In the forests of Pennsylvania, biologists have managed to catch an incredibly rare bird that’s genetically part male and part female, split down the middle and displaying the vibrantly colored characteristics of both sexes.
Researchers from the Carnegie Museum of Natural History caught and banded the unique bird on September 24 at the Powdermill Nature Reserve in Rector, Pennsylvania, according to a statement from the museum.
It belongs to a species known as the rose-breasted grosbeak (Pheucticus ludovicianus), a large seed-eating member of the cardinal family found throughout much of eastern North America. The male of this species is known for its richly colored plumage, but this individual has especially distinctive colored feathers that differ on each side of its body: the right side is rosy red like a male, while the left is brown-orange like a female.
This is because the animal is a rare example of bilateral gynandromorphism, where an animals' external appearance is split down the middle by sex, half-male and half-female.
“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,” Annie Lindsay, bird banding program manager at Powdermill, said. “Bilateral gynandromorphism, while very uncommon, is normal and provides an excellent example of a fascinating genetic process that few people ever encounter.”
You’re no doubt wondering, how does this rare phenomenon occur?
Well, first things first, sex determination in birds is a little bit different from humans. In humans, the females have two copies of the same sex chromosome (XX) and the males have a copy of each (XY), but it’s essentially the opposite away around in birds. The males have a double sex chromosome (ZZ) and the females have one of each (ZW).
Gynandromorphy is thought to occur for different reasons in different species, but for birds, it’s believed to occur when an egg accidentally develops with two nuclei, one carrying a Z and the other a W. If that egg is fertilized by sperm carrying two male Z chromosomes, the egg develops with both ZZ (male) and ZW (female) chromosomes.
The team who discovered this recent gynandromorph bird is now curious to see whether it can successfully breed. Since only the left ovary is typically functional in birds and the left side of this bird is the female side, it's theoretically possible for the individual to breed with a male. However, there's a chance its unusual feathers might trigger a territorial response from other males, which would dampen its chances of successfully courting.
Although the odds of gynandromorphy occurring are exceptionally slim, bilateral gynandromorphism has been seen in a number of different animals. Back in early 2019, a couple (also in Pennsylvania) spotted a northern cardinal (Cardinalis cardinalis) with gynandromorphy that was half-brown and half-red. Earlier this year, scientists also documented the discovery of a gynandromorph bee whose left side – the male side – features a long antenna and smoother mandible, but the right side – the female side – featured a short antenna, a spikey mandible, and a chunky hind leg.