Arctic redpolls all belong to the same species | Polarjournal
In temperate latitudes, redpolls are seen only now and then in winter. Photo: Jyrki Salmi, Wikimedia Commons, CC BY-SA 2.0

There are some species of birds that birders have a hard time identifying because they closely resemble related species. This is also likely to be the case for redpolls, which breed circumpolar in the Arctic and fly only sporadically to temperate latitudes in the northern hemisphere. Depending on the region, redpolls differ in body and beak size and plumage coloration. Therefore, redpolls were divided into three species, whose exact identification is not simple however: Common redpoll (Acanthis flammea), Lesser redpoll (Acanthis cabaret) and Hoary redpoll (Acanthis hornemanni). Scientists at the University of Colorado Boulder have now found that genetically, the three species are actually one single species.

Redpolls are easily recognized by their red head markings, but still confuse birders because their appearance can vary greatly within the species. Some are white and have a small beak, while others are darker and smaller with a larger beak. Originally, these differences were thought to indicate three different species.

The study, published in the journal Nature Communications, has now revealed that a “supergene” is responsible for the differences in plumage colour and morphology. Unlike other birds, redpolls lack what usually signifies different subspecies. Instead, according to Scott Taylor, assistant professor in the Department of Ecology and Evolutionary Biology, director of the University of Colorado Boulder’s Mountain Research Station and co-author of the study, there is a region of the redpolls’ genome that happens to influence the birds’ appearance.

a – Seasonal distribution of redpolls: light red – breeding area, dark red – wintering area. The colored dots indicate the number and location of the specimens, with blue corresponding to the Hoary Redpoll, gold to the Common Redpoll, and green to the Lesser Redpoll. b – “ConStruct Plots” show that genetic clustering corresponds poorly with taxonomy. c – Illustrations of the three ecotypes of the Common Redpoll, showing differences in size, bill and plumage colouration. Figure: Funk et al. 2021

In a previous study from 2015, researchers already recognized that redpolls must be genetically one species that appears with different external characteristics. They weren’t sure why, though. So Erik Funk, a doctoral student in the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder and lead author of the study, re-examined the samples at the time and collected more from other regions such as Greenland, Iceland and Europe to get a more complete picture.

In total, Funk analysed the entire genome of 73 redpolls of all three subspecies. Despite their different appearance, they are genetically almost identical. However, they have a supergene that controls the various traits that make the birds look different. In addition, the researchers discovered a chromosomal inversion – a reversal of part of the chromosome – in one chromosome that allowed this supergene to emerge.

“Often times 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,” Funk said.

These supergenes are also known in many other bird species and also in certain mice. “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 continues.

After finding out why the appearance of redpolls varies, the researchers next plan to explore questions about how the traits are maintained and how that will change with warming in the Arctic for these Arctic specialists.

“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.”

Julia Hager, PolarJournal

Link to the study: Erik R. Funk, Nicholas A. Mason, Snæbjörn Pálsson, Tomáš Albrecht, Jeff A. Johnson, Scott A. Taylor. A supergene underlies linked variation in color and morphology in a Holarctic songbird. Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-27173-z

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