From Many, One: How Many Species of Redpolls Are There?

By Gustave Axelson
March 30, 2015
One species or two? Common Redpoll (left) by Sharon Watson via Birdshare, Hoary Redpoll (right) by Chris Wood.
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The Hoary Redpoll is one of those hard-to-get lifelist-adds that can turn birders into Captain Ahab seeking a little whitish bird. The allure of these little ghost finches has drawn many a lister to places like Minnesota’s Sax-Zim bog—in the dead of winter—just for a chance to lock into a Hoary.

But new research by two scientists at the Cornell Lab of Ornithology presents genetic evidence that reopens questions about the species status of the Hoary Redpoll, long thought to be the frosty cousin of the Common Redpoll. In a paper published this week in the journal Molecular Ecology, Nicholas Mason and Scott Taylor of the Cornell Lab’s Fuller Evolutionary Biology Program show that Hoary Redpolls and Common Redpolls have no differences at all across much of their genomes.

“Based on the samples of DNA we examined for Common and Hoary Redpoll, they’re probably best treated as a single species,” Mason says.

In other words, should this new evidence similarly sway the American Ornithologists’ Union’s checklist committee, all the heroic efforts birders have made to add a Hoary to their life lists may be for naught.

The division of redpolls into different species dates back to before the Civil War. In 1861, legendary ornithologist Elliot Coues (one of the founding fathers of the AOU) described eight separate redpoll species based on their visual appearances. Over time the AOU consolidated Coues’ list, but Hoary Redpoll, which has a snow-white breast, was still considered a separate species from Common Redpoll, which has a brown-streaked breast.

The researchers compared the DNA of 77 redpolls. The evolutionary tree they reconstructed shows that the three redpolls intermixed extensively in their evolutionary past. If they were separate species the branches of the tree would be much more distinct, as shown for their close relative, the White-winged Crossbill. Adapted with permission from Mason and Taylor 2015, Molecular Ecology; White-winged Crossbill by Nick Saunders via Birdshare.

Mason and Taylor looked beyond the plumage into strands of the birds’ DNA in the most extensive look ever at the redpoll genome. Whereas previous genetic analyses of redpolls looked at just 11 regions of the genome (at most), Mason and Taylor examined 235,000 regions. (That impressive number is a testament to the exponential advances in DNA-sequencing technology, but the researchers are quick to note it’s still less than 1% of the total genome.)

In all, the duo compared DNA from 77 redpolls, including specimens from museums around the world, from the Museum of Vertebrates at Cornell University to the Natural History Museum of Geneva in Switzerland. They found no DNA variation that distinguishes Hoary Redpolls from Common Redpolls. Furthermore, another redpoll species found in Europe—the Lesser Redpoll—also had extremely similar DNA sequences. This extreme similarity among all the redpolls stands in marked contrast to studies of other groups of birds—such as Black-capped and Carolina Chickadees—which show differences at many regions of the genome.

In nature, one of the key differentiators among distinct species is assortative mating, that is, members of a group breeding with each other more often than they breed with members of another group. According to Mason, when it comes to Hoary, Common, and Lesser Redpolls, “There are no clear-cut genetic differences, which is what we would expect to see if assortative mating had been occurring for a long time.”

The three current species of redpoll—Common, Hoary, and Lesser—stretch around the Arctic in a continuous swath that isn’t necessarily apparent from a normal map projection. Adapted with permission from Mason and Taylor 2015, Molecular Ecology.

Instead, Mason says the world’s three redpoll species seem to be “functioning as members of a single gene pool that wraps around the top of the globe.”

But how could it be that Hoary and Common Redpolls look so different given that their genetic makeup is basically the same? For that answer, Mason and Taylor delved into the birds’ RNA. (A quick flashback to high-school biology: If DNA is like the body’s blueprints, RNA is like the construction foreman communicating the instructions to build physical features, like hair or feathers.)

The physical differences among redpolls are associated with patterns in their RNA, not their DNA. In other words, the variation we see in plumage and size is probably not a matter of genetic variation, but of genetic expression. It’s kind of like how two humans might have the same gene for brown hair, but one person’s might be lighter than the other’s—that gene is being expressed differently. In the same way, Hoary and Common Redpolls have remarkably similar sets of genes, but those genes are expressed differently, causing the plumage and bill-shape differences we see.

To look simultaneously at both DNA and RNA, Mason and Taylor sampled birds—some with highly streaked plumage, some with white plumage, and some with in-between markings— from a large flock that had gathered in a fellow Cornell Lab employee’s backyard in Cortland, New York. If Hoary and Common Redpolls had long been separate species, then the birds sampled should have mostly fit neatly into two categories, both by visual appearance and genetically. Instead, there were a few birds that definitely fit the visual description of what we call a Common Redpoll, a few birds that definitely fit the pattern for a Hoary Redpoll, and a lot of birds in the middle—with varying degrees of whitish breast and faint brown streaks.

The bird on the left is a classic dark, streaky Common Redpoll, while the bird on the far right is a snowy, small-billed Hoary Redpoll. But many birds lie in between these two extremes. The new research suggests all the Common–Hoary confusion over the years may have been justified. Photos, from left to right, by Sharon Watson, newfoundlander61, Guy Lichter, Stuart Oikawa, and Chris Wood.

“We didn’t find distinct characteristics to separate the redpoll types, but rather a continuum, or a progression, of physical traits,” Mason says. “And many redpolls were somewhere in the middle.”

Next, Mason and Taylor are planning to work their research into an official proposal for the AOU to lump Hoary, Common, and Lesser Redpolls into a single species, based on the genetic evidence. If accepted by the AOU’s Nomenclature Committee, the end result may sting for birders who see a Hoary Redpoll subtracted from their life list. But Taylor hopes his research will change the way people look at redpolls altogether.

“I think this makes them a more interesting bird,” he says. “It means they’re part of an exciting, complicated system that can make a single species look different across different parts of its range.”

For more on redpolls and phylogeny: