Recently Sighted Half-Male, Half-Female Bird May Actually Be Fertile

To the untrained eye a strangely colored bird might have just looked like a beautiful anomaly, but the two amateur birdwatchers who spotted it knew they were witnessing something special.

For the past 25 years Jeffrey and Shirley Caldwell have been feeding and watching the birds that find their way into the backyard of their Erie, Pennsylvania home.

At the beginning of January though the couple began spotting an unusual cardinal that was unlike any bird they had ever seen before.

The bird's half red, half light feathers were split right down the middle.

It's a genetic anomaly known as a bilateral gynandromorph. The phenomenon is so rare that Jeffrey and Shirley couldn't be sure of what they were seeing until the cardinal began flying closer to their house.

"Never did we ever think we would see something like this in all the years we've been feeding," says Shirley.

Once the bird got close enough Shirley was ready to grab some footage of the wonderful creature.

Put simply gynandromorphs are half male and half female.

"This remarkable bird is a genuine male/female chimera," Daniel Hooper, a postdoctoral fellow at the Cornell Lab of Ornithology, wrote in an email to National Geographic.

The trait known as sexual dimorphism also occurs in insects and crustaceans like butterflies and lobsters. In birds it is thought to occur across all species, but in cardinals it is especially noticeable.

Ornithologists refer to them as "half-siders"

"Cardinals are one of the most well-known sexually dimorphic birds in North America," according to Hooper.

"Their bright red plumage in males is iconic—so people easily notice when they look different."

What makes these "half-siders" so different?

Hooper tells NatGeo that most female birds carry one copy of each sex chromosome, called Z and W in birds, whereas males have two copies of the Z chromosome.

It's the opposite in humans where most males have X and Y chromosomes and most females have two X chromosomes.

In birds, sex is determined by the female's eggs which typically carry only one Z or W chromosome, which is then fertalized by the male's Z chromosome sperm.

In dimorphic eggs however the cell develops two nuclei carrying Z and W chromosomes which are then both fertilized by by two Z chromosome sperm.

The result is a chimera who develops a half male ZZ and half female ZW body.

Gynandromorphs like the Caldwell's cardinal aren't unheard of, but they are rare enough that it is usually a big deal when they are spotted.

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The Caldwell's cardinal however may be even more rare.

In most cases gynandromorphs are born infertile and unable to reproduce, but in the case of the Caldwell's bird the left side of its body is female.

While female birds have ovaries on both sides only the left side ovaries are functional. So the Caldwell's bird may be able to reproduce.

And the Caldwells say their cardinal may even have a mate.

Shirley says she often spots her gynandromorph in the company of a male bird.

"We're happy it's not lonely," Shirley says.

It's hard to know for sure if the cardinal will be able to reproduce, but Shirley is certainly hoping so.

"Who knows, maybe we will be lucky enough to see a family in summer!"