But as in the human world, those who mix novelty with familiarity occasionally find success, and their new song circulates through a particular community.
Usually, it stays there, and Dr. Otter and his colleagues figured this was happening only with their birds in western Canada, that “it was just an isolated, peripheral population” doing their own thing, he said.
When they tried to figure out where the song’s range ended, though, they realized birds were singing the song in other areas, too. In 2004, half the birds the researchers recorded in Alberta were singing doublets instead of triplets. By 2014, they all were, “and it was starting to show up as far east as Ontario,” Dr. Otter said.
To get a better sense of the spread, the researchers turned to citizen science birdsong databases. They pulled white-throated sparrow songs from across Canada and the northern United States, and plotted them over time and according to song type. In maps, you can see the doublet song gain prominence, its influence expanding and strengthening. By 2019, it had taken over completely from the Yukon to Ottawa, a certified hit that is currently encroaching on the Northeastern United States.
By tracking the western Canadian birds with geolocators, they found that some of them spend their winters in the southern United States, where they overlap with birds from other places. They’re probably sharing the song there, like a mixtape being passed around.
The possibility that the birds are swapping songs on their wintering grounds “really opens up how we think of song learning,” said Dana Mosely, an ecologist at James Madison University in Virginia who was not involved in the study. It’s also evidence that where birds “winter, where they stop over in migration, and where they breed shapes their behavior,” she said.
For a song to take off like this is highly unusual, said Dr. Otter. It goes against prevailing birdsong theories, which emphasize the benefits of sticking to your own local song type. What’s happening with the sparrows is “kind of like an Australian person coming to New York, and all the New Yorkers start suddenly deciding to adopt an Australian accent,” he said.