Half Male-Half Female Songbird Captured In Pennsylvania

An extremely rare bird with half its body looking like a male and the other half like a female was captured recently at a nature reserve in southwestern Pennsylvania

One of the most enjoyable aspects of birding is that I never know what I’ll find. Oh sure, I have a good idea what to expect, but I never know what I’ll see until I get out there and look.

Such was the situation for a team of bird banders working at Powdermill Avian Research Center (PARC) located in the Laurel Highlands of southwestern Pennsylvania, 55 miles southeast of Pittsburgh. This facility, which is part of the Carnegie Museum of Natural History’s biological research station at Powdermill Nature Reserve, comprises 10 hectares of fields, hedgerows, ponds, wetlands, and streams, providing many opportunities for research with wild birds. PARC scientists conduct bio-acoustic research, evaluate avian perceptions of glass in an experimental flight tunnel to learn how to reduce bird-window collisions, and they operate a bird banding station.

It was late in the afternoon on 24 September so the bird banding team was almost finished for the day when something truly remarkable occurred: upon untangling a struggling bird from their nets, they found a songbird that looked as though it was composed of the lengthwise halves of two birds, one male and one female, that had been glued together into one individual. This bizarre bird was a rare gynandromorph, which is sometimes known more colloquially as a ‘halfsider’.

“We carry walkie talkies around the nets with us, and the field tech radioed back to excitedly say that they had a gynandromorph”, Annie Lindsay, Bird Banding Program Manager and a doctoral candidate at the University of Toledo, said in email. “[A]nd sure enough, when they brought it back, it was a half male, half female Rose-breasted Grosbeak!”

“We were all excited and definitely surprised! Although we all have ‘wish lists’ of birds we hope we’ll see ‘in the hand’ someday, none of us really expected to catch a gynandromorph.”

These birds are incredibly rare: out of more than 750,000 birds captured during almost six decades of bird banding, the Powdermill Nature Reserve’s avian research center has recorded fewer than 10 bilateral gynandromorphs.

According to Ms Lindsay, this was a “once-in-a-lifetime experience.” Except, it would appear that Ms Lindsay has lived two lifetimes already because, astonishingly, this is the second gynandromorph that she’s seen — and captured — at PARC.

“I was at Powdermill in a different capacity when the other bilateral gynandromorph Rose-Breasted Grosbeak was caught in September 2005”, Ms Lindsay explained in email.

“I was working as a field tech then focusing primarily on bioacoustics work (analyzing nocturnal flight calls), so I saw the bird and recognized that it was quite the catch, but I understand better now how rare these birds truly are.”

Indeed. Even most active birders and nature photographers have never seen a live gynandromorph bird, and far fewer still have captured and banded even just one.

Like seeing a unicorn

“The banding team was starstruck by this bird”, Ms Lindsay said in email. “One of the crew said it was ‘like seeing a unicorn’; one described the adrenaline rush lasting the whole day; one talked about how they ‘never thought I’d see something so cool in my life’.”

“The excitement and joy were palpable, and you could feel how special this bird was to them. Watching their expressions of happiness and awe made this capture even more extraordinary.”

Rose-breasted grosbeaks, Pheucticus ludovicianus, are common breeding birds as well as common visitors to backyard bird feeders in southwest Pennsylvania, where this bird was captured. However, most of the local rose-breasted grosbeaks had already departed for southern Mexico, Central America and northern South America for the winter, so this particular individual “was almost certainly a bird that spent the breeding season north of us and was migrating through our area”, Ms Lindsay explained in email.

“I aged the bird as an ‘after-hatching-year’ which means that it did not hatch this calendar year, so it’s at least one year old”, Ms Lindsay explained in email, then added: “We can’t age a songbird beyond knowing that it hatched this year or in a previous year, so this bird could be quite old, or it could be just one year old.”

Although this bird was in fresh autumnal plumage, which is considerably less fancy than its striking breeding plumage, it still shows distinct color differences. On its right side, it has rose-colored speckles on its brown-streaked breast, a rose-colored “wing pit” and black flight feathers — male plumage. On its left side, it has a yellow “wing pit” and brown streaks on its yellow breast plumage — female plumage.

“It was spectacular. This bird is in its nonbreeding [plumage], so in the spring when it’s in its breeding plumage, it’s going to be even more starkly male, female”, Ms Lindsay said.

“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.

But 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.”

But 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. But 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).

Might 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.”

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

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

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