(Avian Reptile and Exotic Pet hospital/Facebook)

Olivia the hen is one of 10,000 chickens who have a rare condition that causes their biology to change their sex.

A year ago, Olivia stopped laying eggs and began to demonstrate qualities of a rooster – including crowing and growing wattles under her chin.

The formerly female chicken, who would suit the name Olive more than Olivia or Oliver, also started to puff out her chest.

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Her Australian owners grew worried about what was happening to their beloved pet chicken.

They took her to a local vet who confirmed that she had the rare condition in which the chicken’s biological makeup alters itself.

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Avian Reptile and Exotic Pet Hospital, who looked after Olive, explained that the syndrome occurs during embryonic development where the ovary stays in an underdeveloped stage.

This underdevelopment means that the “ovary” never becomes an ovary nor a testes.

Rather, it is called an ovotestis. Usually, this lies dormant but in Olivia it began to secret testosterone sparking the chain reaction of male characteristics.

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“You would be forgiven for thinking that this little lady is a rooster, but looks can be deceiving,” the vet hospital wrote on Facebook.

The vet hospital decided that they would correct the ovotestis and now she will have to have a dose of hormone implants every year which will suppress the testosterone.

The hormone blockers have shrunk Olivia’s wattle and she is no longer cowing.