A bird hopping outside the window lately is the strangest that Shirley and Jeffrey Caldwell have ever seen.
Its left side is the taupe shade of female cardinals; its right, the signature scarlet of males.
Researchers believe that the cardinal frequenting the Caldwells’ bird feeder in Erie, Pa., is a rare bilateral gynandromorph, half male and half female. Not much is known about the unusual phenomenon, but this sexual split has been reported among birds, reptiles, butterflies and crustaceans.
No one can be sure the bird is a gynandromorph without analyzing its genes with a blood test or necroscopy, but the split in plumage down the middle is characteristic of the rare event, according to Daniel Hooper, an evolutionary biologist at the Cornell University Lab of Ornithology.
He said that gynandromorphs could theoretically be created through the fusion of two developing embryos that were separately fertilized.
It’s also possible that a female produces an egg that contains both copies of her sex chromosomes, Z and W, and is then fertilized by two sperm, each with a Z chromosome. (While human sex chromosomes are labeled XX for females and XY for males, female birds are ZW and males are ZZ.) Scientists aren’t precisely sure how such an egg yields a chick with both ZW and ZZ cells.
The split runs down the middle of the bird simply because vertebrates develop in a bilaterally symmetrical way. Although one side would largely be ZW and the other ZZ, previous research suggests there is some mixing of cells in the bird’s body.
But in essence, each side of the bird would be largely the brother or sister of the other. Genes other than those that confer gender also are affected.
Sex determination in mammals is controlled by a gene on the Y chromosome that stimulates the development of testes, the hormones of which regulate development of the rest of the organism. That’s why gynandromorphism is so rarely seen in mammals, Dr. Hooper said.
He doesn’t see any reason that cardinals would be more likely to be of mixed sex than other creatures, but their color contrast by gender makes them particularly noticeable.
Female cardinals are taupe-colored and quieter than their brightly hued mates. In addition to their red color, male cardinals sing more often and with more complicated tunes, both to declare their territory and to attract females.
In 2008 Brian Peer, a professor of biology at Western Illinois University in Macomb, Ill., began studying a cardinal with a similar split down the middle. Over the next two years, he made more than 40 visits to the backyard of a retired high school biology teacher whose bird feeder had attracted a right-half female, left-half male bird — the opposite of the Caldwells’ cardinal.
An expert in the behavior of cowbirds, Dr. Peer, was hoping to see whether the cardinal would behave more like a female or a male. Unfortunately, he never saw the bird with others, though he disagreed with the notion that the cardinal was lonely — many cardinals never successfully mate in the wild, he said.
Dr. Peer watched the bird over two winters, but it eventually was pushed out of the teacher’s yard by a male cardinal that aggressively defended its territory. The gynandromorph wasn’t seen again.
Gynandromorphs are believed to be infertile, although the cardinal in the Caldwells’ yard appears to have paired off with a male bird. Dr. Hooper said it’s too soon to know whether that male is the mixed-bird’s father or mate, and whether it will stick around for mating season.
While birds have a pair of ovaries, the only functional one is on the left side — which in this cardinal is female, so it is theoretically possible that it could lay eggs, Dr. Hooper said. He would expect any offspring to be genetically conventional, because its egg cells would have only one sex chromosome.
Dr. Hooper said he would love to be able to study the bird in-depth, to learn more about its genetics and also to understand how its brain functions: In gynandromorphs, half of the brain, too, is female, and half male.
Male songbirds have many more neural connections in their brains to allow them to sing complex tunes, and he wonders how a half-and-half brain would affect this cardinal’s ability to learn, evaluate and produce song, as well as its desire to do so.
“I would imagine,” he said in an email, “there simply just isn’t a complete neural network for producing a song or the right hormonal cocktail in the brain circulating to motivate the bird to sing one even if it could.”
Butterflies can also be gynandromorphs, said Josh Jahner, a postdoctoral researcher at the University of Nevada, Reno, either half-and-half or by even more varied proportions.
In his research, Dr. Jahner found that the wings of gynandromorphic butterflies are similar to the wings of typical butterflies — though male and female colorations appear on the same insect. But each gynandromorph’s genitalia is different from every other’s, Dr. Jahner said. Figuring out why may help scientists understand the rules of development.
For her part, Shirley Caldwell is enjoying both the attention and the opportunity to watch the unusual cardinal and to look for patterns in its daily activities. “It’s been very rewarding as far as learning about the bird,” she said. “It’s a once-in-a lifetime thing. And it’s fun.”