Half Male-Half Female Songbird Captured in Pennsylvania
By: 𝐆𝐫𝐫𝐥𝐒𝐜𝐢𝐞𝐧𝐭𝐢𝐬𝐭, Scientist
Tags: Genetics, Birds, Developmental Biology, Science, Ornithology

“Bilateral gynandromorphism, while very uncommon, is normal and provides an excellent example of a fascinating genetic process that few people ever encounter”, Ms Lindsay added.

A bilateral gynandromorph results from a developmental accident

In birds, sex is determined by sex chromosomes, just as in mammals. But unlike mammals, where females are XX and males are XY, female birds are ZW whilst males are ZZ (lots more details here). So the sex chromosome — either W or Z — determines the resulting chick’s sex.

To develop properly, the ovum sheds half of its chromosomes in a small package called a polar body before it is fertilized. But if both the egg’s nucleus and its polar body are fertilized by two different sperm cells (ref), and these nuclei start dividing separately, each side of the bird’s body develops its own sex because one side has female chromosomes (ZW) whilst the other side has male chromosomes (ZZ).

Because rose-breasted grosbeaks are sexually dichromatic, the differences between the sexes are readily apparent, especially when the bird attains breeding plumage. Thus, these gynandromorphs are readily visible, whereas gynandromorphs of other bird species where males and females look the same are almost impossible to spot.

This raises the question: is this bilateral gynandromorph grosbeak capable of breeding and raising chicks?

Can a bilateral gynandromorph breed?

Well, it’s complicated.

“Since usually only the left ovary is functional in birds, and the left side of this bird is the female side, this bird theoretically could produce young if it successfully mates with a male”, Ms Lindsay explained. “The bird’s ability to reproduce could also depend on whether it sings like a male, which would potentially attract females and elicit a territorial response from other males.”

But this is speculation at this point because there are no documented cases of bilateral gynandromorph birds successfully breeding in the wild nor in captivity.

Last year, I interviewed ecologist Brian Peer, a professor of biology at Western Illinois University, about a bilateral gynandromorph Northern cardinal that he studied (more here) and here’s what he told me then:

“I’m not sure that it is capable of breeding”, Professor Peer cautioned in email. “Because the female side is on the left doesn’t necessarily mean that the ovary is also on that side. The study by Zhao et al. (2010; ref) that we cited indicated that the [physical] appearance doesn’t always correspond with the gonad [location]. They found two birds that appeared male on the left half, but one individual had an ovary on that side. A third individual was female on the left side and had a combination testis-ovary structure.”

Such a combination structure, known as an ovotestis, is a gonad with both testicular and ovarian aspects and thus, is not fertile.

As Professor Peer explained to me last year, although we don’t know much about the breeding capabilities of gynandromorph birds, there is little evidence to suggest that they are fertile. Professor Peer did mention that gynandromorph songbirds can exhibit important courtship behaviors, like singing, that make it possible for them to form a pair bond, as documented in a study of a captive gynandromorph zebra finch that behaved as a male, yet also produced infertile eggs (ref).

Could this bird reappear next year?

“It’s unlikely that we’ll see this bird next spring, but not impossible”, Ms Lindsay said in email. “Most of the birds we recapture year after year are local breeders or winter residents, but less so with birds that are migrating through Powdermill.”

Annie Lindsay examines a migratory songbird in the lab before releasing it. (Credit: Powdermill Nature Reserve / Carnegie Museum of Natural History)

After photographing and banding the bird, Ms Lindsay took it to the lab where she measured its wing length and plucked two feathers from each side of the bird (four total) for genetic analysis, before releasing it to continue on its migratory journey.

Has anyone reported seeing this special bird?

“No local birders reported seeing the bird, either before or after it was caught and banded”, Ms Lindsay replied in email.

It appears that this extraordinary songbird simply vanished, like a mythical unicorn.