PITTSBURGH (KDKA) — A birdwatching couple in Erie spotted and photographed one of the most rare and elusive occurrences in nature.

Jeffrey and Shirley Caldwell photographed a “half-male, half-female” cardinal in their own backyard, National Geographic reported.

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The anomaly is known as a bilateral gynandromorph, and the cardinal’s colors are split down the middle.

Never did we ever think we would see something like this in all the years we’ve been feeding,” Shirley Caldwell told National Geographic.

Here’s how it happens says National Geographic:

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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.

National Geographic says one of the most exciting things about the bird’s discovery is that it may be able to reproduce.

Typically most are infertile and isolated, but this cardinal has female traits on her left side, the side which carries the only functional ovary in birds.

The cardinal has even been observed accompanied by a curious male.

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“Who knows, maybe we will be lucky enough to see a family in summer!” Shirley Caldwell said.