Let’s Talk About Chicken Sex Reversal

You may hear a crow from the coop, even without a rooster, if one of your hens undergoes chicken sex reversal and starts to develop male traits.

by Moira McGhee
PHOTO: Moira McGhee

A little red hen struts around the pen, clucking away, when suddenly it lets out a weird, warbly cock-a-doodle-doo. Did that chicken just crow? Probably, and it’s not as uncommon as you might think.

Chicken sex reversal can occur in hens for various reasons, but ultimately, it’s tied to an ovary that quits working and the output of the one that never worked. Genetically, the hen is still female, but she may no longer look or act like one.

Inside a Normal Hen

Like other females of a species, hens have two ovaries. However, only one of these ovaries works, usually the left ovary, and it’s perfectly normal for chickens.

Jacquie Jacob, poultry extension project manager in the department of animal and food sciences at the University of Kentucky, explains that female vertebrates develop two reproductive organs. This symmetry usually remains into adulthood.

“In most avian species, however, there’s a failure of one ovary and its corresponding oviduct to develop,” Jacob says. “The result is normally the presence of the left ovary and oviduct reaching functional development in the adult. The right oviduct is underdeveloped or rudimentary.” 

Jacob has a Ph.D. in poultry nutrition and more than 40 years of experience working in poultry extension and outreach. She previously worked in Mozambique, Africa, for four and a half years on a poultry project, in Florida for five years and Minnesota for seven years before settling in Kentucky, where she’s been for more than 12 years.

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She specializes in small and backyard flocks and youth poultry programs, especially 4-H and embryology in the classroom.

Richard Blatchford, an assistant professor of extension, small- to large-scale poultry, in the department of animal science at the University of California, Davis, specializes in husbandry, behavior and welfare of poultry. He works with broilers, layers and backyard flocks and speaks to groups about backyard flock behavior and nuisance behavior that he helps them fix.

“Female birds only have one functional ovary, and the other ovary doesn’t create ova, but it does release some testosterone,” he says. “If the functional ovary is fine, it’s not enough testosterone to make any big changes in the bird. She appears to be a female, she lays eggs, she goes on with her life as she should.”

Read more: Check out these fun chicken facts, including sex reversal and more.

Why the Change?

Hens undergo chicken sex reversal, physically changing into a rooster, due to a malfunction in their working ovary. Numerous things can prompt this malfunction in the chicken, leading to a spontaneous sex reversal. Something must happen to the functional ovary to cause it to become inactive before this change occurs.

“This could be something like a disease,” Blatchford says. “She could have ovarian cancer, which is pretty common. She could have some other tumor on her ovary. It could be caused by an injury.

“Sometimes, it could just be age and her body just kind of gives out. There could be lots of reasons why the ovary becomes nonfunctioning. For whatever reason, the ovary stops working.”

Jacob adds that an ovary damaged by tumors or disease causes the hen’s hormone levels to drop and the testosterone takes over. The increased testosterone gives the chicken secondary male characteristics and, sometimes, the right oviduct develops into a testes-ovary.

“Essentially what happens is they stop producing estrogen,” Blatchford says. “So, the testosterone from the nonfunctional ovary becomes the major sex hormone. Because they now predominately have testosterone circulating, they start to develop secondary sex characteristics of males.”

Both experts agree that chicken sex reversal appears to be more common in older birds. This makes sense as their bodies naturally produce less estrogen as they age, and they’re more prone to disease, tumors and other issues with their functioning ovary.

“We typically see this when the birds reach, I’d say most commonly around 5 to 6 years of age,” Blatchford says. “Their reproductive system is starting to break down normally, and they just become prone to other types of diseases. So that’s when you see things like ovarian cancer and other types of tumors occurring on the ovary.

“This is mostly because she’s been using it a lot. It’s starting to break down.”

However, Blatchford stresses that just because a bird is aging doesn’t mean she’s eventually going to experience chicken sex reversal and start showing signs of becoming a rooster. Jacob agrees that although it’s more usual for older hens to have sex reversal, it can occur any time after a chicken starts laying.

“It can happen in younger individuals,” Blatchford says. “It just kind of depends on the underlying pathology that has to occur for the change to happen. There’s some genetic, heritability component.”

chicken sex reversal

Crowing & Other Odd Behavior

Hens that undergo chicken sex reversal usually only develop some rooster traits. One of the most common traits is crowing, which could be bad in urban areas where roosters aren’t allowed. Trying to explain that there’s not a rooster present may be difficult with all that crowing going on.

According to Jacob, hens are physically capable of crowing without any male changes occurring. But they usually don’t want to because of the estrogen in their bodies. She said it’s rare for a hen to start crowing for no reason. Generally, it means there’s a problem with their hormones.

