Gynandromorph Rose-breasted Grosbeak

Just for the record, and from WESA Pittsburgh, we have a gynandromorph Rose-breasted Grosbeak  (Pheucticus ludovicianus): a bird that’s part male and part female. In this case the bird appears to be largely, but not completely, divided down the middle, similar to the gynandromorph Northern cardinal I wrote about in 2012.

A rare bird has been found at Powdermill Nature Reserve in Westmoreland County.

The newly banded Rose-breasted Grosbeak is a gynandromorph, meaning that it is part male and part female. This particular Grosbeak is male on the right side and female on the left, making it a bilateral gynandromorph.

Researchers at the Carnegie Museum of Natural History said less than 10 bilateral gynandromorph birds have been documented in the reserve’s 64-year bird banding history. The reserve’s only other documented Rose-breasted Grosbeak bilateral gynandromorph was banded in 2005.

Annie Lindsay, Powdermill’s bird banding program manager, said finding the gynandromorph is a “once-in-a-lifetime experience.”

“One [of the banding team members] described it as ‘seeing a unicorn’ and another described the adrenaline rush of seeing something so remarkable. They all are incredibly grateful to be part of such a noteworthy and interesting banding record,” said Lindsay in a press release.

The fact the bird is a gynandromorph is discernible [sic] to the naked eye as it has physical traits of both male and female Grosbeaks. On the right, male side of its body, it has ruby wing pits and a ruby breast spot, along with black wing feathers. On the left it has yellow wing pits and a brownish, speckled wing.

At first, the color appears split down the middle insofar as the “wingpits” and breast color are concerned (see photos below of normal male and female), but the head of the bird shows no black on the male side, which it should if this was a truly “split” gynandromorph like the cardinal. Even young males have darker heads, but this bird has a full female head. Ergo, it appears to be a “more-than-half female” grosbeak.  Researchers are waiting to see if it acts like a female or male; that is, can it produce eggs? Will it sing a male song? My prediction is that if the head is female, the chances are higher that the brain is female, and it will act like a female—if it can find a mate.

This Rose-breasted Grosbeak gynandromorph bird possesses both male and female physical traits, including different colored wing pits. Male Grosbeak have reddish pits, while females have yellow. ANNIE LINDSAY / CARNEGIE MUSEUM OF NATURAL HISTORY

Normal male:

Normal female:

How do these part-male/part-female birds form? I discuss possibilities on the gynandromorph cardinal post, and, in the comments, readers suggest some other possibilities, but we don’t know for sure.  It could involve chromosome loss, a non-genetic developmental accident, fertilization by “unreduced” sperm, and so on. Looking at the chromosomes on the male versus female parts of the bird might give a hint.

h/t: Bruce Lyon


  1. GBJames
    Posted October 9, 2020 at 9:15 am | Permalink

    Woke gender theorists will love this one!

  2. jezgrove
    Posted October 9, 2020 at 9:18 am | Permalink

    Fascinating – and a beautiful looking bird!

  3. ThyroidPlanet
    Posted October 9, 2020 at 9:34 am | Permalink


  4. AD
    Posted October 9, 2020 at 9:55 am | Permalink

    Possible typo “changes are higher that the brain is female” should be “chances are higher”

  5. Randall Schenck
    Posted October 9, 2020 at 10:06 am | Permalink

    Another thing that might be interesting is how other bird of the species act around this one.

  6. Paul Matthews
    Posted October 9, 2020 at 10:14 am | Permalink

    Hmm. This bird just seems really weird to me. The head pattern of females and young males of this species are pretty similar. I’m not completely sure that the head on the “male side” is female. But the streaks on the upper breast of the male side show a female pattern despite the incipient red patch —- young males have far fewer streaks. And what’s with all the white flight feathers on the wing of the female side? These would be brown in a female.

    This species frequently hybridizes with the closely related Black-headed Grosbeak (Pheucticus melanocephalus). I was wondering whether this could be a hybrid, but that doesn’t fit either.

    • Nicolaas Stempels
      Posted October 9, 2020 at 12:19 pm | Permalink

      Could be a mosaic?

  7. Keith
    Posted October 9, 2020 at 10:20 am | Permalink

    Such a beautiful bird! And really fascinating developmental biology.

  8. Posted October 9, 2020 at 10:27 am | Permalink

    Very cool, thanks!

  9. Mark R.
    Posted October 9, 2020 at 12:38 pm | Permalink

    Amazing. That would be an exciting find.

  10. Posted October 9, 2020 at 12:43 pm | Permalink

    Cool! Maybe it can develop eggs, but laying them would be another matter if its reproductive track is part male.

  11. Posted October 9, 2020 at 2:11 pm | Permalink

    Can this happen in mammals, or is there something about bird genetics that makes it more likely for them?

  12. Posted October 9, 2020 at 8:13 pm | Permalink

    Very interesting!

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