Scientists from the Powdermill Nature Reserve, part of the Carnegie Museum of Natural History in Pittsburgh, caught and banded the rare Rose-breasted Grosbeak in September.
The bird is a gynandromorph, which is any organism that contains both male and female characteristics. In this case, the bird is split roughly down the middle with male characteristics on the right and female on the left.
The sexes of Rose-breasted Grosbeaks can be determined by their colour, as the male has pink “wing pits” and black wing feathers, while the female has yellow wing pits and browner wing feathers.
The rare songbird found by researchers has the male pink wing pits, breast spot and black wing feathers on its right side, while its left side displays the female yellow wing pits and brown wing feathers.
According to Powdermill’s Avian Research Centre, it has recorded fewer than 10 bilateral gynandromorphs in its 64 years of bird banding.
Annie Lindsay, a manager in the bird banding program, said: “The entire banding team was very excited to see such a rarity up close, and are riding the high of this once-in-a-lifetime experience.
“One of them 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.
“Bilateral gynandromorphism, while very uncommon, is normal and provides an excellent example of a fascinating genetic process that few people ever encounter.”
Whether or not this unusual bird can reproduce is unclear, but possible in theory. Female birds have only one functional ovary, which is typically located in the left side of the body - as this rare bird’s left side is the female side, it could theoretically reproduce if it successfully mates with a male.
Its ability to reproduce could also depend on whether it sings like a male. If it does, it could potentially attract females but provoke a territorial response from other males, said the researchers.
Gynandromorphy is not uncommon in birds and species of insects, crustaceans, and snakes. Since not all bird species have significantly different male and female plumage, there could be more gynandromorphic birds out there than is known.
But finding a gynandromorph that shows distinct differences in the appearance of the male and female side is nearly one in a million, Stephen Rogers, the birds collection manager at the museum told National Geographic.