When a chicken begins crowing, it’s often not a full crow. “It’s kind of a weird sounding crow,” Blatchford says. “For people who’ve had roosters, they know when they first start crowing, it kind of sounds weird. That’s kind of the sound that comes out because she’s not used to doing it. Plus, the acoustics or the vocal systems of the two sexes are slightly different. She just doesn’t have the crow a rooster has, and it’s probably always going to sound a little off.”

Blatchford adds that people often notice other behavior changes in hens that are changing before the birds start crowing. The hen may become more alert or start bossing the other birds around. She might try to get them to go places or follow her. Essentially, she starts doing things you’d expect roosters to do. 

“She may even start mounting other birds, but that doesn’t always happen,” Blatchford says. “The hen may become much more protective of the flock though. Sometimes they become more aggressive toward people, which is usually only a behavior roosters exhibit.

“But the biggest difference is she probably won’t lay any eggs.”

Read more: Your chickens might lay these types of abnormal eggs.

Eggs No More

For hens that undergo chicken sex reversal and start taking on rooster traits, their egg-laying days are usually behind them. Jacob says changed hens won’t ever lay eggs again. “The changed chicken is typically infertile,” she says. “No eggs or sperm — at least none that reach the outside of the body.”

Blatchford confirms that, for the most part, hens that undergo the chicken sex reversal stop laying eggs because of the underlying issue with the ovary. However, people have told him that some hens do still lay eggs.

“I’m not entirely sure what’s going on there,” he says. “But for the most part, hens will stop laying when the change happens. However, it’s not because they’ve changed into a rooster. It’s because their ovary is no longer working, so they’re not producing eggs.”

chicken sex reversal
Moira McGhee

Backyard Flock Issues

Blatchford also notes that backyard chicken flocks seem more prone to having hens undergo sex reversal. Primarily, it’s because the phenomenon occurs more frequently in older birds. He says the condition is much more common with backyard flocks because we allow the birds to age. In comparison, chickens raised as farm animals or in commercial productions get slaughtered when they’re relatively young.

“Weight is also a big issue,” he says. “Most backyard birds are overweight and that can be an issue with the reproductive system and the ovaries. That’s probably why I hear about sex reversals a lot because most backyard birds are overweight. We feed them too much good stuff.”

Even foods we think are healthy, sometimes they aren’t healthy if the birds are eating a lot of it. Kitchen scraps, usually some sort of vegetable or fruit, are something the birds really like. But eating a lot of scraps over time isn’t good for them. It actually packs on the pounds.

Some chicken-keepers have never even heard of a chicken turning into a rooster, much less have seen this sex reversal happen. However, it’s becoming more common for backyard flock owners to mention seeing the change occur.

While it’s difficult to gauge how often this phenomenon happens, it seems to be happening a lot. “It seems to be more common than it used to be,” Jacob says, “[but we’re] not sure why.”

Blatchford says that it’s difficult to know what the percentage is in the population that we see this, he does get asked about this a lot. “So, I assume it happens quite frequently,” he says.

Both experts agree that the reverse doesn’t occur. A rooster won’t suddenly become a hen and start laying eggs, so don’t expect to hear a cock-a-doodle-cluck-cluck-cluck any time soon! 

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Special Care Requirements

Backyard chicken-keepers tend to treat their chickens as pets and are greatly concerned about their well being. Therefore, some common questions are whether they need to do anything for their hen-turned-rooster and will it die sooner than its flock mates.

Jacquie Jacob, poultry extension project manager in the department of animal and food sciences at the University of Kentucky says the lifespan of a chicken could be affected because a disease or tumor may have caused the sex reversal. However, the hen generally doesn’t require any special treatment.

“It really depends on why the change happened,” says Richard Blatchford, an assistant professor of extension, small- to large-scale poultry, in the department of animal science at the University of California, Davis.

“For instance, she may live out her life fine. She may just be more like a rooster and not lay eggs but live out her life. However, she may have a shorter lifespan if it’s a cancer issue or some other disease that may cause mortality. As far as special treatment, she’s still a chicken. She’s still doing her thing, so you really don’t have to change anything you’re doing.”

Due to the concern that a disease or cancer caused the change, backyard chicken-keepers may want to take their hen to a vet to learn the underlying issue and/or request care. Unfortunately, this may not help.

“The vet may not actually be able to tell exactly what’s happening either,” Blatchford says. “Usually after the birds have died, there’s a necropsy performed. This is when you can actually see what’s happening with the organs. Then, you can kind of backtrack, and determine what disease was happening, why her ovary wasn’t functional and why she was doing all these weird things for a hen.”

You’ll have to let nature take its course for the most part. Jacob says that once a chicken begins morphing into a rooster, it’s usually too late to stop or reverse what’s happening. Plus, treatment is pretty limited for backyard flocks. Many vets won’t even treat chickens.

If there’s no sign of illness, Blatchford says the male behavior isn’t necessarily something to worry about. 

This article originally appeared in the September/October 2022 issue of Chickens magazine.

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