[rossdavidh]

"At first Mr Hill wondered if the bird was leucistic - a term that means the specimen would have a loss of pigmentation in its feathers, but would not be half-female, half-male.

But after seeing mobile phone pictures, he suspected it had what is called bilateral gynandromorphism, which is when a bird would have both a functioning ovary and a functioning single testis."

So, uh, it would have been interesting if they had mentioned WHY the ornithologist thought it was gynandromorphic rather than leucistic? I'm not an ornithologist but it seems like a pigment mutation is a simpler explanation...

[souprock]

Yep. Unless you check the gonads and the DNA on both sides, any explanation will be uncertain.

I note that birds use the WZ sex system, which is like the opposite of the mammalian XY sex system. Males are ZZ. Selective silencing of genes on the Z chromosome for dosage compensation could cause spacial variation across the body. We see something like this in the XY sex system with Barr bodies, leading to coloration differences such as those of the calico cat.

It's also possible to have a plain old male-male chimera, or a plain old mutation, with the cells in one part failing to produce the proper color.

[sp332]

Leucistic cardinals don't just look like female cardinals. https://lostpineslife.com/leucistic-northern-cardinal-winter...

[elmomle]

I suspect it had to do with sex-associated traits were split down the middle. I'm only an amateur, but I can see that the right side has a male's long tail feathers while the left side does not. I imagine there are other details that would be apparent to an expert.

[_cloudkate]

Came here to say something similar. It seems fairly obvious (to an amateur like me, I guess) that the lack of color on one side would simply be from a pigmentation issue. But the description of the event by the photographer as a "once-in-a-lifetime, one-in-a-million encounter" makes it seem like there's a very clear reason he narrowed out pigmentation.