Rare bird with half male and half female plumage photographed in Colombia

Zoologists say the newly observed green honeycreeper is an example of a bilateral gynandromorph.
A bird called a green honeycreeper stands on a tree branch. It has blue male plumage on its left side and green female plumage on its right.
Green honeycreepers are found in humid forests from Mexico south towards Brazil and eat fruit, arthropods, and plant nectar. John Murillo

A zoologist from the University of Otago in New Zealand spotted a rare bird with distinct half-male and half-female plumage. Hamish Spencer and amateur ornithologist John Murillo took photos and video of a wild green honeycreeper (Chlorophanes spiza) while on vacation in Colombia. A report on the find was published in the Journal of Field Ornithology and represents the second recorded example of gynandromorphism in this species in more than a century. 

[Related: These female hummingbirds don flashy male feathers to avoid unwanted harassment.]

A bilateral gynandromorph is an animal that is born with one male half and one female half. The animal is usually divided down the middle with characteristics of two sexes in one body. Bilateral gynandromorphism has been documented in other animals including bees, butterflies, spiders, and stick insects. Cardinals and the rose-breasted grosbeak have also been documented with this division, but bilateral gynandromorphs are believed to be rare.

“Many birdwatchers could go their whole lives and not see a bilateral gynandromorph in any species of bird. The phenomenon is extremely rare in birds, I know of no examples from New Zealand ever,” Spencer said in a statement. “It is very striking, I was very privileged to see it.”

A bilaterally gynandromorphic green honeycreeper. The bird has blue male plumage on its left side and green female plumage on its right.
A bilaterally gynandromorphic green honeycreeper. CREDIT: John Murillo.

Studying gynandromorphs are important for our understanding of how biological sex is determined in birds and their sexual behavior. Male green honeycreepers have predominantly blue plumage. Female green honeycreepers have green plumage. The observed bird has both. 

“This particular example of bilateral gynandromorphy–male one side and female the other–shows that, as in several other species, either side of the bird can be male or female,” said Spencer. 

While there are a range of theories of how gynandromorphic animals form, scientists believe that it could occur in birds when a female egg cell develops with two nuclei. For mammals, male sex cells generally have one copy of each sex chromosome (X and Y) and females have two copies of the X chromosome. In birds, it’s the opposite and their sex chromosomes are designated as Z and W instead of X and Y. The female’s egg cells will have a single copy of each (ZW) chromosome in their nucleus, while the male’s sperm will have two Z’s. According to ornithologist Daniel Hooper, gynandromorphism likely occurs if a female egg cell develops with two nuclei—one with a Z and one with a W. It is then “double fertilized” by the two Z-carrying sperm, resulting in a gynandromorph.

[Related: Sex and gender binaries don’t tell the entire story of life.]

The team did not observe any courtship behavior or take any blood or tissue samples to study its chromosomes. It was also not clear if the bird was fertile or reproduced over the 21 months that the bird was reportedly present in the forest in Villamaría, Colombia. The bird tended to keep to itself and acted as the other members of its species. 

There are five species of honeycreepers. Green honeycreepers live in southern Mexico, south towards Brazil, Colombia, and Trinidad. Green honeycreepers are generally found on the edges of evergreen forests, gardens, and plantations in Central and South America. They are only five to five and a half inches long and weigh less than a pound. Green honeycreepers eat fruit and some arthropods and some nectar from plants. Scientists also believe that they are doing well and are categorized as Least Concern on the IUCN Red List.