Sex Reversal Is More Common in Birds Than Previously Thought, Suggests Study of Australian Species

Researchers find that about 5 percent of birds studied have a mismatch between genetic and physical sex—including one male bird that seems to have laid an egg

Sara Hashemi | Daily Correspondent

August 14, 2025

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Sex determination in birds is not quite as clear as scientists once thought, according to a new study that finds a higher than expected rate of “sex reversal,” meaning the animal has the genetics of one sex, but its physical features don’t match.

A team of scientists dissected almost 500 birds from five Australian species: Australian magpies, laughing kookaburras, crested pigeons, rainbow lorikeets and scaly breasted lorikeets. The animals had all died at wildlife hospitals for reasons unrelated to their sex, such as injuries or disease. To determine each bird’s genetic sex, the researchers tested their DNA, and they also examined the specimens’ reproductive organs for comparison.

To the team’s surprise, they found that 24 of the birds were sex reversed. The kookaburras had the highest rate of sex reversal, at 6.9 percent, and the lowest rate was seen in the magpies, at 4 percent. Their results were published in the journal Biology Letters on August 13.

“This indicates that sex determination in wild birds is more fluid than we thought—and can persist into adulthood,” says Dominique Potvin, study senior author and an animal ecologist at the University of the Sunshine Coast in Australia, in a statement.

Potvin was skeptical of the results at first. “I was thinking, ‘is this right?’” she says to Graham Readfearn at the Guardian. “So we rechecked, and rechecked and rechecked. And then we were thinking, ‘oh my God.’”

Overall, around 92 percent of the sex-reversed birds were genetic females with male reproductive organs. But the researchers also discovered one genetically male kookaburra with large follicles and a distended oviduct, suggesting it had recently laid an egg.

Sex reversal has been documented across much of the animal kingdom, but scientists haven’t often seen it in birds and mammals. Some sex-reversed fish, amphibians and reptiles have been found. And it doesn’t always happen for the same reasons: Some reptiles, for instance, will experience sex reversal at high temperatures. Sometimes, sex reversal can be the result of endocrine-disrupting chemicals in the environment.

The study thus raises questions about the reason behind this change in the magpies, kookaburras, pigeons and lorikeets. “Now that we know discordance occurs, the next big question will be, what is driving this discordance in birds?” says Clare Holleley, an environmental biologist at Australia’s Commonwealth Scientific and Industrial Research Organization who wasn’t involved in the research, to Phie Jacobs at Science. “Is it chemicals, is it environmental stress or some other factor that can alter developmental trajectories?”

So far, the research hasn’t provided answers. But understanding why sex reversal occurs can have important implications for conservation and bird research. Clancy Hall, a study co-author and researcher at the University of the Sunshine Coast, says that sex reversal may impact the ability of some birds to reproduce. “This can lead to skewed sex ratios, reduced population sizes, altered mate preferences and even population decline,” she explains in the statement.

Potvin tells Science that she hopes her work will lead other scientists to explore sex reversal in bird species. She wonders how the phenomenon might impact other characteristics of birds, such as their vocalizations.

“We know a lot about the physiology,” she adds. “We don’t know how it impacts the individuals and especially the populations as a whole.”

Sara Hashemi | Read More

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.