It’s called a ‘bilateral gynandromorph.Half of its body is male and the other half is female. Its colors split right down the middle.

The photos (below) of the half-male, half-female Northern Cardinal were taken in Grand Valley, Pennsylvania.

According to National Geographic, this rarity likely occurs within all species of birds, but we likely only notice it in species where the males and females look different from each other.

So how does this happen?

Sex determination in birds is different than in mammals. 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).

Pubmed.com says that gynandromorphy occurs in the very early stages 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.

Gynandromorphs also occur in insects, spiders, crustaceans, but they are extremely rare and can be hard to detect without the aid of bi-coloring.

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