It was a once-in-a-lifetime thing for this couple in Pennsylvania when they saw a scarce bird at their feeder. Cardinal: It turned out to be a half-male, half-woman cardinal. They are both red, but their plumage is different. That’s not the case with the one Jeffrey and Shirley Caldwell saw. It was half red and half dull brown.

This is the first time Shirley Caldwell has seen something like this in all the years she has been feeding.

“The left side is a man. The one in front of him is a woman.” Two winters in a row, the bird came to the feeder of a retired high school biology teacher who used to work at the school. Several times, I was able to watch it, and I noticed that it didn’t get along with other cardinals, nor did I hear it make any sounds. To get blood samples for further research, we tried to catch them with mist nets. We saw every bird in the neighborhood except this one! The birdwatcher called by the Caldwells to look at the rare bird said that it didn’t return for the third winter.

It’s called a “Bilateral Gynandromorph,” meaning that half of the bird’s body is male and the other half is female. It is a real male/female chimera, says Daniel Hooper, a postdoctoral fellow at the Cornell Lab of Ornithology.

The most interesting thing about this bird is that it can have babies. “Most gynandromorphs are infertile, but this one may be fertile because the left side is female, and only the left ovary in birds is working,” said Mr. Hooper, who led the study.

Indeed, the rare cardinal is never alone because almost all the time, a male is near it. It’s not lonely, Shirley said.

This isn’t the first time a cardinal has made the world wonder what it looks like. Last year, a yellow cardinal called a “one in a million” was found in Alabama.