What makes birds two-sided like this, both male and female in the same body?
It’s a very rare condition and it only happens when there’s an embryo error in the bird’s sex chromosomes, W and Z. The resulting oddity is a “bilateral gynandromorph.”
Learn how it occurs in this last-day-of-the-year article … Anatomy: W and Z
(photo credits: evening grosbeaks at the Smithsonian by ap2il via Flickr, Creative Commons license. Northern cardinal courtesy of Western Illinois University. Click on the captions to see the originals.)