On the branches of a eucalyptus tree, a kookaburra tilts its head and lets loose its strange, rolling call. Its DNA insists it is male. Its body disagrees.

This is not a fluke, but a pattern that researchers have now traced across species. In a new study of almost 500 wild birds in southeast Queensland, scientists found that up to 6% had undergone “sex reversal” — developing the reproductive organs of one sex while carrying the chromosomes of the other.

The phenomenon, long known in fish, reptiles, and amphibians, had been considered rare in birds. Now, it appears far more common, and far more puzzling.

“Sex determination in wild birds is more fluid than we thought — and can persist into adulthood,” said Dominique Potvin, an ecologist at the University of the Sunshine Coast and co-author of the research.

A Biological Riddle

The rules of avian sex determination seem simple. Males carry two Z chromosomes, females one Z and one W. In humans, females have two X chromosomes and males have one X and one Y chromosome. These genetic instructions are set during embryonic development and remain fixed for life. Or so scientists believed.

The researchers used DNA tests to determine the genetic sex of five bird species — laughing kookaburra, Australian magpie, crested pigeon, rainbow lorikeet, and scaly-breasted lorikeet — all of which had died after being admitted to wildlife hospitals. Then came the dissection knives.

Inside 24 of the birds, the researchers were baffled to find sexual traits incompatible with those predicted from DNA. A genetically male kookaburra showed a distended oviduct — the passageway for eggs. Two genetically female crested pigeons bore both ovarian and testicular tissue. And in case after case, genetically female birds had fully formed male reproductive organs.

“I was thinking, is this right?” Potvin recalled during an interview with The Guardian. “So we rechecked, and rechecked and rechecked. And then we were thinking, ‘Oh my God’.”

The Invisible Hand

In animals where sex is easily swayed — like sea turtles whose gender can hinge on the temperature of their nest — environmental nudges are part of the script. But birds and mammals were supposed to follow a stricter code, their sexes locked in by genetics.

That code can apparently be hacked. The most likely culprits, many scientists suspect, are endocrine-disrupting chemicals, or EDCs — pollutants that interfere with hormones, found in everything from pesticides to wastewater.

“The most likely explanation of the masculinisation is some environmental stimulation, probably anthropogenic chemicals,” said Kate Buchanan, an evolutionary biologist at Deakin University who was not involved in the study.

Buchanan has detected EDCs in insects that feed birds, and has seen how they can reshape both physiology and behavior. For instance, male starlings sang more elaborate songs, but paid for it with a weaker immune system.

If chemicals are at work here, they might be pushing the development of genetic females toward the male form — and possibly away from reproduction. Even if the changes were reversible, Buchanan said, “it would probably knock them out of being reproductive.”

Consequences in the margins

For a single species, a few individuals switching sex might seem negligible. But the numbers could matter.

“This can lead to skewed sex ratios, reduced population sizes, altered mate preferences, and even population decline,” said lead author Clancy Hall.

The effect could ripple into conservation work. Ornithologists often identify a bird’s sex through DNA, plumage, or behavior. If any of these cues can be wrong as much as 6% of the time, population models could be skewed — especially in endangered species where every breeding individual counts.

And this may be only the beginning of the puzzle. The birds in this study all came from wildlife hospitals. Whether the same pattern holds in healthy, free-living flocks is unknown. So too is whether sex reversal might, under some circumstances, serve as an adaptive advantage rather than a liability.

Somewhere in the tangled branches of Australia’s forests, these anomalies go on living and breeding — or not breeding — as the case may be. For now, they remain an enigma, one more reminder that in the natural world, the categories we draw are not always the ones nature uses.

The findings appeared in the journal Biology Letters.