Spontaneous Sex Reversal – Is That My Hen Crowing?!

A Spontaneous Sex Reversal Can Result in a Hen Exhibiting Aggressive Rooster Behavior


Have you heard a hen crowing when you know you don’t keep roosters? Jeff Goldblum’s groovy scientist character in the 1993 blockbuster, “Jurassic Park,” comments that “life finds a way” and that somehow the all-female population of cloned dinosaurs would reproduce. Well, life is stranger than fiction and your backyard chicken CAN undergo a spontaneous sex reversal and become a rooster!

A hen is born with two ovaries like a female human (sort of). The left ovary in a hen grows and develops. It is this left ovary that produces all of the necessary estrogen in a hen’s body that regulars the production of ova (though these are called oocytes in chickens) and their release into the oviduct tract. The right “ovary” in a hen doesn’t actually develop at all as the bird grows. Rather this gonad sex organ (i.e. right “ovary”) remains small, dormant and undeveloped.

Hen Anatomical Model - photo by Lisa Bruce

Hen Anatomical Model – photo by Lisa Bruce

A spontaneous sex reversal occurs in a hen when her left ovary becomes somehow damaged or fails to produce the necessary levels of estrogen. A hen’s left ovary is the only organ in her body producing estrogen. Without the left ovary properly functioning in a hen, the estrogen levels in her body will drop to critically low levels and conversely testosterone levels will rise. Without proper estrogen levels, how do chickens lay eggs? The hen will no longer produce eggs.

More disturbing though, a hen who’s left ovary has failed and consequently has elevated testosterone levels in her body, will actually physically transform to take on male characteristics. Such a hen will grow a larger comb, longer waddles, male-patterned plumage, and spurs. Moreover, this hen will also adopt aggressive rooster behaviors — such as a hen crowing.

You might be thinking to yourself, just because a hen with high testosterone levels grows spurs, long waddles and takes to crowing like a rooster — does not make her, in fact, a rooster. It just makes her a very butch hen. If that was all that happened in a spontaneous sex reversal of a hen — you would be correct. There is more to it though!

When a hen’s left ovary fails and sufficient testosterone levels are reached in her body, the hen’s dormant right side gonad becomes activated. When the dormant, right-side gonad is switched on, it develops into a male sex organ, called an ovotestis. Scientists have found that an ovotestis will produce sperm. A sexually reversed hen with a “turn-on” ovotestis, will actually try to mate with the other hens in the flock. There is conflicting information as to whether a hen that has undergone a spontaneous sex reversal and developed an ovotestis can sire offspring. At least one account of a sex-reversed hen fathering chicks exists on the web.

Dr. Jacqueline Jacob, a poultry expert (whose Ph.D. is in Poultry Science) wrote a very informative paper on the spontaneous sex reversal phenomenon in chickens. Dr. Jacobs discusses this rare condition in episode 018 of the Urban Chicken Podcast. Listen HERE to learn more about this truly fascinating and bizarre chicken phenomenon. There are links to several news articles about spontaneous sex-reversed chickens in the show notes of this episode.

Recently, a couple of Urban Chicken Podcast listeners wrote to report about hens crowing and behaving like roosters all of the sudden in their flocks. You can read more about these stories sent to me about spontaneous sex reversal in backyard hens HERE.

One last thought on this subject, there are rare cases of roosters reportedly also being able to undergo sex reversal — thereby becoming hens and even laying eggs. The cases of rooster to hen sex reversal is so extremely rare that it is not fully understood and is a topic that is still hotly debated.

Have you ever heard a hen crowing? Let us know.


Originally published in 2014 and regularly vetted for accuracy.

  • We have backyard chickens and our 3yr old Welsumer has started crowing. She is still producing eggs. Is there a way to raise her estrogen level or is just inevitable that she will transform

  • After my rooster was killed, one on my hens started crowing and even mounting several hens. She was always a big girl to begin with too. I’ve heard that this can happen when a hen takes on the role of the rooster as the new protector, but the fact that she mounted them was weird because I wondered what her intentions were…just to dominate?

  • This seems to be somewhat common in flocks that lose their roosters. A hen will start behaving roosterishly and stop laying. I wonder if there is a predisposition to the condition that responds to an environmental trigger.

  • I’ve had at least 4 hens do this over the years. I have one old, nearly blind 9 year old hen in the house who is doing it now. Her hackle and saddle feathers are turning white too, not sure if that an age thing or sex change. She’s a Buff Orpington.

