Can birds change sex? The curious story of Mary/Marty the duck…

Can birds change sex? The curious story of Mary/Marty the duck... thumbnail
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Can birds change sex? When one of our pet female ducks began to look and act like a male we set out to find the answer. Here’s the curious story of Mary/Marty the duck and the explanation for how birds can sometimes appear to change sex!

In our article, How to tell boy and girl ducks & ducklings apart, we detail how to differentiate between male and female ducks at various stages of development. We also made a clear distinction between sex and gender, which we’ll do again here for clarity’s sake: 

“Sex” refers to reproductive organs. “Gender” refers to a critter’s sexual identification, e.g. primarily a function of the brain. Confusingly, these terms are often used interchangeably even though they’re quite different, biologically speaking. 

Now that we’re clear about sex vs. gender terminology, let’s continue forward…

Primary vs secondary sex characteristics

As in other vertebrates, birds have primary sex characteristics (which are universal) and secondary sex characteristics which may vary by species and even breed (in domesticated birds).

In ducks — as in other vertebrates — the primary sex characteristic is their internal and external genitalia. For example, males have testes and females have ovaries. Same with humans. 

Secondary sex characteristics in our Welsh Harlequin ducks include:

  • Adult males (aka drakes) have iridescent green head feathers/coloration when they’re in their nuptial plumage; females do not. 
  • Drakes have orange feet and legs; females have grey/brown feet and legs.
  • Females quack loudly; adult males make a raspy low-pitched sound. 
  • During nuptial plumage, drakes have curled “drake feathers” on the top of their tails; females do not. 
Young male/drake Welsh Harlequin ducks from our very first flock.

Young male/drake Welsh Harlequin ducks from our very first flock.

Thus, it’s pretty easy to sex adult ducks with your ears and eyes (no hands required)! This is similar to how you intuitively “sex” new humans you encounter visually by observing secondary sex characteristics such as presence of facial hair, deep voice, breast shape/size, etc.

However, as we’ll discuss below, things aren’t always as simple as they seem on the surface… 

What causes a bird to become male or female? 

In humans, XY chromosomes determine your sex. XX = female; XY = male. In humans, the default sex is female

In birds (including ducks), ZW chromosomes determine sex. ZZ = male; ZW = female. The default sex in birds is male. 

In birds/ducks, sexual differentiation is controlled genetically and initiated in embryonic life. Exactly when is the sex of an individual bird determined? According to researchers, “the sex of the offspring is determined just before ovulation during the first meiotic division.”

This process happens via the excretion of sex steroid hormones which either masculinize or feminize the body. Obviously, these effects persist after hatching and as the birds develop into sexual maturity. 

An interesting side note: some bird species engage in sex ratio adjustment/manipulation, meaning rather than producing roughly 50%/50% male/female offspring, they can produce more or less of a particular sex from breeding season to breeding season. Potential reasons for this phenomena are complex and not fully understood.

Equally interesting, on rare occasions, some individual birds are even gynandromorphic, meaning they are both male and female. In such cases, the secondary sex characteristics are split right down the middle of their bodies — one side of the body appears male, the other female. Nature! 

Can birds change sex? 

Ok, now onto the curious case of Mary/Marty, the transsexual duck (not from Transylvania)…  

We’ve lived with a flock of domesticated Welsh Harlequin ducks for a decade (with both male and female flockmates). Over that time period, our ducks have provided us with quite a bit of education into the strange, mysterious, and hilarious world of ducks. 

Just when we think we’ve seen it all, our ducks throw us a curveball (a figure of speech, not a testicle). Case in point: Marigold, one of our female Welsh Harlequins… We shortened her name to Mary, but imagined she had trouble pronouncing r’s so changed her name again to Mawy, pronounced Ma-wee. She doesn’t mind what we call her so long as we feed her tomatoes and fresh greens. I digress… 

Three years ago, Mawy was late “turning off” her egg maker at the end of egg laying season. Wild Mallards might produce 24 eggs in a year, but domesticated ducks can produce hundreds of eggs. In this case, Mawy was likely approaching 300+ days straight of egg production, which is an enormous energy and nutrient tax on a duck’s body. 

(See: What to feed backyard ducks to optimize their health and longevity.) 

Thus, we finally made Mawy go broody to stop egg production. We didn’t know it at the time, but that would be the last year she ever laid eggs. 

In the years since, we’ve noticed that Mawy took on some interesting features that are typically only seen in Welsh Harlequin drakes. For instance:

  • her feet/legs turned orange,
  • some of her head feathers turned iridescent green, and
  • she even grew some curly drake feathers on her tail! 
Mawy the duck with arrows showing some of her male features: drake curl, orange feet, green head plumage.

Mawy the duck with arrows showing some of her male features: drake curl, orange feet, green head plumage.

Mawy and our drake, Sir Winston Duckbill aka Winnie the Screw, also began to fight each other at every opportunity. For reference, when we’ve had multiple drakes, they constantly fight each other during breeding season, but our drakes and females never fight each other.

