Using wing bar patterns to identify sex in Crowned Eagles Stephanoaetus coronatus: A misleading field characteristic

Abstract

Almost universally, raptors exhibit some degree of sexual size dimorphism, with many species also having substantial sexual dimorphism in plumage patterns. The monophyletic clade ‘booted eagles’ generally do not, except for the Crowned Eagle Stephanoaetus coronatus, which are believed to have sex-specific wing patterns suitable for field identification. We challenge this long-established norm of field identification of sexes of Crowned Eagles based on their wing bar patterns. Photographs of seven adult Crowned Eagles were translated to a standardised digital template to facilitate a systematic comparison. There was significant individual variation in the wing pattern of Crowned Eagles, and these did not appear to be linked to sex. Alternative explanations for the underwing patterns include plumage polymorphism and individual plumage variation. We demonstrated that Crowned Eagles cannot be reliably sexed by the number of bars on the wing and we established a foundation for ongoing investigation, especially regarding the stability of this pattern in ageing individuals.