Sexing first-calendar-year Carrion Crows Corvus corone from biometrics reveals variation between years in post-fledging sex ratio

Sex is an important parameter to consider when studying population dynamics and movement ecology, for example, though sex determination is often difficult in young birds of sexually monomorphic species, with large biometric overlap between sexes. We aimed at determining the sex of first-calendar-year (1cy) Carrion Crows, in order to study temporal trends in sex ratio. We performed molecular sexing of a sample of 48 females and 45 males, creating a statistical framework to confidently discriminate sexes using four morphometric variables. Although biometrics overlapped between sexes, we developed a discriminant function which separated the sexes with high accuracy (88%). Then we applied this discriminant function to biometrics obtained from all 1cy individuals captured, to determine their sex. We found a female-biased overall sex ratio of the captures: female Carrion Crows might therefore be more numerous than males, or more prone to enter traps. There were significant variations in sex ratio between years, but not between months.