A bird hopping outside the window lately is the strangest that Shirley and Jeffrey Caldwell have ever seen.
Its left side is the taupe shade of female cardinals; its right, the signature scarlet of males.
Researchers believe that the cardinal frequenting the Caldwells’ bird feeder in Erie, Pa., is a rare bilateral gynandromorph, half male and half female. Not much is known about the unusual phenomenon, but this sexual split has been reported among birds, reptiles, butterflies and crustaceans.
No one can be sure the bird is a gynandromorph without analyzing its genes with a blood test or necroscopy, but the split in plumage down the middle is characteristic of the rare event, according to Daniel Hooper, an evolutionary biologist at the Cornell University Lab of Ornithology.
He said that gynandromorphs could theoretically be created through the fusion of two developing embryos that were separately fertilized.
It’s also possible that a female produces an egg that contains both copies of her sex chromosomes, Z and W, and is then fertilized by two sperm, each with a Z chromosome. (While human sex chromosomes are labeled XX for females and XY for males, female birds are ZW and males are ZZ.) Scientists aren’t precisely sure how such an egg yields a chick with both ZW and ZZ cells.
The split runs down the middle of the bird simply because vertebrates develop in a bilaterally symmetrical way. Although one side would largely be ZW and the other ZZ, previous research suggests there is some mixing of cells in the bird’s body.
Read the rest here: https://www.nytimes.com/2019/02/09/science/cardinal-sex-gender.html