Sexual size dimorphism and morphological sex determination in the Yellow-browed Bunting (Emberiza chrysophrys)

Sex-differential migration patterns may coevolve with sexual size dimorphism, and reliable field sexing is necessary to understand this relationship. At stopover sites near the wintering grounds, Yellow-browed Buntings (Emberiza chrysophrys) exhibit relatively low levels of protandry and sexual size dimorphism among bunting species. Plumage-based criteria for this species are further complicated at autumn stopovers, where molt progression varies, and the difficulty of sexing and ageing can vary across sites and seasons. Although sexual size dimorphism (SSD) in this species has been reported, morphological sexing has not been validated with molecular methods, and large-scale ringing programs are unable to feasibly genotype all individuals. We therefore aimed to quantify SSD and develop practical morphometric discriminant functions using molecularly validated sex determinations, thereby enabling a better assessment of sex differential migration patterns and their relationship with morphology in this species. We measured 50 migratory first-year Yellow-browed Buntings captured during autumn stopover on Daecheong Island, South Korea, and confirmed sex through molecular assays. Among the 50 first-year birds (24 males, 26 females), sexual size dimorphism was strongest in wing length (Cohen’s d = 3.00) and tail length (d = 2.11). For discriminant function analysis, a simplified wing-only model (D0) achieved 92.0% leave-one-out cross-validation accuracy, as did the wing-and-tail model (D2) and the stepwise-selected model (D3, wing-only). All models relied principally on wing length as the primary discriminator. These morphometric discriminant functions enable accurate, rapid sexing of Yellow-browed Buntings in the field without genetic testing. They facilitate demographic monitoring and sex-specific ecological studies at stopover sites and can be applied to individuals with ambiguous field identification characteristics, thereby improving our understanding of sex differential migration patterns and their coevolutionary relationship with morphology in Yellow-browed Buntings.