Unfortunately for Winnie the Screw, Mawy has a 2+ lb weight advantage, so their scuffles usually ended in his defeat. (We’ve since separated Winnie into his own enclosure during the day to maintain more peaceful flock dynamics.) 

Side note: our female ducks (including Mawy) frequently have sex with each other, so sexual activity is not helpful in sexing a duck. 

What the duck? Is Mary the duck a boy or a girl? 

We set out to solve the biological mystery of Mary the duck. Based on what we’ve read about birds/ducks from research literature and biologists, we think we may have solved the riddle… 

Ducks have one ovary, and when Mary stopped laying eggs a few years back, it was likely because her ovary was somehow damaged. No functional ovary means no oestrogen release. 

Since there’s nothing suppressing the male genes on Mary’s Z chromosome, she’s at least partially reverting to the default sex in ducks/birds: male. Biologists have seen this happen to birds in nature as well, with some female birds even developing testis once their vestigial ovary masculinises. According to the BBC, in at least one very unusual outlier case, a previously female chicken became a male and sired viable offspring. Nature strikes again!   

Mawy with her female Welsh Harlequin friends.

Mawy with her female Welsh Harlequin friends.

It’s also interesting to note that since male is the default sex in birds/poultry, it’s likely impossible for a male bird to ever become a female. Avian sex transitions are a rare, one-way street.

In Marigold the Duck’s case, she is still a female based on her genes/chromosomes, even though hormonal shifts are causing her to take on secondary sex characteristics typical of a male. As a paper titled Sex Reversal in Birds put it: “Increasingly, it is being recognised that sex reversal is not a clear cut phenomenon but can reflect a partial decoupling of sexual phenotypes at various levels (chromosomal sex, gonadal sex, brain sex, plumage, or other sexually dimorphic aspects of anatomy).” 

So in answer to the question “can birds change sex?”: yes, but it’s quite rare. Still rare (but more frequent than full sex changes) hormonal changes cause female birds’ secondary sex characteristics to change to male secondary sex characteristics.  

Should we change Mawy’s name to Marty (or Mawty – cursed r’s!)? She doesn’t seem to care either way, so long as we keep feeding her fresh garden greens and tomatoes. 

Mary/Marty the transsexual duck looking even more like a drake in spring 2022.

Mary/Marty the transsexual duck looking even more like a drake in spring 2022.

Other poultry parents chime in

We posted an abridged version of Mary’s story to our facebook page and Instagram and were surprised to hear from quite a few other chicken and duck parents who had similar experiences… 

Here’s what others had to say: 

  1. “So cool! Thinking about my hen that crows. She does [still lay eggs]. It’s just a dominant hen taking on some of the male roles in my flock with no cocks. They also mount each other from time to time.” 
  2. “Okay, this confirms it for me! Two of my Harlies have also developed orange feet and stopped laying eggs My question is, what could have damaged their ovaries? We did have an unfortunate incident involving the duck flock and an azalea bush.” 
  3. “When one of my first adopted hens stopped laying (age unknown, but we had her for about three+ years) she began acting like a Roo – tidbiting, weirdo crow.”
  4. “I had the same thing happen with one of my runner ducks!”  
  5. “We’ve had 2 Harlequins in the past 4 years do this. I thought I was losing my mind (hubby thought so too, or that I was just full of (s***). But not, it really happens!” 
  6. “A similar thing happens in chickens. We went through it with one of our hens… she stopped laying, started crowing, grew streamer feathers like a rooster, and began to fight with our other rooster. Biology is so fascinating!”
  7. “This is Eddie Gizzard [referencing photo], a duck I adopted from an egg farm after the exact same thing you described happened! They stopped laying eggs and developed a gorgeous green stripe, their feet turned orange, the tail curl appeared, and their voice changed to somewhere between a quack and a raspy sound. Nature is full of such gorgeous diversity.” (The duck is 2 1/2 years old.)
A photo of

A photo of “Eddie Gizzard” (mentioned above) shared by Rob Reed on facebook. Here again you can clearly see male features on a female duck.

Are sex changes more common in domesticated poultry than wild birds?

Domesticated poultry are bred and fed for high egg production, which increases the likelihood of ovarian damage. On average, domesticated ducks (or other fowl) also likely live longer than their wild counterparts. 

Given these circumstances, we’d hypothesize that sex changes in domesticated poultry likely occur at a much higher rate than might be seen in wild birds. This hypothesis is based on our own experience and the accounts of other poultry owners we’ve talked to, e.g. it’s anecdotal evidence not an actual statistical analysis. 

However, perhaps the story of Mawy (or Mawty) might give ornithologists and biologists an opportunity to better study the dynamics of sex changes in birds. Vewy intwesting!

If you have a bird(s) exhibiting the signs of a possible sex change, please share details of your story in the comments!  


