White-throated sparrows across Canada are abandoning an old song in exchange for a trendy new tune. The reason remains a mystery.
Most birds have distinct calls that tend to stay the same. It's how birders can recognize a species without seeing it. But new research shows these tunes can change.
Over the course of two decades, white-throated sparrows across western and central Canada have changed one of their songs, replacing a three-note call with a two-note one. The new tune started in British Columbia and spread west—now, most of Canada's birds are singing it. And it's still spreading in Quebec, more than 2,000 miles from where it originated.
Although some bird calls undergo slow evolutions, this rapid shift in a bird's song has never been observed before, says Ken Otter, lead author of the study, published July 2 in the journal Current Biology.
"There is nothing that we know of that's spread like this," Otter says.
As the song sweeps west to east, ornithologists wonder what makes the song so catchy—and if the trend will continue. The finding was made possible by crowd-sourced birdsong recordings, which are uncovering patterns that may have previously gone unnoticed.
A SONG IS BORN
Birdsongs are not just pleasant to listen to, they're also rich with information, such as the health and fitness of the speaker. Like other birds, male sparrows sing to establish territory and to entice females. It's only the males that sing certain tunes, and they learn them during a critical window early in their development.
Otter, who studies bird behavior and communication at the University of Northern British Columbia, first noticed that something was up with sparrow calls in the late 1990s. He was doing fieldwork in British Columbia, just west of the Rocky Mountains, with a colleague who usually studies eastern populations of the species.
"We were walking around... and he suddenly said, 'Your sparrows sound weird.'" Otter hadn't noticed it before but agreed—they did sound different.
White-throated sparrows have this classic song that's supposed to sound like it goes, 'Oh, my sweet Canada, Canada, Canada,'" he explains. "And our birds sound like they're going, 'Oh, my sweet Cana– Cana– Cana– Canada.'"
The new song trend emerged by the 1990s in northern British Columbia, where Otter and his colleague first heard the "weird" call. From there, it crept east, moving across Alberta, Saskatchewan, and Manitoba.
In 2004, about half the sparrows in Alberta were singing the new doublet ending, but by 2014, every sparrow in the area had made the shift. By 2015, every sparrow west of central Ontario was singing the doublet ending. It didn't stop there. In western Quebec, nearly 2,000 miles from where the song began, it's still spreading.
Knowing that bird songs must be learned from others, Otter and his colleges suspected that eastern and western sparrows may be crossing paths.
In 2013 and 2016, they strapped geolocators to 50 male sparrows breeding in Prince George, British Columbia, to track their seasonal migration path and areas where they winter.
Otter says he expected the western sparrow populations to travel directly south to their overwintering areas in California. Instead, the birds crossed the Rocky Mountains, meeting up with eastern populations in the southern Great Plains of the United States, in Texas, Oklahoma, Arkansas, and Kansas. This convergence of western and eastern sparrows may act as a tutoring ground for young males, which could learn the new song before returning to their respective breeding ranges.
Using two decades of citizen-recorded data, including more than 1,785 recordings, Otter and his team were able to map the song's spread. Charting the new song in blue and the old song in red, Otter's maps show a cascade of blue dots crashing east from 2000 to 2019. Only a thin ribbon of red dots—birds singing the old song—still clings to the eastern edge of the country.
"It's cool to realize that this sort of happenstance pattern of migration allowed [some sparrows] to then hear birds singing the other form of song"—and then spread—"like a viral contagion," says Jeffrey Podos, who studies birdsong at the University of Massachusetts Amherst and was not involved in the study.
Podos isn't surprised the birds are learning from each other, but he admits that the pace at which the new song spread is "somewhat surprising."
"It's like a blue wave," he says.
TRENDY TWEET
New variations of songs crop up constantly, but the vast majority of these aren't picked up by other birds.
"For some reason, some birds just went deviant," says Podos, describing the advent of the new doublet-ending song. "You figure it would have just died on the vine, but somehow other birds must have found it interesting."
Otter and his team didn't find that birds singing new doublet-ending songs were better at wooing mates or defending territories, so it doesn't appear to be advantageous or deleterious. This just adds to the mystery of the song's virality.
"The only thing that we can think of is that the females might have a preference for something that's slightly novel," Otter says.
It's possible that sweeping evolutions in songs like this have happened before but went undetected. Otter's work relied on recordings from eBird and Xeno-Canto, databases which contain birdsongs recorded and uploaded by people around the world.
Bob Planqué, a cofounder of Xeno-Canto and mathematics professor at Vrije Universiteit in Amsterdam, says this crowd-sourced information is "a tremendous boon to academia." One reason this model lends itself so well to studying birds, says Planqué, is that recording songs is easy and accessible. Planqué says hundreds of papers a year rely on Xeno-Canto data, which includes over half a million recordings.
Crowd-sourced science is "like having thousands of research assistants spread out across the continent," Otter says. "It's allowing researchers to tap into a totally different avenue of research [and] to look at this on a very big scale that was never there before."
