Your post this morning on Venus the Cat reminded me of a bilateral gynandromorph Northern Cardinal we found a couple years back in nearby Rock Island, IL. I’ve attached several photos of the bird. As you can see, the left side is male and the right female. For two winters the bird appeared at the feeder of a retired high school biology teacher. I was able to observe it on several occasions, and noticed that it didn’t associate with other cardinals, nor did I hear it produce any vocalizations. We attempted to capture it with mist nets so that Rob Fleischer and I could get blood samples for further study, but we caught every bird in the neighborhood except this one! Alas, it never returned the third winter.
It’s not known exactly how gynandromorphy happens in birds, Arnold said. The predominant theory is that an error occurs in the formation of an egg, which normally carries one chromosome to unite with the single chromosome carried by the sperm. But if an egg accidentally ends up with two chromosomes — a Z and a W — and if this aberrant egg is fertilized by two Z-carrying sperm, the bird that results will have some ZZ cells and some ZW cells, he explained.