Rare Half-Male, Half-Female Cardinal Spotted In Pennsylvania

Bird is what's known as a bilateral gynandromorph, with male and female traits. This happens when female egg with two nuclei is 'double fertilized' by two sperm. The amazing male-female chimera was spotted in a Pennsylvania backyard. Male cardinals have bright red coloring; females have a much duller appearance.

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. 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.