Anatomy Genetics Physiology

Gender Bending Chickens

Spontaneous Sex Reversal In Hens

Facebook chicken groups are full of posters lamenting that their favourite pullet turned into a rooster. Not quite – that bird was always male, but misidentified until sexual maturity. There are others that wonder if their birds truly are transgender, in that, they clearly had characteristics of one sex, but later took on ones of the opposite sex. That actually can happen and here is how.

Sexual dimorphism is the differences in appearance between males and females of the same species, such as colour, shape, size and structure. Those traits are created by the inheritance of sex-linked genes responding to varying amounts of sex hormones, like testosterone. This results in the same genotypes (genetics) producing different phenotypes (appearances), depending on sex.

If you’re familiar with the chicken reproductive system you’ll know that hens have only one functional ovary – usually the left one. The right ovary and oviduct are present in female embryos, but don’t develop. If you pardon the pun, it is like putting all your eggs in one basket by relying on only one working ovary. So what happens if it gets damaged or diseased?

An ovarian cyst, tumor or diseased adrenal glands can cause the left ovary to regress and no longer produce the necessary levels of estrogen in the hen’s body. As estrogen drops to critically low levels, the hen’s testosterone increases triggering residual tissue in the right ovary to develop in the absence of the functional left ovary. This regenerated right gonad is known as an ovotestis and may contain some tissue characteristics of the ovary, testes, or both.

An ovotestis secretes androgen as well as estrogen. As a result, the hen develops male secondary sex characteristics: larger comb, longer wattles, male patterned feathering, spurs and crowing.

The hen can’t completely become a rooster. The transition is limited to developing physical characteristics that will make her look male, while still remaining genetically female. All my readings indicated that hens would no longer lay eggs, but there were conflicting reports whether she could produce sperm and father chicks or not.


There were very few online photographs of before and after transitioned chickens, but I did find some that involved other birds species: ducks, geese and peacocks.

Credit: Geoff Bennett, Evening Post

So what about roosters becoming feminized? Nope, that can’t happen.

Particular chromosomes determine the sex of an offspring. In humans, the sex chromosomes are called X and Y: females are XX and males are XY. In chickens, sex chromosomes are Z and W. ZW is female and ZZ is male.

The Z chromosome controls male characteristics. Estrogen released by the functioning ovary inhibits the Z chromosome genes that would trigger male hormones and characteristics in hens. Estrogen in birds has a similar function to testosterone in humans, which restricts the production of female hormones. If female hormones in hens are suppressed it enables those birds to become masculinized.

Roosters are ZZ and can’t be feminized because they lack the W chromosome, which is linked to the genes that determine the development of ovaries.

Many of us have hens with spurs or who crow in the absence of a rooster or even mount other hens. These are normal behaviours of biologically female chickens. Spontaneous sex reversal is rare and only happens with hens taking on male physiological characteristics.

If the ‘sexed’ chicks you bought as pullets grow up to be roosters that was a mistake on the hatchery’s part. If you were sure your favourite chick was female, but grew up to be a rooster that was probably wishful thinking on your part.

Credits: Feather Site; Small & Backyard Poultry; Urban Chicken Podcast. Featured Photo: Olivia-turned-Oliver (Credit: Pink News UK).

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