“The national pride of Canada is hinging on this,” jokes Jeff Podos, a biologist at the University of Massachusetts at Amherst, who was not involved with the study. More seriously, says Podos, seeing changes in a feature as basic as the number of notes in a song is striking. “Just to be able to see the speed of this change is really amazing,” he adds. Scientists still don’t know how the song variant managed to spread so completely across an entire country.
Otter and his team think the Cana variant of the song originated in western Canada between 1960 and 2000. Archival recordings of white-throated sparrows before that all ended in the classic Canada. Once the team heard the strange new variant in Prince George, they started looking east, especially in Ontario’s Algonquin Park, where scientists have long been monitoring a population of white-throated sparrows. They didn’t find a male singing Oh sweet Cana in Algonquin Park until 2005. They found two more in 2007. Over the years, the number steadily increased until 44 of 92 males they recorded in 2017 were singing Oh sweet Cana. Meanwhile, in Alberta, in western Canada, surveys in 2004 and then 2014 found that the Cana variant had completely replaced Canada between the two surveys. Hundreds of additional birdsongs uploaded by birders to sites such as eBird and Xeno-canto corroborated the findings.
Read: The quiet disappearance of birds in North America
The new song variant had clearly spread west to east, but how? From 2014 to 2016, Otter and his team were able to recover nine sparrows that they tagged with geolocators. White-throated sparrows spend most of the year in Canada and the northeastern United States, but they migrate to warmer places during the winter. These geolocators showed that birds from both western and eastern Canada spent the winter in overlapping areas in eastern Texas, Oklahoma, Arkansas, and Kansas. “It gave us the idea that maybe what's happening is these birds are singing on the wintering grounds and becoming song tutors for birds that live east of the Rockies,” Otter told me. Birds from the West were meeting young sparrows from the East—and possibly passing on Oh sweet Cana.
Young sparrows indeed have a sensitive period when they learn to sing, says Jill Soha, a birdsong researcher at Duke University, who was not involved in the study. Once this period is over, their songs remain mostly fixed for the rest of their life. But Soha wonders whether the young sparrows, who hatch in the spring and summer, would already be too old to learn by the time they migrate to their wintering grounds. “From lab studies,” she says, “they shouldn’t be learning new songs after about 100 days of age.” If young white-throated sparrows really are picking up the Cana song variant over the winter, that would challenge the conventional wisdom on how the birds learn. Otter said he thinks mixing over the winter has to play a role because Oh sweet Cana could not have spread so quickly, based on models, if it were simply diffusing from west to east.