 So, if a bird's egg develops with two intact nuclei, the second nucleus can also be fertilized. It's sort of like when you get a chicken egg with two yolks. But in this case, it's like the yolks have fused. And the result of these fused embryos is a chick that's half male and half female, just like that half red and half brown cardinal. As far as we know, half-siders do not reproduce.