Colombia is a birdwatching paradise. Its incredibly diverse ecosystems, which include mountain ranges, mangrove swamps, Caribbean beaches and Amazon rainforests, are home to multiple 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 birding 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, a local farm owner called to his attention a Green Treecreeper, a small songbird common in forests ranging from southern Mexico to Brazil.
But this particular green creeper had some very unusual plumage. The left side of her body was covered in shimmering spring green feathers, the classic coloration for females. Its right side, however, was iridescent blue, a telltale sign 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 is a relatively rare and little understood phenomenon. The bird seen by Dr Spencer in Colombia is only the second known case of bilateral gynandromorphism in a greencreeper – 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 showed the opposite pattern, with female plumage on the right and male plumage on the left. )
It is not entirely clear how this condition occurs, but one of the most popular theories is that it results from an error during the production of eggs in female birds. Female birds have two different sex chromosomes, designated W and Z, while males have two Z chromosomes. An error during the production of egg cells could result in two fused or incompletely separated cells, one with a W chromosome and one with a Z chromosome.
If these fused cells are fertilized by two different sperm, each carrying a Z chromosome, the result could be a bird with a female’s WZ chromosomes in some cells and a male’s ZZ chromosomes in others. “And so you get a bird that is half and half,” Dr. Spencer said.
John Murillo, an amateur ornithologist who owns a small farm and nature reserve in Colombia, first spotted the gynandromorphic treecreeper in October 2021. He has become a regular visitor to the farm’s bird feeding station, stocked with fresh fruit and sugar water. When Dr. Spencer and his birding tour arrived at the farm more than a year later, Mr. Murillo pointed out the unusual bird and shared some photos he had taken of it.
“These are the best photos of a wild gynandromorphic bird I have ever seen,” Dr Spencer said. “I thought: the world needs to see them.”
Photos have been included a document that Dr. Spencer and several other scientists wrote about the unusual honeycreeper, which was published in The Journal of Field Ornithology in December. (Mr. Murillo was one of the authors.)
The internal characteristics of the bird remain a mystery. In some, but not all, previously studied cases, gynandromorph birds had internal sex 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 creeper has never been observed engaging in any courtship or mating behavior. He tended to avoid other green creepers and often stayed away from the feeding station until the other birds had left. “The bird tended to be a bit solitary,” Dr Spencer said.
However, he appeared to have remained there, repeatedly visiting the feeding station over a period of almost two years. “This bird had been around for a long time,” Dr. Spencer said. “There wasn’t any kind of obvious disadvantage, except maybe in finding a mate.”
