Sex Reversal: Male Birds Can Lay Eggs

For wild birds, sex reversal may be more common than ornithologists originally believed.

A male kookaburra

A new study of roughly 500 birds from five species found that up to six percent had the physical features of one biological sex, but the genetic make up of the other. In one instance, a genetically male kookaburra had a distended oviduct. The scientists’ only possible conclusion? The male bird had recently laid an egg.

“This indicates that sex determination in wild birds is more fluid than we thought - and can persist into adulthood,” said Dominique Potvin, a behavioural ecologist, ornithologist, and evolutionary biologist from the University of the Sunshine Coast in Queensland, Australia.

Sex reversal is well known in fish, amphibians, and reptiles. However, it is rarely documented in wild birds and mammals.

“One of the key findings was that 92 percent of sex-reversed birds were genetically female but had male reproductive organs,” said Potvin. “We also discovered a genetically male kookaburra who was reproductively active with large follicles and a distended oviduct, indicating recent egg production.”

According to the team, the causes of sex reversal in these birds is unclear. More studies are needed to better understand environmental triggers.

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