Sex Reversal Turns Conventional Avian Biology Upside Down
A first-of-its-kind study reveals that up to 6% of free-living Australian birds may possess reproductive features mismatched to their genetic sex - a finding with profound implications for conservation and ecology
Breaking the biological mould
In what may be a groundbreaking shift in ornithological understanding, research led by the University of the Sunshine Coast has discovered that sex reversal occurs more frequently in wild birds than previously thought. In a survey of nearly 500 birds across five species - including magpies, kookaburras, pigeons and lorikeets - scientists found that up to 6 % exhibited a discordance between their genetic sex and their reproductive anatomy. This includes individuals with male organs yet female DNA, and vice versa.
The study, published on 13 August 2025 in *Biology Letters*, revealed that 92 % of these sex-reversed birds were genetically female but had male reproductive structures. Remarkably, researchers also documented a genetically male kookaburra showing clear signs of recent egg production, including well-developed follicles and a distended oviduct.
Challenging established methods
Traditionally, researchers determine bird sex based on DNA tests, plumage, or reproductive behavior. This study, however, demonstrates that such methods may be misleading in up to 6 % of cases due to sex reversal.
Dr Clancy Hall, the study’s lead author, emphasised the implications: “Biases in sex identification could skew population models, reproductive success assessments and conservation planning,” potentially impacting threatened species in unexpected ways.
Environmental influences under scrutiny
While sex reversal is known in fish, reptiles and amphibians, it is rarely documented in birds. The causes in this study remain unclear, but researchers suspect environmental factors - such as exposure to endocrine-disrupting chemicals or elevated stress hormones - may play a key role, especially since some of the birds came from peri-urban, agricultural or chemically exposed areas.
Biologist Kate Buchanan commented that most sex reversals tend toward female-to-male direction, aligning with female being the "default" sex in birds, and pointed to anthropogenic influences as a likely trigger.
Conservation and data integrity at stake
The presence of sex-reversed individuals may skew sex ratios, hamper reproductive success, and even contribute to population decline - especially in vulnerable species.
The researchers note that predictive models, species recovery plans, and breeding programmes must account for this phenomenon to avoid inadvertent mismanagement.
Setting a baseline for future research
This study establishes a vital baseline for sex-reversal prevalence in wild bird populations. Researchers advocate for expanded studies across regions and species to explore how widespread the phenomenon truly is, and to investigate potential environmental triggers. Such insights could reshape scientific approaches to avian biology, conservation strategies, and population monitoring.
August 2025
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