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 rare but not altogether uncommon. 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.)
HOW DOES IT HAPPEN?
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 (ZZ). 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.