As It Happens

This rare male-female cardinal is 'something pretty special'

Split right down the middle, the bright red side is male and the brownish yellow side is female.

The bright red side is male and the brownish yellow side is female

This photo by Shirley Caldwell shows a cardinal in Eerie, Penn., that is both male and female. (Submitted by Shirley Caldwell)

Story transcript

In 25 years of watching the birds who stop by to snack on her backyard feeders, Shirley Caldwell of Erie, Penn., says she's never seen anything like this two-coloured cardinal.

Split right down the middle, the bird is bright red on its right side and a muted yellowish brown on the left, with only a few feathers overlapping. 

"I knew I was looking at something different and something pretty special," Caldwell, who snapped photos of the cardinal, told As It Happens host Carol Off. 

"It is a half-male, half-female individual."

The cardinal is what scientists call a gynandromorph — an organism that displays both male and female sex characteristics.

Daniel Hooper, a postdoctoral fellow at the Cornell Lab of Ornithology, told National Geographic that gynandromorphs likely exist across all species of birds, but are especially noticeable in species that display sexual dimorphism, when males and females look significantly different.

"Cardinals are one of the most well-known sexually dimorphic birds in North America — their bright red plumage in males is iconic — so people easily notice when they look different," he said.

Chromosomes and colour 

It all comes down to chromosomes.

Female birds have a single copy each of a Z and W chromosome, while males have ZZ. 

This cardinal was likely the result of a female egg cell that developed with two nuclei — one with a Z and one with a W — which was then "double fertilized" by two Z-carrying sperms, Hooper told the magazine. 

Its red side is male, while the yellow is female. (Submitted by Shirley Caldwell)

The result is a single cardinal that is literally split down the middle, both in colour and in chromosomes. 

The cells on its red side likely contain male ZZ chromosomes, while its yellow side likely has cells with ZW chromosomes.

Different, but far from lonely 

Being a gynandromorph can sometimes mean a lonely existence for birds, as many are unable to reproduce and therefore don't attract mates.

But this bird is far from lonely, and may even be able to lay eggs. 

Female birds only have one working ovary, usually on their left side. Since this cardinal is female on the left, it's possible it has a fully functional female reproductive system. 

In fact, Caldwell believes the cardinal already has a mate. 

"This one does have a male that it travels with," she said. "When the gynandromorph does leave, the male follows with it. I always see them together."

She's keeping a close eye on the pair to see if they hatch any eggs. 

"I'm really going to be paying attention," she said. 

Written by Sheena Goodyear. Interview with Shirley Caldwell produced by Sarah Jackson.