The cardinal pictured above is a rare split-sex gynandromorph: It has bright red male plumage on its left side, and its other half is covered with comparatively drab, brownish-gray female plumage. Natural gynandromorph butterflies, lobsters, and chickens, for example, exist but researchers have rarely observed them extensively in the wild. So, a duo led by Brian Peer of Western Illinois University observed the bilateral gynandromorph northern cardinal (Cardinalis cardinalis) around bird feeders for more than 40 days between December of 2008 and March of 2010 in northwestern Illinois. In that time, the bird never paired with another cardinal, and the team never heard it vocalizing. The bird was also never subjected to “unusual agnostic behaviors” from other cardinals. The findings were published in the Wilson Journal of Ornithology this month. 

With most mammals, sex chromosomes (XY for males, XX for females) trigger the formation of either male or female gonads, which then release hormones during early developmental stages that direct the sex of other cells. Birds, on the other hand, have a ZW sex determination system -- with females having ZW and males being ZZ -- and hormones don’t play nearly as big of a role. In a 2003 study of zebra finches who had brains that were literally half genetically male and half genetically female, researchers found that the sex differences were neural in origin, and not gonadal. When three gynandromorph chickens were examined in detail in 2010, that team found that the female half is mostly made up of normal female cells with female chromosomes; likewise, the male side is comprised of mostly male cells with male chromosomes. Their cells, it would seem, follow their own instructions, and not those of the gonads. 

[h/t Science]

Image via Peer Lab