A study of five common Australian species, including kookaburras, magpies and lorikeets, found around 6% of birds had the chromosomes of one sex but the reproductive organs of another.
The findings indicated a surprisingly high number of birds had reversed their sex after birth, said researchers from the University of the Sunshine Coast (USC).
“This indicates that sex determination in wild birds is more fluid than we thought and can persist into adulthood,” said study co-author Dominique Potvin, who shared the results with ornithologist friends and said they were “mind-blown”.
The study, which involved DNA tests on almost 500 birds, found that the overwhelming majority of sex reversals involved genetically female birds growing male gonads.
“We also discovered a genetically male kookaburra who was reproductively active with large follicles and a distended oviduct – the passageway for an egg – indicating recent egg production,” Potvin said.
Two genetically female crested pigeons had both testicular and ovarian reproductive structures, the research, published last week in the peer-reviewed journal Biology Letters, also found.
Sex reversal is well known in certain species of reptile and fish but is thought to be rare in wild birds and mammals. Scientists have documented how pollutants and even warm temperatures can trigger sex reversal in frogs.
The cause of sex reversal in wild birds was not clear, this new study said, but it could be due to environmental factors, such as hormone-disrupting chemicals building up in wild areas.
“Understanding how and why sex reversal occurs is vital for conservation and for improving the accuracy of bird research,” added Potvin.
Lead author Clancy Hall, also from USC, said sex-reversed birds could affect reproductive success that should raise concerns about the impact on threatened species.
“This could lead to skewed sex ratios, reduced population sizes, altered mate preferences, and even population decline,” she highlighted.
“The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study.”