An avian GYNANDROMORPH is a bird that is half male, half female. It is believed to happen when a fertilized egg first divides (egg meosis), and the sex chromosomes separate abnormally, with ZW (female cells) chromosomes on one side, and ZZ (male genotype) on the other side. That is, as the egg is dividing, it gets fertilized by separate sperm. So one side of the bird has heterogametic (ZW) female cells, and the other has homogametic (ZZ) male cells.
The resulting adult bird looks bilaterally half male, half female – i.e., one side (including the bill) has the coloring of a male adult, and the other side has the coloring of a female adult of that species. That is, it is sexually dimorphic (the left and right sides are different from each other.) Juveniles look mostly like females.
Gynandromorphy is extremely rare. It does not occur in mammals because hormones override the genetic differences on the right and left sides. But it has been documented in various birds, insects and other critters like crustaceans.
Two bilateral gynandromorphic Northern Cardinals (Cardinalis cardinalis, in Pennsylvania) and Green Honeycreeper (Chlorophanes spiza, seen in Villamaria, Columbia) have been documented. It is not known whether any bluebird gynandromorphs have shown up.
“The bird’s internal characteristics remain a mystery. In some, but not all, previously studied cases, gynandromorphic birds have 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,” although the Green Honeycreeper in the picture above has not seen engaging in courting or mating behavior, and tends to be anti-social at the feeding station. (NYT
MORE INFORMATION & RESOURCES:
- Glossary of ornithological terms
- Leucistic and Albino Bluebirds
- Weird Coloration
Photo by James R. Hill III, of a bilateral gynandromorphic Northern Cardinal in Pennsylvania. - White bluebird eggs
- John Murillo et al. 2023. Report of bilateral gynandromorphy in a Green Honeycreeper (Chlorophanes spiza) from Colombia. Journal of Field Ornithology 94 (4): 12; doi: 10.5751/JFO-00392-940412
- This Bird Is Half Male, Half Female, and Completely Stunning – The New York Times (nytimes.com), by Emily Anthes
- Sex determination in birds is a little different than in mammals. In mammals, males have one copy of each sex chromosome (X and Y) while females have two copies of the X chromosome. In birds, it’s the opposite. Their sex chromosomes are called Z and W, and it’s the females that have a single copy of each (ZW), whereas the males have two of the same (ZZ). Sex cells’ nuclei, including sperm and eggs, usually have only one copy of either chromosome — males produce only Z-carrying sperm, and females produce either Z- or W-carrying eggs. Gynandromorphy, like that in this cardinal, occurs when a female egg cell develops with two nuclei — one with a Z and one with a W — and it’s “double fertilized” by two Z-carrying sperm.” – Dr. Daniel Hooper, from A second bilateral gynandromorph Northern Cardinal in northwest Pennsylvania! — Erie Bird Observatory
Every kind of oddities make life more interesting, less boring!
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