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When “She” Becomes “He” – Spontaneous Sex Reversal in Chickens

Sometimes chicken observers see birds do odd things… and few seem stranger than when a hen transforms into a rooster. It’s not common, and a backyard flock owner may never experience it, but it happens. Automatic, or spontaneous, sex reversal has been studied by many scientists including Dr. Jacqueline Jacob and Ben Mather, and Richard Blanchford. The change doesn’t happen overnight, but a person could have six hens in the spring and five hens and what appears to be a rooster months later. 

Roosters crow, usually have pointed feathers, grow long spurs, and sport a larger comb than females of the same breed. Hens generally have softer and more rounded feathers and a tiny spur. They never welcome dawn with enthusiastic crowing.

Unlike mammals, which have two ovaries, female birds have only one functional ovary on the left side.  The right ovary is an incomplete organ and doesn’t develop in a healthy bird. But, if a hen’s functional ovary is injured or develops a cyst or cancer the tiny right one becomes what scientists call an ovotestis. It can contain tissues common in ovaries, testes, or both. An ovotestis may secrete androgens that cause the birds comb and wattles to grow and feathers to change to the male pattern. She may crow. Crazy, right?

So, is she now a rooster? Dr. Jacob and F. Ben Mather, in an article on the topic, state that she’s genotypically female but phenotypically male. Big words that sound like the poor bird is a blend of both genders. There are reports of these birds producing semen capable of fathering offspring but most will not do so, nor will they lay eggs. The reverse switch doesn’t seem to happen. Roosters don’t automatically become hens.

One thing is certain. Most ordinances specify that roosters may not be kept in suburban or urban back yards. Scientists may not consider a crowing chicken with male feathers and a spur a total rooster, but whoever enforces the ordinance will almost certainly declare the bird illegal. Any neighbor woken up by predawn crowing will agree. So, a hen that seems to transform into a rooster may be a candidate for rehoming.

Resources: Backyard Poultry; University of Florida Extension