Tuesday, March 5, 2019

Abnormal Plumage, Part I: Bilateral Gynandromorphs

Bilateral Gynandromorph in the Bell Museum collection, between a normal male and female.  
A long time ago in a galaxy far, far away—well, my own backyard in 1986—I spied an oddity among the scores of Evening Grosbeaks at my feeder. I almost rubbed my binoculars in disbelief when I realized I was looking at a bilateral gynandromorph—a bird that was half male and half female. The left side appeared to be in perfect female plumage, the right almost perfectly male, though the tail was pale and the forehead wasn't quite as black as on other males. I studied it for five minutes, but then the flock flew off in a flurry of wings and I lost it. The male wing was probably longer than the female wing, but I didn’t notice how it flew in the bustle, and I never saw it again.  

I knew about bilateral gynandromorphism because there's a gynandromorph Evening Grosbeak at the Bell Museum in Minneapolis, and Bud Tordoff wrote about it in the journal of the Minnesota Ornithologists’ Union, The Loon, in the Spring 1983 issue of Volume 54. The only way it’s possible to recognize a gynandromorph bird is when it belongs to a species with a strong difference between male and female plumage. It’s rare to spot one, and they’re usually hard to keep track of or photograph before they disappear. 

But this winter a gynandromorph cardinal turned up in Shirley Caldwell’s backyard in Erie, Pennsylvania. The bird is amazingly cool-looking, so of course photos of it went viral, and stories about it appeared in the New York Times, Forbes, and more. 

Shirley Caldwell's photo of a bilateral gynandromorph Northern Cardinal. 
Most of the news stories focused on the unique elements of bird physiology and morphology that allow such a thing to happen. The secondary sex features of us humans are influenced much more heavily by hormones than by genetics; in birds, it’s the opposite. Plumage characteristics and wing-length on the female side are completely different from those on the male side, not because of the hormones produced by the bird, which after all flow in the bloodstream throughout the body, but because of how male and female cells respond differently to those hormones in birds. 

A 2010 study investigating three bilateral gynandromorph chickens, "Somatic sex identity is cell autonomous in the chicken," by D. Zhao & D. McBride, published in Nature, established that gynandropmorph birds are not the result of something affecting the chromosomes early in development, but rather, that these birds are genuine male:female chimeras—that is, single animals produced by the merger of two different fertilized ova. In birds, usually if two ova are fertilized at the same time, the result is fraternal twins developing in a double-yolked egg; they usually die before hatching. Chimeras are entirely different. 

Gynandromorph chicken from paper in Nature
It’s easier to do scientific research on chickens than on wild birds, which are protected by law and hard to track in the field. Most birds, including chickens, have only one functional ovary, on the left side. In the chicken study, of two gynandromorphs that appeared male on the left half, one had an ovary on that male side, but couldn’t produce eggs. The bird that appeared female on the left side had an ovary on the left side and a testis on the right. 

A gynandromorph cardinal was discovered in Illinois several years ago—in that individual, the “female” side was the right side. Intriguingly, although that bird was tracked for 40 days, it was never heard vocalizing at all, and it kept to itself away from other cardinals. This year’s Erie cardinal is female on the left side, so it may have a functional ovary. And this one seems to have paired with a male—the two are fairly inseparable, and vocalize back and forth as normal mated cardinals do.

Shirley Caldwell was interviewed for the Forbes article. She's been taking photos and making videos of this bird, and is documenting the behaviors and vocalizations. With luck, she’ll be able to track the birds into the nesting season and find out if the pair mates and produces eggs and ultimately viable young. Field studies are far trickier to do than studies of captive birds, but I’m hoping this star-crossed, or genetically-crossed, bird has a long and happy life.