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Annie Lindsay says it’s not unusual for the research team at Powdermill Nature Reserve in Rector, Pa. to see rose-breasted grosbeaks during fall migration. The birds breed in the region and are now headed to southern Mexico, Central America and northern South America for the winter.

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But on September 24, avian researchers found a rare rose-breasted grosbeak in their net — a bilateral gynandromorph. That means the bird is male on one side of its body, and female on the other. The line between the two is right down the middle.

“It was actually towards the end of the banding day,” said Lindsay, bird banding program manager at the research station. “So we almost missed it.”

In a bilateral gynandromorph, each side of the body has its own genome, and also its own sex. In this case, the grosbeak is female on its left side, and male on its right. Linsday says you can tell mostly by the way the bird looks. Female grosbeaks are brown. “They have a thick white line above their eyes, and their wingpits, their underwing coverts, are yellow,” she said.

On the male side, the wing feathers are black, and under the wing is pink. There are also some pink spots on the chest. The black wing on this grosbeak also measured longer than the brown side, which is consistent with the male of the species. 

According to Lindsay, gynandromorphs are really uncommon, but completely normal. The phenomenon occurs when an egg doesn’t shed some of its DNA in a package called a polar body. “Both the polar body and the egg’s nucleus are fertilized,” she said. “That’s how this happens.”

Can It Reproduce?

The grosbeak might be able to reproduce, because in songbirds, the functional ovary is on the left side where this bird is female. Assuming it has an ovary, it could theoretically mate with a male and produce an egg. But, there are a lot of “ifs” involved.

“So if that bird sings like a male, it would be potentially trying to attract females, and also eliciting a territorial response from other males, essentially saying ‘keep away, this is my spot,’ ” Lindsay said. “And territorial males will either respect that or it would be more of an aggressive response, not a great mating response.”

Powdermill researchers took some feather samples from both sides of the bird for genetic analysis. The grosbeak was then banded and released like any bird at the banding station during fall migration, except for a short photo shoot and the filming of a quick TikTok video. 

Less than ten gynandromorphs have been caught in the bird banding program’s nearly 60 year history at Powdermill, which is part of Carnegie Museum of Natural History. Lindsay was there in a different role 15 years ago when another bilateral gynandromorph, also a rose-breasted grosbeak, was caught. “To be fair, back then, I didn’t realize how significant and how cool that was,” she admitted.

For her, the best part of this year’s find was watching the reaction of the Powdermill team as they realized what they had. “You know, the science is important, but awe and wonder are important, as well.”

Other News from Powdermill

The gynandromorph is just one of several exciting events that have happened this year at Powdermill, according to Lindsay. A record number of red-eyed vireos have been recorded this fall at the station, and they also caught a Brewer’s sparrow, a species that rarely comes east of Colorado and Wyoming. It turns out that it’s the first record of the bird in Pennsylvania.

This story is part of our series, Wild Pennsylvania. To check out all of the other stories in the series, click here

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