This Bird Is Half Male, Half Female, and Completely Stunning

Colombia is a bird watcher’s paradise. Its stunningly diverse ecosystems — which include mountain ranges, mangrove swamps, Caribbean beaches and Amazonian rainforests — are home to more avian species than any other country on Earth.

So when Hamish Spencer, an evolutionary biologist at the University of Otago in New Zealand, booked a bird-watching vacation in Colombia, he was hoping to spot some interesting and unusual creatures.

He got more than he bargained for. During one outing, in early January 2023, the proprietor of a local farm drew his attention to a green honeycreeper, a small songbird that is common in forests ranging from southern Mexico to Brazil.

But this particular green honeycreeper had highly unusual plumage. The left side of its body was covered in shimmering spring-green feathers, the classic coloring for females. Its right side, however, was iridescent blue, the telltale marker of a male. The bird appeared to be a bilateral gynandromorph: female on one side and male on the other.

“It was just incredible,” Dr. Spencer said. “We were lucky to see it.”

Gynandromorphism has been documented in a variety of birds, as well as insects, crustaceans and other organisms. But it’s a relatively rare and poorly understood phenomenon. The bird Dr. Spencer saw in Colombia is only the second known case of bilateral gynandromorphism in a green honeycreeper — and the first documented in the wild.(The only previous example was reported more than a century ago and was based on a museum specimen, Dr. Spencer said. That bird displayed the opposite pattern, with female plumage on the right and male plumage on the left.)

It is not entirely clear how the condition comes about, but one leading theory is that it results from an error during the production of egg cells in female birds.Female birds have two different sex chromosomes, designated W and Z, while males have two Z chromosomes. An error duringegg cell production could result in two fused or incompletely separated cells, one with a W chromosome and one with a Z chromosome.