This northern cardinal has a birth defect that made it both male and female. The right side of its body is female, the left side is male.
This cannot happen in humans because our sexual characteristics are determined by hormones but in birds each cell has a sexual identity that’s determined early in embryonic development.
On rare occasions something goes wrong during the first cell division and an individual bird is born a bilateral gynandromorph. In other words, side-to-side (bilaterally) exactly half the body is female (gyn) and the other half male (andro). The dividing line is always vertical from head to tail. To understand how this happens, read this 2010 blog post on bird chromosomes: Anatomy: W and Z
In bird species where males and females look the same it’s hard to tell this has happened but in sexually dimorphic species like the northern cardinal or evening grosbeak, it’s easy to see.
This particular cardinal from Rock Island, Illinois is now famous because he-she was studied extensively by Professor Brian Peer and Robert Motz of Western Illinois University. Their findings — “Observations of a Bilateral Gynandromorph Northern Cardinal (Cardinalis cardinalis)” — were recently published in The Wilson Journal of Ornithology and featured in Science magazine.
(photo courtesy Western Illinois University)