  • Thank you so much for this article! I thought I was imagining things when this happened in my flock. I had sorted out and sold all of the roosters, but then weeks later, we had a rooster! The hen even developed the skirt and tail feathers of a rooster. Thank you for confirming this phenomenon!

  • This explains so much about our last two backyard hens. One was aggressive towards people – would not submit, and would actually turn to face whoever walked up to her. She had longer saddle feathers and made crowing style noises regularly. We got the hens as young adults, and I suspected from day one that the previous owners had sexed her incorrectly and given us a rooster. Nope, Delaware breed, which is sex-linked. She did lay an egg every day, and did not display any other male characteristics. Maybe this is what happens if they have a hormonal imbalance?

  • Yes my wife has had two he is that crowed. We videotape d the later one.

  • I have a Buff Orpington hen that will crow in the winter during her molt. She does lay eggs for me still but my rooster is not particularly nice to her even tho she will squat for him. She has done this over the past two winters. It always freaks me out at first when I hear a crow I don’t recognize in my barn. I took this hen as a rescue so I really don’t know how old she is but I have had her for 4 years myself. A crazy thing-for her and me!

  • My hen crows occasionally, but she is the only chicken we have. She was the only egg that hatched out of 11 incubated, and we raised her from hatching. Other than crowing she shows no other rooster characteristics. She’s very friendly, and even squats sometimes when I pet her back. And she lays at least one egg a day for me. I think maybe she’s just lonely. Also we have other people in our neighborhood who have roosters and can her them crowing, she may be trying to get their attention or just mimic them. I’m only guessing here

  • I had 10 roosters for 5 years when the ordinance in my city changed and I had to re-home them. Within a few days of them leaving, my buff cochin hen started flapping her wings and crowing loudly every morning. I was shocked! I had to put her in the house so I wouldn’t get in trouble! Soon my Australorp hen started mounting another hen! I also have a Brahma hen that crows occasionally too. Got to love chickens!

  • My black sex linked hen TOTALLY crows! And exhibits other male characteristics to include aggressive roo behaviors, nubs where spurs would be at the moment, but she also struts like a roo. We have separatedone her from the rooster and other hens currently because she attacks them.

  • Yes we have one. Our lovely little Hamburg who used to be aggressive, has grown two amazing spurs, stopped laying (she is 4, so I know she is a hen), and crows every day for the past 5 weeks. She crows during the day if the hens are squarking and spends her days sitting in the coop whilst the others are in the nesting boxes. She’s not aggressive anymore, if anything she is very passive. Sadly my area is a no go for crowing, so I need to get something sorted.

  • We had what we thought was a rooster. When growing it had more masculine looking comb and it crowed. Well this morning we caught her laying eggs! 2/day at that! Such an odd little bird. Lol

  • Oh wow interesting read, my little Ladybird started crowing a few weeks ago, her baby was sitting on eggs and we swapped them for chicks cause we are in town and do not have a rooster. Guess she is now our rooster.

  • Has anyone found anything that helped? We just brought our two hens (who have always had roosters around before) to a new town where they are now “urban chickens”. Roosters are not allowed here. One, Dot, has slowly started crowing. She’s gotten better and better at it, and now she’s starting to develop male traits like pointed neck and saddle feathers and a few longer iridescent tail feathers. She has spur nubs that are growing. She’s four years old and laid everyday when she was younger. (She’s been a free range chicken without supervision for the last year and a half, so we don’t know what her laying has been like more recently.)

    So far, we have tried a No Crow collar. We’ve tried separating her from the other hen. We’ve tried putting her in a very low-ceilinged cage so she can’t stand up fully. In the short cage, she still crows by stretching her neck horizontally. My husband thinks the collar and the short cage reduce her volume somewhat but I don’t really notice a difference. We now put her in the garage at night which is dark so she usually doesn’t start crowing until we take her out again in the morning.

    It’s been cold and neither of our hens are laying yet. We found one egg, that looked like Dot’s a few weeks ago, but no more.

    Some anonymous neighbor has already complained to Animal Control. Animal Control has asked us to take her to an avian vet to see if they will certify that she is indeed a she. We have an appointment on Saturday.

    Depending on expense and chances of success we might a) ask about an oophorectomy of the offending right ovary (ovotestis now?), b) try to find a farm with roosters to board her for several months to try to change her back, c) look into hormone treatment??, d) re-home her. I’m really hoping to avoid option d.

    Does anyone have any stories about whether they succeeded or failed at turning their hooster back into a hen?


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