Prevalence and implications of sex reversal in free-living birds.

The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study. Recent evidence indicates that avian sex determination is more flexible than once believed, with sex-reversed individuals identified in domestic bird populations-that is, individuals exhibiting gonadal and morphological traits of one sex while possessing the chromosomal make-up of the opposite sex. The presence of these individuals can challenge the reliability of traditional sexing methods that rely solely on external morphology, internal anatomy or genetic markers. These methods, when used in isolation, fail to identify sex-reversed or intersex individuals, potentially overlooking their impact on population dynamics. In this pioneering study, we investigated the prevalence of sex-reversed individuals in five common free-living avian species in Queensland, Australia. By comparing internal and external morphological characteristics with polymerase chain reaction results from sex-linked molecular markers, we identified sex-reversed individuals in all five species, with rates ranging from 3 to 6%. Our findings suggest that sex reversal is a common and potentially widespread phenomenon in avian species.