A bird skipping by the window lately is the weirdest Shirley and Jeffrey Caldwell have ever seen.

His left side is the mole shade of female Cardinals; on the right, the scarlet signature of the males.

Researchers believe that the Cardinal attending the Caldwells bird feeder in Erie, Pennsylvania, is a rare bilateral, half-male and half-female female gynandromorph. The unusual phenomenon is poorly known, but this sexual division has been reported in birds, reptiles, butterflies and crustaceans.

No one can be sure that the bird is a gynandromorph without analyzing its genes with a blood test or a necroscopy, but the division in plumage in the center is characteristic of this rare event, according to Daniel Hooper, biologist of the evolution of Cornell University Lab of Ornithology.

He said that gynandromorphs could theoretically be created by the fusion of two separately fertilized developing embryos.

It is also possible for a woman to produce an egg containing both copies of her sex chromosomes, Z and W, and then to be fertilized by two spermatozoa each with a Z chromosome. (While human sex chromosomes are labeled XX for females and XY for males, female birds are ZW and ZZ males.) Scientists do not know exactly how such an egg produces a chick with ZW and ZZ cells.

The division is done in the middle of the bird simply because vertebrates develop bilaterally and symmetrically. Although one side is largely ZW and the other ZZ, Previous search suggests that there is a mixture of cells in the body of the bird.

But essentially, each side of the bird would be largely the brother or sister of the other. Genes other than those that confer gender are also affected.

The determination of sex in mammals is controlled by a gene located on the Y chromosome that stimulates the development of the testes, whose hormones regulate the development of the rest of the body. This is why gynandromorphism is so rarely seen in mammals, said Dr. Hooper.

He sees no reason for Cardinals to be more likely to be mixed-sex than other creatures, but their sex-color contrast makes them particularly remarkable.

The cardinal are taupe and quieter than their brightly colored companions. In addition to their red color, male cardinals sing more often and with more complicated tunes, both to declare their territory and to attract women.

In 2008, Brian Peer, a professor of biology at Western Illinois University in Macomb, Illinois, began studying a cardinal with a similar division in the middle. Over the next two years, he visited the backyard of a retired high school biology teacher more than 40 times, whose bird feeder had drawn a male, half-right, male bird-unlike Cardinal of Caldwell.

[[[[Like the Science Times page on Facebook. | Sign up for the Science Times Bulletin.]

Mr. Peer, a specialist in cowhide behavior, was hoping to know if the cardinal would behave more like a woman or a man. Unfortunately, he never saw the bird with other people, although he disagreed with the idea that the cardinal was alone – many cardinals did not never mated to the wild, he said.

Dr. Peer observed the bird for two winters, but was eventually pushed out of the teacher's yard by a cardinal who defended his territory aggressively. The gynandromorph was not reviewed.

The gynandromorphs would be infertile, although the Cardinal of the Caldwell Court seems to have associated with a male bird. Dr. Hooper said it was too early to know if he was the father or partner of the mixed bird and whether he would stay for the mating season.

While birds have a pair of ovaries, the only functional is on the left side – which in this cardinal is a woman. So it is theoretically possible that he can lay eggs, said Dr. Hooper. He would expect that any offspring would be genetically conventional because his oocytes would have only one sex chromosome.

Dr. Hooper stated that he would like to be able to study the bird in depth, learn about its genetics and also understand its brain function. In the gynandromorphs, half of the brain is female and the other half male.

Male songbirds have many more neuronal connections in their brains that allow them to sing complex tunes. He wonders how a half-half brain would affect the cardinal's ability to learn, evaluate and produce songs, as well as his desire to do so. .

"I imagine," he said in an email, "there is simply no complete neural network to produce a song or the appropriate hormonal cocktail in the circulating brain to motivate the patient." Bird to sing one, even if he could. "

Butterflies can also be gynandromorphs, said Josh Jahner, a postdoctoral fellow at the University of Nevada, Reno, according to even more varied proportions.

In his research, Dr. Jahner discovered that the wings of gynandromorph butterflies are similar to those of typical butterflies, although the colors of males and females appear on the same insect. But the genitals of each gynandromorph are different from those of others, Dr. Jahner said. Understand why can help scientists understand the rules of development.

Shirley Caldwell appreciates both the attention and the opportunity to look at this unusual cardinal and look for patterns in her daily activities. "It's very rewarding to get to know the bird," she said. "It's a unique thing in life. And it's fun.