Bird-spotters, twitchers, nature lovers, people spend years spying on wildlife in the hope of seeing, perhaps even capturing on camera, a rare and wondrous sight. A couple from Pennsylvania did just that, when they noticed a very unusual bird hanging out in their garden.
One side of the bird, a northern cardinal (Cardinalis cardinalis), had the spectacular scarlet plumage so iconic of the male, and the other half, the soft brown-green of the female, split down the middle.
“Never did we ever think we would see something like this in all the years we've been [bird] feeding,” Shirley Caldwell told National Geographic, after she and her husband Jeffrey spied the bird in their garden in Erie, Pennsylvania.
The bird popped up a few weeks ago, and she snapped the picture of it while it was chilling in a dawn redwood tree just 9 meters (30 feet) from their kitchen window.
So, is it really half-male, half-female? Yes. Although rare, bilateral gynandromorphism – where a species' external appearance is split down the middle, half male, half female – has been seen in a variety of organisms, including birds, insects, and crustaceans.
In fact, it possibly occurs more often than thought, and we only notice it when it's really obvious, like in a species that is sexually dimorphic, where there are differences in the appearance of adult males and females.
Sex determination in birds is basically the opposite of humans. Instead of the females having two copies of the same sex chromosome (XX) and the male having a copy of each (XY), in birds it’s the other way around. Birds’ chromosomes are Z and W, so the female has ZW and the male has ZZ. So males’ sperm only carries Z, while females produce eggs with either Z or W.
Gynandromorphy occurs differently in different species. In birds, it’s thought that it happens when an egg develops with two nuclei, one carrying a Z, the other a W. If it gets fertilized by two ZZ sperm, then the embryo will carry both ZW and ZZ cells.