A gynandromorph, as the name implies, is an animal that is part male and part female, with the sex-specific parts usually demarcated cleanly.  Here’s Brian’s description of how he saw the bird:

Brian Peer took these photos of a “gynandromorph”  Northern cardinal - Cardinalis cardinalis - he found a couple years back in nearby Rock Island, IL. 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. Brian 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. He attempted to capture it with mist nets so that Rob Fleischer and he could get blood samples for further study, but they caught every bird in the neighborhood except this one! Alas, it never returned the third winter.