Palliatus) Suitable for the Field

There is no published method for visually sexing American Oystercatchers (Haematopus palliatus) in the field without having the bird in-hand or using expensive telephoto equipment. This study describes and verifies the accuracy of a method requiring only binoculars or a spotting scope to sex American Oystercatchers by dimorphism in body size, bill length, bill color, and back plumage. From 2023–2024, 50 individuals breeding in Virginia, U.S.A. were visually assigned a sex in the field. After capture, DNA testing confirmed the visually assigned sex in 98% of cases. To further validate the method, in 2024, 27 of these birds (14 males, 13 females) were captured, photographed to record back plumage and bill color and measured. Photos were color-analyzed using ImageJ. Males had significantly darker backs, with shorter, darker red bills and smaller bodies. There was 100% sexing classification accuracy using these colormetrics and morphometrics. Photography techniques and ImageJ analysis had been previously verified using 23 American Oystercatcher museum specimens (eight males and 15 females). This visual technique provides an easy, cost-effective way to discriminate the sexes. It might be applied to enrich breeding productivity data collection and identify sex-specific behaviors and life history strategies.