Researchers with a team monitoring bird populations at Powdermill Nature Reserve, in Rector, Pennsylvania, netted a surprise on September 24: a rose-breasted grosbeak with bizarre coloring. It had the bright scarlet feathers of a male grosbeak on one side of its body and the canary yellow plumage of a female on the other.
When they saw the robin-size songbird’s split coloring, it was immediately clear that the grosbeak was what scientists call a bilateral gynandromorph—an animal that appears half male and half female.
“There was no question about it,” says Annie Lindsay, bird banding program manager at Powdermill.
Measurements also revealed that the bird’s right wing was slightly longer than the left, typical of the difference between male and female grosbeaks.
While gynandromorphy simply means that an animal has both female and male characteristics, bilateral gynandromorphs often appear more dramatically different because those characteristics are separated down the middle of their body; the separation may be internal as well as external. And bilateral gynandromorphy is different from hermaphrodism, in which an organism has both male and female reproductive organs but may appear on the outside to be either male or female.
Although hermaphrodism is natural among many creatures, such earthworms and snails, gynandromorphy is rare and, in birds, seems to occur when cells fail to divide properly early on in development. (Learn more about how butterflies with the condition develop.)
In fact, since Powdermill began keeping records nearly six decades ago, Lindsay says, only five of roughly 800,000 captured birds have been documented as likely gynandromorphs. The only other bird with bilateral gynandromorphy Lindsay says she’s seen during the past 15 years was also a rose-breasted grosbeak.
“I think this time around I have more knowledge about just how rare this is,” she says. “Just watching everyone else react—it was like pure joy.”
One in a million?
“That bird is quite famous,” says Danny Bystrak, a wildlife biologist at the Patuxent Wildlife Research Center’s Bird Banding Laboratory, in Maryland, a branch of the U.S. Geological Survey that serves as the central clearinghouse for data from bird banding operations across the country.
Looking back at all the records for the past 15 years, the odds of discovering a gynandromorph in a bird-banding operation seem to be “almost exactly one in a million,” Bystrak said in an email.
But there’s a caveat to those long odds. Not all bird species have males and females with noticeably different plumage, Lindsay says. More gynandromorphs likely are flying around out there than we realize.
Bird collections in museums also shed light on the phenomenon. Over his career, Stephen Rogers, collection manager for the Section of Birds at Carnegie Museum of Natural History, in Pittsburgh, Pennsylvania, has prepared some 16,000 birds for the museum’s collections. That entails drying and preserving the birds and taking notes on each specimen’s physical appearance, outside and inside, including its reproductive organs.
Among the museum’s more than 27,000 specimens, Rogers found records for only four gynandromorphs—three of them birds of species that have no color differences between males and females.
“The one-in-a-million figure would only be for birds that show distinct differences,” Rogers says. “Cardinals, grosbeaks, and the like.”
If you add in all the gynandromorphs that are hiding in plain sight, the odds go up to around 146 in a million.
Gynandromorphs make headlines when they’re discovered, such as a bilateral gynandromorphic cardinal from 2019 and the honeybee with the eyes of a male and the body of a female found earlier this year, but the phenomenon is difficult to study.
Bystrak says he can find no record of a gynandromorph ever being re-captured by a banding operation, so it’s unclear, for example, if the color differences persist after the birds molt.
There’s also the question as to what sort of future the grosbeak might have. Female birds, like humans, have two ovaries, but only the left one is functional in songbirds. Since the grosbeak’s left side seems to be female, Lindsay says it’s possible that the bird could reproduce—though it would also have to behave like a female to attract a mate.
Not much is known about the behavior of gynandromorphs, but in 2009 and 2010, scientists were able to observe a northern cardinal in Illinois with bilateral gynandromorphy and noted that it seemed to survive into adulthood without difficulties. Yet the cardinal never seemed to perform any songs or vocalizations, nor was it seen with a mate.
In the end, we may never know what becomes of the grosbeak, a migratory bird. It was released the day it was caught and could be as far away as South America by now.