Jeffrey and Shirley Caldwell have been attracting birds for 25 years with carefully tended backyard feeders. But the lifelong Erie, Pennsylvania, residents have never seen a creature so wondrous as the half-vermillion, half-taupe cardinal—its colors split right down the middle—that first showed up a few weeks ago in the dawn redwood tree 10 yards from their home.

In fact, they weren't sure they saw it correctly until it came in closer. "Never did we ever think we would see something like this in all the years we've been feeding," Shirley Caldwell says. The anomaly is known as a bilateral gynandromorph. In plain language: Half its body is male and the other half is female. "This remarkable bird is a genuine male/female chimera," says Daniel Hooper, a postdoctoral fellow at the Cornell Lab of Ornithology, in an email.

Gynandromorphs, known as "half-siders" among ornithologists, are uncommon but not unheard of. They likely occur across all species of birds, Hooper says, but we're only likely to notice them in species where the adult males and females look distinct from each other, a trait known as sexual dimorphism. "Cardinals are one of the most well-known sexually dimorphic birds in North America—their bright red plumage in males is iconic—so people easily notice when they look different," Hooper says. (Further reading: This yellow cardinal is one-in-a-million.) (This gynandromorphic cardinal has been seen eating in the backyard of bird-lovers Shirley and Jeffrey Caldwell. Shirley snapped a photo of the rare bird through her kitchen window as it perched on a tree at the end of her yard.)

Hooper says sex determination in birds is a little different than in mammals. In mammals, he says, males have one copy of each sex chromosome (X and Y) while females have two copies of the X chromosome. In birds, it's the opposite. Their sex chromosomes are called Z and W, and it's the females that have a single copy of each (ZW), whereas the males have two of the same. Sex cells' nuclei, including sperm and eggs, usually have only one copy of either chromosome—males produce only Z-carrying sperm, and females produce either Z- or W-carrying eggs.

Gynandromorphy like that in this cardinal occurs when a female egg cell develops with two nuclei—one with a Z and one with a W—and it's "double fertilized" by two Z-carrying sperm. The chimeric individual then develops with half of its body as a male ZZ and the other half as a female ZW. If you were to examine a cell from the bright red male side, it would have cells with ZZ chromosomes. If you looked at a cell from the left, it would have cells with ZW chromosomes. This phenomenon happens in birds, many insects, and crustaceans. (Be sure to check out this butterfly that's half male, half female with colors split down the middle and this half-orange, half-brown lobster.)