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  • Reply
    Sophie Laurie
    June 3, 2023 at 5:53 pm

    I have a white Campbell duck who has stopped laying and developed a curly tail. I also have a goose that I always assumed was make from his plumage and who used to exhibit male behaviour. This year I separated him out from the flock as he was being bullied and noticed that he was laying eggs – just two or three. I gave him some to incubate as he went broody and he has just hatched out two goslings!

    • Reply
      Aaron von Frank
      June 4, 2023 at 7:01 am

      Woah, so interesting – especially your gander! Thanks for sharing.

  • Reply
    Joan Wakefield
    November 26, 2022 at 12:58 pm

    I have a white duck that developed curly tail feathers two months ago. She is one of a dozen White Layer Ducks I purchased from Metzger Farms about ten years ago. I recently noticed that her head and neck are bigger than they used to be and her chest is bulking up, too. I know this is not my imagination because it’s easy to compare her with her sisters, who are still quite feminine. Just today, I noticed that her voice is changing.

    • Reply
      Aaron von Frank
      November 27, 2022 at 7:39 am

      Interesting! It sounds like you might be experiencing the same thing as we are with our Mary/Marty duck. Marty is now by far our largest duck and she presently looks far manlier than our drake! Do you happen to know if she (your White Layer) still produces any eggs – or how long it’s been since she did produce eggs?

    • Reply
      November 28, 2022 at 8:40 am

      Us too, Joan! One of our Blue Swedish who is about 2 years old. She’s always been the smallest of the bunch (so much so that I have had to separate her from the drake at times). One of our other females died this summer and suddenly “Unicorn” seemed much more hardy. At first I wondered if the other duck hadn’t been letting her eat, but now she is starting to fight with other ducks. None of them have laid eggs since about September, so we’ll see what spring brings!

  • Reply
    Rick Dog
    August 29, 2022 at 1:15 pm

    My Welsh has pretty royal blue feathers on it’s wings and I am hoping its a girl. But I thought only males had efflorescent feathers?
    We had a hen that looked petite and started setting on other hens eggs and protected them like a maniac. She hadn’t laid any eggs yet. She didn’t look like a rooster or any other hens, but 1 day she jumped into the air10 or 15 feet and crowed like a sick rooster! I told my grandma about it and she said that her little mommy would chop its head off haha So Idid and practiced butchering a chicken! I didn’t get very far!

    • Reply
      Aaron von Frank
      August 30, 2022 at 10:26 am

      Female Welsh Harlequins have a gorgeous blue-purple wing bar. Males do as well. Generally speaking, male Welshies tend to have more ruddy feather colorations on their chests, although females can take on similar colorations, especially when they’re in their eclipse plumage. It’s pretty easy to distinguish between male and female Welsh Harlequins when they’re in their nuptial plumage, since: 1) the males develop their bright green head/neck colorations and females don’t, and 2) males develop a drake curl on the top of their tail. However, with our transexual duck Mary/Marty, someone who wasn’t familiar with her situation would swear she’s a drake because she has a green head/neck and even has a drake curl atop her tail. Visually, she looks exactly like a drake. She still sounds like a female with her loud quack vs the typical raspy blurp-blurp-blurp sound that drakes make.

      Interesting story about your chicken!

  • Reply
    Robin Nolan
    August 14, 2022 at 7:57 pm

    Hi Aaron, I think this is happening to my duck, Cricket, right now! She’s a two year old Welsh Harlequin who suffered an egg-related infection earlier this summer. Bloodwork suggested salpingitis, which we treated with antibiotics. She stopped laying eggs about two months ago, which I assumed was due to her fighting infection and then molting. Today, when I saw her standing next to my other WH duck and drake, I was struck by how dramatically her coloring has changed. Her feet are becoming orange, flecks of dark green feathers are emerging on her head, and a distinct band of white is forming around her neck. I remembered reading this article last year so I came back, reread it, and yep, all the signs seem to be there!

    • Reply
      Aaron von Frank
      August 21, 2022 at 5:27 pm

      How interesting, thanks for sharing, Robin! Perhaps so, but it may be until next spring or so before you know for sure. When they molt and go into their eclipse plumage, Welsh Harlequin drakes and ducks/hens can look pretty similar. If she’s still not laying or looking more like a female by next spring, you might well have a transexual duck on your hands — or whatever terminology should be applied to a female duck (based on chromosomes/genes) who is displaying the secondary sex characteristics of a drake. Please keep us posted!

  • Reply
    August 2, 2021 at 12:06 am

    This happened to us with our French Rouen. When she was 12 years old, she stopped laying eggs and all her colors changed to that of a male Rouen. I just thought it was duck menopause at the time. She lived to be 17 years old.

    • Reply
      Aaron von Frank
      August 2, 2021 at 7:25 am

      How interesting! Thanks for sharing, Mary. Unrelated, but that’s one of the oldest ducks we’ve heard about.

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