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 during egg cell 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, each of which carries a Z chromosome, the result might be a bird with the WZ chromosomes of a female in some cells and the ZZ chromosomes of a male in others. “And so you get a bird that’s 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 honeycreeper in October 2021. It 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 bird-watching tour arrived at the farm more than a year later, Mr. Murillo pointed out the unusual bird and shared some photos he had snapped of it.
“They’re the best photos of a wild gynandromorphic bird that I’ve ever seen,” Dr. Spencer said. “I thought, The world needs to see these.”
The photos were included in a paper 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.)