science

The green honeycreeper is only the second of its species ever observed with this condition—and the first recorded in more than 100 years

Researchers have spotted an “extremely rare” green honeycreeper in Colombia that’s half female and half male. The bird’s plumage is divided directly down the middle, with blue feathers typical of males on its right side and the emerald-green feathers of females on its left. This individual is only the second of the species ever recorded exhibiting this trait—called bilateral gynandromorphism—and the first in more than 100 years.

“Many birdwatchers could go their whole lives and not see a bilateral gynandromorph in any species of bird. The phenomenon is extremely rare in birds. I know of no examples from New Zealand, ever,” Hamish Spencer, a zoologist at the University of Otago in New Zealand who witnessed the bird while on vacation, says in a statement. “It is very striking. I was very privileged to see it.”

Bilateral gynandromorphism occurs in a variety of species—including spiders, butterflies, crustaceans and lizards—though it’s never been recorded in humans. In humans, biological sex is usually determined by our chromosomes. Those born female typically have two X chromosomes, while those born male tend to have an X and a Y chromosome. Birds are the opposite. Their chromosomes are designated as Z and W—females have a Z and a W, while males have two Z chromosomes.