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,”