In early 2021 bird watchers in the eastern United States photographed an unusually northern cardinal. The plumage on one side was looking like a typical male cardinal and the other side appearing like a typical female. This condition is called bilateral gynandromorphism. In birds the genetic system for sex is different from mammals. Female birds normally have Z and W sex chromosomes, while males have two Z chromosomes. Bilateral gynandromorphs arise when an egg ends up with two nuclei and both are fertilized, so the developing embryo contains both male and female cell lines. As development proceeds the groups of cells from each nucleus stay mostly separate, producing an adult in which one side of the body is genetically male and the other side is genetically female. The change shows up most clearly in species where males and females look very different, because plumage and sometimes other features contrast sharply down the middle. These occurrences are extremely rare in wild birds. Scientists know of only a handful of documented cases across different species. In such birds the internal anatomy can also reflect the two sexes with reproductive tissues corresponding to each side, although it is not clear how often these individuals can reproduce.

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