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Male White-throated Sparrows usually whistle a simple yet elegant tune: Oh sweet Canada Canada Canada. But that's not the song biologist Ken Otter has heard the local sparrows singing since 2000. Instead of the thin whistle typically ending in three syllables, male sparrows in Prince George, British Columbia, sang songs that ended in two syllables: Oh sweet Cana Cana Cana.

Between 2000 and 2019, this rare dialect went “viral,” Otter says, traveling more than 1,800 miles eastward into Canada and the United States. As it spread, the new two-syllable song replaced the widespread three-syllable song, the University of Northern British Columbia researcher and his colleagues report in a published today in Current Biology. 

Such rapid uptake of a new tune is almost unheard of in male songbirds, the authors say. Most birds belt out the same tunes their neighbors and kin do because there’s potentially little advantage to getting creative. Establishing and defending territories, for example, might be hard if a male sang a song unfamiliar to other male birds; likewise, attracting a mate could be difficult if the female doesn’t recognize a male’s song. “You would predict dialects to persist in a location for extremely long periods of time,” Otter says, “because you get punished for not conforming.” But there are exceptions: In Yellow-rumped Caciques, for example, if the tried-and-true song becomes too ordinary and stops attracting females effectively. In that case, a little creativity could help him stand out.

Otter and his team first launched this research because they study the evolution of birdsong and wanted to focus on a widespread, relatively common species. White-throated Sparrows fit the bill. By listening to archived recordings of male White-throated Sparrows from the 1950s and '60s, they learned that the triplet-ending song was ubiquitous across Canada. But somewhere between the '60s and 2000s, the doublet-ending song emerged in a sparrow population in British Columbia, west of the Rockies, and by 2002 all recorded males in this region whistled the two-syllable tune. 

At first they reckoned that the spread would have stopped there, with the mountain range acting as a physical barrier that prevents the crooners that sing doublets from traveling east. “We were thinking that it was a distinct population that was cut off from other White-throated Sparrows east of the Rockies,” Otter says. A had documented appearances of the doublet-ending song in Manitoba, Ontario, Québec, and Maine—all east of the Rockies—but these were only sporadic. 

Maybe, Otter thought at the time, they were looking at evolution in action. To find out, he and his team curated an enormous collection of male songs over a much larger region. They gathered audio uploaded to online community science platforms like eBird and Xeno-Canto and combined them with their own field recordings as well as those donated by other scientists. Altogether the team gathered nearly 1,800 songs of White-throated Sparrows throughout their breeding and wintering ranges.

After analyzing the collection, they found that in 2004 nearly half of male White-throated Sparrows in Alberta, which is east of the Rockies and neighbors British Columbia, sang the novel song—and by 2014, that figure jumped to 100 percent. By 2015, this new tune had invaded all regions west of central Ontario, completely replacing the triplet-ending song, and by 2019, the song traveled into Québec. “It looks like it took a slow time to takeoff, but once enough birds were singing it, it really took off very quickly,” Otter says.

Emily Hudson, a biologist at Vanderbilt University in Nashville who wasn’t involved in the research, is surprised by the rapid uptake of this new tune. “It’s wild that this dialect has swept across the continent and in our lifetime,” she says.

How did the song travel so quickly? The answer is simple: migration. White-throated Sparrows spend their summers breeding in the Northern Hemisphere and then migrate to wintering grounds in the coastal and southern United States. Otter’s team tracked the migratory paths of 41 male sparrows by fitting them with geolocators in 2013 and 2016. It turns out both eastern and western birds used the same wintering sites in Texas. That’s most likely where young, impressionable birds hear different dialects and start to copy them, Otter says. A tune that’s whistled by more birds may be picked up more often.

Otter doesn’t yet know whether ditching the old tune for a new one helps the male sparrows in any way. In , male birds responded just as aggressively to both dialects. “The males treat them almost the same,” Otter says. If females shift their preference for what they consider to be an attractive song, the researchers wonder whether more males will adopt the new tune. Hudson also wonders if some songs may be easier to sing than others, which might make them more popular among young males.

The researchers are lucky because they have a brand-new dialect to study as they try to better answer these questions. In the last five years, White-throated Sparrows in Prince George have started singing a modified version of the doublet-ending song. Only time, and a lot of audio recordings, will tell how soon the predominant doublet-ending tune gets overthrown and replaced by an even fresher version.