Male birds laying eggs? Australian study shows gender reversal is more common in wild birds than previously believed

A new study of wild birds in Australia has revealed a surprising finding: a genetically male kookaburra was found to have laid an egg. The research suggests that sex reversal, where an animal’s genetic sex does not match its physical characteristics, may be more common in birds than previously thought.

Associate Professor Dominique Potvin from the University of the Sunshine Coast (UniSC) in Queensland, Australia, co-authored a study that revealed sex reversal in wild Australian birds. The research was published in the journal Biology Letters on August 13, 2025.

Researchers studied roughly 500 birds across five species: Australian magpie, laughing kookaburra, crested pigeon, rainbow lorikeet, and scaly-breasted lorikeet.


The birds had been treated for injuries or illnesses at wildlife hospitals in southeast Queensland and examined posthumously.

The study found that up to six per cent of birds displayed a mismatch between genetic sex and physical features. Some were genetic males with female reproductive organs, while others were genetic females with male characteristics. A small number even showed both testicular and ovarian structures.

“One of the key findings was that 92 per cent of sex-reversed birds were genetically female but had male reproductive organs,” said Dominique Potvin, behavioral ecologist and co-author of the study published in Biology Letters.

A male kokaburra laid an egg

The study highlighted one remarkable case: a genetically male kookaburra had a distended oviduct and large follicles, indicating recent egg production.

“This indicates that sex determination in wild birds is more fluid than we thought, and can persist into adulthood,” Potvin quoted by Popular Science.

Implications for bird research and conservation

Sex-reversed birds can complicate population studies and conservation efforts. Misidentifying a bird’s sex based on DNA, plumage, or behavior could be inaccurate up to six percent of the time.

This is particularly significant for threatened species, where skewed sex ratios could affect reproductive success and population stability.

What causes sex reversal?

The causes of sex reversal in birds remain unclear. Scientists suspect environmental factors, such as stress hormones or endocrine-disrupting chemicals, may play a role. For instance, the study found the male kookaburra in a semi-urban area, where exposure to such chemicals could influence reproductive development.

Similar phenomena are well-documented in fish, reptiles, and amphibians. In birds, however, this is a rarely observed occurrence that challenges traditional sex determining methods.