Sex reversal is more common in wild birds than expected

Some wild birds can develop the body of one sex while carrying the DNA of the other. New evidence suggests this happens more often than scientists realized, and it may matter for species already under pressure.

Researchers led by the University of the Sunshine Coast (UniSC) examined nearly 500 birds from five familiar species and found that up to 6% showed a mismatch between their physical sex and their genetic sex. 

The work points to a hidden source of error in bird studies and a potential risk for threatened populations.

Females with male reproductive organs

“This indicates that sex determination in wild birds is more fluid than we thought – and can persist into adulthood,” said co-author Dominique Potvin, an associate professor at UniSC. 

The birds – including magpies, kookaburras, pigeons, and lorikeets – were examined after death at wildlife hospitals in southeast Queensland. 

Each bird’s reproductive organs were identified, and DNA tests determined genetic sex. 

“One of the key findings was that 92% of sex-reversed birds were genetically female but had male reproductive organs,” Potvin said.

A single case demonstrated how dramatic the reversal can be. “We also discovered a genetically male kookaburra who was reproductively active with large follicles and a distended oviduct, indicating recent egg production.”

How the team defined sex reversal

The team classified sex-reversed birds in three ways. Some were genetic males with a complete feminine phenotype. Some were genetic females with a complete masculine phenotype. Others showed a mix of ovarian and testicular features.

Importantly, the mismatches were not limited to juvenile birds. Many were adults, meaning the reversal had persisted beyond early development.

Sex reversal is not a new idea in biology. It is well documented in fish, amphibians, and reptiles. But it has rarely been confirmed in wild birds and mammals. 

This study offers a baseline for Australian birds, showing that the phenomenon is present across multiple species living in the same region.

Sex reversal and bird conservation

“Understanding how and why sex reversal occurs is vital for conservation and for improving the accuracy of bird research,” Potvin said. 

If a population carries more functional males than its DNA suggests – or vice versa – managers could misjudge breeding potential. That matters for small or declining groups, where every breeding season counts.

Lead author Clancy Hall, a UniSC doctoral student. warned about the demographic effects. “This can lead to skewed sex ratios, reduced population sizes, altered mate preferences, and even population decline,” he said.

Distinguishing male from female

Traditional cues like plumage, size, and behavior can mislead. DNA tests are often treated as the gold standard. The new results show that even genetic markers do not always match a bird’s functional sex. 

“The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study,” Hall said. “We’ve now established that DNA evidence does not always reflect the suspected sex.”

“Bird researchers often take small DNA samples from blood or feathers and make decisions based on the results showing male or female, but this actually could be wrong up to 6% of the time.”

That error rate might seem small. It is not, if it clusters in particular species or habitats, or if census sizes are low. For threatened birds, a few percentage points can tip the balance between stability and decline.

What causes sex reversal?

The study did not pin down causes. The authors highlight several possibilities for future investigation.

Endocrine-disrupting chemicals can interfere with hormones needed for sex development. Chronic stress can elevate hormone levels and shift development, too. 

The environment around some birds suggests exposure is also plausible. For example, the reproductively active, genetically male kookaburra was found in a peri-urban agricultural zone, where such chemicals may accumulate.

More research will be needed to sort out timing and triggers. Does reversal arise in the egg, during nestling growth, or later in life? Are some species more susceptible than others? Do heat waves or drought play a role through stress pathways? 

The answers to these questions will help conservationists target the most effective protections.

Future research directions

The immediate step is methodological. Field biologists may need to pair DNA testing with direct examination of reproductive anatomy when possible, or develop noninvasive hormonal assays that better reflect functional sex. 

Museum and hospital specimens, like those used here, can anchor the science, even as live-animal tools improve.

The broader step is ecological. If sex reversal is common enough to shift local sex ratios, managers will need to account for it when modeling population growth, setting harvest limits, or designing recovery plans.

The study’s message is clear and practical. Birds do not always fit neatly into genetic boxes. Their bodies – and their populations – can be more fluid. Recognizing that reality will make research more accurate and conservation more effective.

The study is published in the journal Biology Letters.

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