Halfsider: a bizarre half-male half-female bird

What is it like to live as opposite-sex twins that are fused into one living being? A paper shares observations of the behaviour and social life of one such bird living in the wild

by GrrlScientist for The Guardian | @GrrlScientist

β€œHalfsider” Northern cardinal, Cardinalis cardinalis. (Female plumage on the bird’s right side, male plumage on the left.) Photographed sometime between December 2008 and March 2010 in Rock Island, Illinois. (doi: 10.1676/14–025.1). Photograph: Brian Peer & RobertΒ Motz

Imagine looking out your window one morning and seeing a bird at your feeding table that looks as if a male and female of the species had been cut in half lengthwise and two opposite-sex sides had been carefully sewn together to create one individual. Perhaps you’d suspect a prank; maybe a local artist had skilfully painted the plumage on one half of a female bird to look like a male of the species? Or perhaps you’d start worrying about what sorts of illegal mind-bending substances might have been added to your food or drink?

After you became convinced that such a bird was not a product of your imagination, you might start to wonder what its life is like. Are these half male-half female birds confused? What might it feel like to go through life as a β€œhalfsider”?

Such β€œhalfsider” birds are occasionally seen, but long-term observations of the behaviours and social life of a bilateral gynandromorphβ€Šβ€”β€Šas they are more properly knownβ€Šβ€”β€Šare rarer than the birds themselves. In a published paper, two birders, Brian Peer, a Professor of Ecology and Curator of Birds and Mammals at Western Illinois University, and Robert Motz, share their long-term observations of a free-living β€œhalfsider” Northern cardinal, Cardinalis cardinalis, made during more than 40 nonconsecutive days between December 2008 and March 2010.

β€œOur observations are among the most extensive of a bilateral gynandromorph bird in the wild”, Professor Peer and Mr Motz write in their paper.

The team report that the cardinal never appeared to pair up, nor did they ever hear it sing. Nor did the bird respond aggressively to recorded Northern cardinal songs that were played to it. Yet despite this bird’s seemingly solitary and silent life and bizarre appearance, Professor Peer and Mr Motz never observed its flock mates behaving aggressively towards it.

This contrasts with behaviours observed for a bilateral gynandromorphic zebra finch, Taeniopygia guttata, that popped up in a university laboratory more than a decade ago (doi: 10.1073/pnas.0636925100).

Figure 1. Zebra finch gynandromorph (A) with male plumage on its right side (B) and female plumage on its left side ( C ). A few black male feathers can be seen on the left breast in A. (doi: 10.1073/pnas.0636925100)

In that paper, the authors report that their gynandromorph finch sang a typical song and a female it was paired with produced infertile eggs. The researchers also reported that their zebra finch gynandromorph was attacked when housed with other males. But there is one important difference between that finch and this cardinal: that zebra finch was genetically male on its right side and genetically female on its leftβ€Šβ€”β€Šimportant when considering avian physiology and morphology (read a discussion of why that difference is important in birds).

Professor Peer and Mr Motz were unsuccessful when they tried to capture the β€œhalfsider” cardinal to obtain blood and tissue samples for further study. But previously published research on bilateral gynadromorph chickens found mostly male cells in the side with male plumage and mostly female cells in the side with female plumage (read more).

Bilateral gynadromorphs result from an error during early embryonic development, when two embryosβ€Šβ€”β€Štwinsβ€Šβ€”β€Šfuse into one individual. These twin embryos may either be the opposite sex or the same sex, but this phenomenon is only visible in species where males and females are visibly distinct, or when the two fused embryos have differently coloured plumage.

In halfsider birds, cells on each side of the fused embryo develop based on their chromosomal makeup, regardless of the hormonal milieu. In contrast, human embryos develop based upon the hormonal milieu that their cells are exposed to, regardless of their chromosomal makeup. For this reason, gynandromorphism doesn’t occur in humans or other mammals. In addition to birds, bilateral gynadromorphs sometimes pop up in a variety of spineless creatures (crustaceans, arachnids, and insects).


Peer B.D. & R.W. Motz (2014). Observations of a Bilateral Gynandromorph Northern Cardinal (Cardinalis cardinalis), The Wilson Journal of Ornithology, 126 (4):778–781 | doi: 10.1676/14–025.1

Agate R.J., J. Wade, S. Mann, J.C. Wingfield, C. Schanen, A. Palotie & A.P. Arnold (2003). Neural, not gonadal, origin of brain sex differences in a gynandromorphic finch, Proceedings of the National Academy of Sciences, 100 (8):4873–4878 | doi: 10.1073/pnas.0636925100

Additional information:

Chue J. & C.A. Smith (2011). Sex determination and sexual differentiation in the avian model, FEBS Journal, 278 (7):1027–1034. doi: 10.1111/j.1742–4658.2011.08032.x [OA PDF]

Gender-Bending Chickens: Mixed, Not Scrambledβ€” an in-depth discussion of the developmental process that creates an avian bilateral gynandromorph.

Half-siders: A tale of two birdiesβ€Šβ€”β€Šdiscusses a pet budgerigar bilateral gynadromorph (includes several videos and photographs) and also mentions gynandromorphs that have been observed in other groups of animals.

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Originally published at The Guardian on 5 January 2015.