A few weeks ago, Ms Caldwell was looking out her kitchen window and noticed something unusual in the dawn redwood tree, Metasequoia glyptostroboides, standing at the corner of her property in Erie, Pennsylvania: a northern cardinal, Cardinalis cardinalis, with a truly astounding color pattern. When the bird faced one direction, it was a male, cloaked in all his scarlet finery, but when it faced the opposite direction, it was a female, easily identified by her subdued tan plumage. But when this bird faced the Caldwells, it was half red and half tan; its colors divided lengthwise down its middle. It was almost as if two birds, one male and the other female, had been sᴘʟɪᴛ ɪɴ ʜᴀʟf and the halves had been neatly stitched together.
Impossible! Well, except … this is what happened. In birds, s.ᴇ x is determined by s.ᴇ x chromosomes, just as in mammals. But unlike mammals, where females are XX and males are XY, female birds are ZW whilst males are ZZ. So the s.ᴇ x chromosome — either W or Z — contained in each avian ᴏᴠᴜᴍ determines the resulting chick’s s.ᴇ x.
Thus, this peculiar bird is the product of male and female fraternal twin ᴇᴍʙʀʏᴏs, resulting from two different ova fᴇʀᴛɪʟɪᴢᴇᴅ by two different sᴘᴇʀᴍs. Somewhere between the 2-cell and the 64-cell stage of development, these male and female ᴇᴍʙʀʏᴏs that had been developing alongside each other inside the same eggsʜᴇʟʟ ceased to develop separately and fused into just one ᴇᴍʙʀʏᴏ. This bizarre bird is that ᴇᴍʙʀʏᴏ — all grown up. It exemplifies an unusual phenomenon, a fascinating developᴍᴇɴᴛᴀʟ mistake, known in scientific circles as a bilateral gynandromorph, and amongst veterinarians and pet bird breeders — and even by some bird watchers — as a half-sider. Because northern cardinals are a s.ᴇ xually dimorphic species, where males are scarlet and females are tan, it was easy to see that this peculiar bird is both male and female. (Recognizing a bilateral gynandromorph is almost impossible when looking at species where males and females appear identical.)
Annie Lindsay, the bird banding program manager at the reserve in Rector, Pa., remembers getting the call on her walkie-talkie about the rare sighting and rushing to see it first-hand.
Everyone was buzzing with excitement about the rare and beautiful creature, but had to keep their wits about them so they could follow their procedures of tagging it, taking measurements, and quickly releasing it back into the wild.
One researcher compared the fleeting moment to “‘seeing a unicorn.” Another described feeling an incredible adrenaline rush.
“We only had it in the hand for a few minutes and then we released it,” Lindsay told As It Happens host Carol Off. “It was incredibly rare to see this. So we were all very, very excited.”
In Powdermill’s 60-year history, they have recorded more than 800,000 birds. But they’ve only come across five gynandromorphs — at least, that they know of.
“We would only be able to detect that if it’s a species that the males and females look different,” Lindsay said. “There are a lot of birds that we catch here that the males and females look the same, and so we wouldn’t necessarily be able to detect that.”
In the case of the rose-ʙʀᴇᴀsᴛed grosbeak, males have a rose-red lining on the underside of their wings, while females sport a yellow colour. This particular bird had one of each.
“The pink side is ɢᴇɴᴇᴛɪᴄally male, and that yellow side is ɢᴇɴᴇᴛɪᴄally female,” Lindsay said.
Whether a gynandromorph can ᴍᴀᴛᴇ depends on a number of different factors. In this case, the researchers don’t know enough about the bird to say for sure.
But Lindsay suspects it’s likely it could ᴍᴀᴛᴇ with male rose-ʙʀᴇᴀsᴛed grosbeak, as its female side is where its ᴏᴠᴀʀʏ would be.
“So if this bird does have a functional left ᴏᴠᴀʀʏ, it’s possible that it could find a male ᴍᴀᴛᴇ and reproduce that way,” she said. “There are some considerations, though. The bird would have to act like a female. Its behaviour would have to be female-like in order to attract a male ᴍᴀᴛᴇ. And so we don’t know if that’s how the bird behaves.”
In 2019, As It Happens interviewed birdwatcher Shirley Caldwell, who captured photos of a gynandromorph cardinal in Erie, Pa., with colouring split right down the middle — bright red male on its right side and a muted yellowish brown female on the left.
That particular bird did appear to have a male paramour.
“When the gynandromorph does leave, the male follows with it,” Caldwell said at the time. “I always see them together.”