This bird is half male, half female and completely breathtaking

Colombia is a birdwatcher’s paradise. The stunningly diverse ecosystems – including mountain ranges, mangrove swamps, Caribbean beaches and Amazon rainforests – are home to more bird species than any other country on earth.

So when Hamish Spencer, an evolutionary biologist at the University of Otago in New Zealand, booked a birdwatching holiday in Colombia, he hoped to spot some interesting and unusual creatures.

He got more than he bargained for. During an outing in early January 2023, the owner of a local farm turned his attention to a green honeycreeper, a small songbird common in forests from southern Mexico to Brazil.

But this particular green honeycreeper had very unusual plumage. The left side of its body was covered in glistening spring green feathers, the classic color for females. The right side, however, was iridescent blue, the telltale mark of a male. The bird turned out to be a bilateral gynandromorph: female on one side and male on the other.

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

Gynandromorphism has been documented in a variety of birds, as well as insects, crustaceans, and other organisms. But it is a relatively rare and poorly understood phenomenon. The bird that Dr. Spencer saw in Colombia is only the second known case of bilateral gynandromorphism in a green honeycreeper – and the first to be 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 showed the opposite pattern, with female plumage on the right and male plumage on the left.)

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

If those fused cells are fertilized by two different sperm cells, each carrying a Z chromosome, the result can be a bird with the WZ chromosomes of a female in some cells and the ZZ chromosomes of a male in others. “And that’s how you get a bird that’s half and half,” said Dr. Spencer.

John Murillo, an amateur ornithologist who owns a small farm and nature reserve in Colombia, first spotted the gynandromorphic honeycreeper in October 2021. He became a regular visitor to the farm’s bird feeding station, which was stocked with fresh fruit and sugar water . When Dr. Spencer and his birdwatching trip arrived at the farm more than a year later, Mr. Murillo pointed out the unusual bird and shared some photos he took of it.

“They are the best photos of a wild gynandromorphic bird I have ever seen,” said Dr. Spencer. “I thought: the world needs to see this.”

The photos are included a newspaper that Dr. Spencer and several other scientists wrote about the unusual honeycreeper, which was published in December in The Journal of Field Ornithology. (Mr. Murillo was one of the authors.)

The bird’s internal features remain a mystery. In some, but not all, previously studied cases, gynandromorphic birds had internal sexual organs that matched their external plumage, with an ovary on one side and a testis on the other. Past observations suggest that some gynandromorphic birds can successfully court mates and reproduce.

But this particular green honeycreeper was never observed to engage in courtship or mating behavior. It tended to avoid other green honeycreepers and often lingered behind the feeding station until other birds had left. “The bird tended to be a bit of a loner,” said Dr. Spencer.

Still, it seemed to stick around, visiting the feeding station repeatedly over a period of almost two years. “This bird had been around for a long time,” said Dr. Spencer. “It didn’t have any obvious disadvantage, except possibly in finding a mate.”