A new birdsong unexpectedly spread across North America

• Studying birdsong might help scientists to understand how humans develop dialects, says Angelika Nelson, an ornithologist at the Landesbund für Vogelschutz in Bavaria, Germany, who studied white-throated sparrow song and wasn't involved in the study.

Birdsongs can change over time but those changes are typically limited to a region and its dialect. In the case of the white-throated sparrows of North America, their song changed across the continent.

• The traditional three-note-ending song, which dominated the repertoire of sparrows west of the Rockies, was abandoned for one that ends in two notes. (The mnemonic is Oh my sweet, Can-a-da, Can-a-da, Can-a-da! for the three-note tune versus Oh my sweet, Can-a, Can-a, Cana-da!)
• The song spread across North America and, as of 2019, only birds in the easternmost regions of the continent continue to sing the triplet-ending song.

"This would be like, if you were from Kentucky, and you move to Seattle, and everybody starts thinking, 'hey, this Kentucky accent sounds awesome.' And suddenly 10 years later, everybody in Seattle has a Kentucky accent," says Ken Otter, an ornithologist at the University of Northern British Columbia and an author of the study.

• "It's completely at odds with the expected norm for how regional song variants would establish and solidify. Instead this is actually spreading."

What they did: Otter and his colleagues used 1,785 male white-throated sparrow, Zonotrichia albicolis, bird song recordings collected by citizen scientists to show the spread of the new song.

• Birds migrate and spend the winter with birds from other regions and the researchers thought perhaps the younger birds were learning the new song from their seasonal friends and taking them back to their breeding grounds.
• The team tracked the location of sparrows with backpack geolocators and found those from the west, where the song was first observed, were overwintering in the southern U.S. with birds from farther east, where the song was later observed.
• "It took 9 years (2005–2014) for the song variant to go from approximately 1% to 22% of males adopting, but then only 3 years (2014–2017) to go from 22% to nearly 50%, suggesting that the cultural spread may be exponential once a critical number of males have begun adopting the new variant," the authors write in Current Biology.

• "To our knowledge, this is an unprecedented rate of song-type transition in any species of birds."

The intrigue: It's unclear why the new song is preferable.

• Males sing to defend territory, which allows them to attract a mate. At the same time, females use birdsong to assess a male's prospects as a mate.
• The researchers think females may have a preference for novel songs — but not too novel.
• What to watch: Evidence for that comes from another song that has emerged — and spread — in the west. (This one is a doublet too but the first note is modulated.)
• "As of this year, it's completely replaced the traditional doublet. And so there was nothing super special about the doublet," says Otter. "It was just something different."