New research: Redpoll species are all one

Updated, December 9, with a comment at the end from the American Ornithological Society.

For birders struggling to figure out what kind of redpoll they’re watching, new research from the University of Colorado at Boulder says don’t worry — the different species are actually all one and the same.

This new research, out recently in Nature Communications, finds that redpolls aren’t multiple species, genetically speaking. Instead, the three recognized species (Common, Hoary, and Lesser Redpoll) are all just one with a “supergene” that controls differences in plumage color and morphology, making them look different.

This research builds on findings from 2015 suggesting this might be the case but without having a clear idea as to why. This time, with greater genetic technological capability, the researchers looked at the full genome of the different species and found that what commonly signifies different subspecies in birds doesn’t apply to redpolls.

“I think, solidly now, the new paper shows that there is widespread gene flow across the (redpoll’s) genome, except for this one region, and it just so happens this one region influences the way they look,” said Scott Taylor, assistant professor in the Department of Ecology and Evolutionary Biology, director of CU Boulder’s Mountain Research Station, and an author of the paper.

Redpolls are a type of “winter finch,” or a finch that lives in the northernmost reaches of the globe and flies south sporadically in winter. While known for their characteristic red marking on their heads, appearance within the species can vary widely. Some redpolls are white with small bills, and others have streaked breasts and larger and darker bills. Initially, it was thought that these differences signified three different species of the bird — like is the case with many other bird species — but those “species” can be difficult to differentiate.

“Redpolls have been taxonomically confusing for a long time, and we only get to see them every once in a while in the winter,” Taylor explained. “They aren’t a bird you’re guaranteed to see at your feeder, so when it happens, people get excited, and they want to know what they’re looking at.”

In 2015, however, then-postdoc Taylor and graduate student Nicholas A. Mason, now an assistant professor at Louisiana State University, found preliminary genetic data suggesting they aren’t different species. Instead, they are all the same species, just with different appearances — but the researchers weren’t quite sure why.

A more complete picture

These new findings picked up where that previous research left off. Erik Funk, a graduate student at CU Boulder and the lead author of the paper, re-examined the original samples and adding a few more from other areas, including Greenland, Iceland, and parts of Europe, to get a more complete picture.

Altogether, Funk examined the full genomes of 73 individuals from all three currently recognized species.

What he found is that, despite the differences in appearance, the birds are almost identical genetically but with a “supergene” that controls the different traits that make the birds appear different. In particular, the researchers found a chromosomal inversion (part of one chromosome is flipped) that allowed this supergene to be created.

“Oftentimes, we assume that a lot of traits can act independently, meaning that different traits can be inherited separately from one another, but this particular result shows that sometimes these traits are actually tightly linked together. At least for these birds, they’re inheriting a whole group of traits together as one,” said Funk.

And redpolls aren’t alone with these supergenes. Many species, from other birds to certain types of mice, are now known to have these supergenes.

“It seems to be less common, but I think one of the things that we are learning as we have access to more sequence data now is that maybe they’re not as uncommon as we once thought,” Funk said.

While the researchers now have an answer about why this species’ appearance varies, questions remain as to how. So, how are these traits maintained? And how will that change as the arctic warms? Both are questions the researchers hope to dig into next.

“Sometimes birders get mad if you take birds off their list, but I think it makes the redpolls even more interesting,” Taylor said. “Understanding the genetic basis of the trait makes it make much more sense now, which I think is pretty cool.”

Update, December 9: A spokesperson for the American Ornithological Society says its taxonomic committee (which decides whether to split or lump bird species) “is open to reconsidering the issue and will do so when a new proposal is submitted.”

Thanks to the University of Colorado at Boulder for providing this news.

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