Colombia is a bird watcher's paradise. Its amazingly diverse ecosystems, including mountain ranges, mangrove swamps, Caribbean beaches, and the Amazon rainforest, 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 bird-watching holiday to Colombia, he hoped he might spot some interesting and unusual creatures.
He got more than he bargained for. During one field trip in early January 2023, a local farm owner drew the attention of a green hummingbird, a small songbird common in forests from southern Mexico to Brazil.
But this particular green honey bee had very unusual plumage. The left side of its body was covered in shiny, spring-green feathers, the classic color of a female. However, the one on the right was iridescent blue, indicating that it was a male. The bird appeared to be a bilateral gynandromorph. One side is female and the other side is male.
“It was absolutely incredible,” said Dr. Spencer. “We were lucky to see it.”
Gynandromorphism has been recorded in a variety of birds as well as insects, crustaceans and other organisms. However, this is a relatively rare and poorly understood phenomenon. The bird Dr. Spencer saw in Colombia is the second case of bilateral gynandromorphism in a green hive, and the first recorded in the wild. (The only previous example was reported more than 100 years ago and was based on a museum specimen, Dr. Spencer said. The bird showed the opposite pattern, with female feathers on the right and male feathers on the left.)
It's not entirely clear how this condition occurs, but one leading theory is that it's caused by an error during egg production in female birds. Female birds have two different sex chromosomes, designated W and Z, while males have two Z chromosomes. An error occurred while Egg production can result in two fused or incompletely separated cells, one with the W chromosome and the other with the Z chromosome.
If these fused cells are fertilized by two different sperm, each carrying a Z chromosome, the result may be a bird with some cells having the female's WZ chromosome and other cells having the male's ZZ chromosome. “So you get half and half birds,” Dr. Spencer said.
John Murillo, an amateur ornithologist who owns a small farm and nature reserve in Colombia, first spotted the Gynandro-type honeycreeper in October 2021. The bird has become a regular visitor to the farm's bird feeding station, which is stocked with fresh fruit and sugar water. When Dr. Spencer and his bird-watching tour arrived at the farm a little over a year later, Mr. Murillo pointed out the unusual bird and shared some of the photos he had taken.
“They are the best photographs I have ever seen of a wild gynandro-type bird,” Dr. Spencer said. “I thought the world needed to see this.”
The photo is included in a paper Dr. Spencer and several other scientists wrote about this unusual honeycomb bird, published in December in The Journal of Field Ornithology. (Mr. Murillo was one of the authors.)
The bird's internal characteristics remain a mystery. In some, but not all, previously studied cases, gynecomastia birds had internal genitalia matching their external plumage, with ovaries on one side and testes on the other. Past observations have shown that some gynomorphous birds are able to successfully mate and reproduce.
However, this particular green bee has never been observed engaging in courtship or mating behavior. It tended to avoid other green hummingbirds and would often hang back at the feeding station until the other birds left. “The bird tended to be a bit lonely,” Dr. Spencer said.
Despite this, it seemed to stay, making repeated visits to the feeding station over a period of nearly two years. “This bird has been around for a long time.” Dr. Spencer said. “There was no apparent disadvantage of any kind other than perhaps finding a mate.”