Current hypotheses suggest songs sung by birds of a particular species remain the same within a region, and distinct between regions. These songs persist between bird populations over extended periods of time. However, when researchers analysed the songs of 1,785 male white-throated sparrows (Zonotrichia albicolis) recorded across North America over two decades, they found the spread of a novel song that is being sung by these birds across Canada. The song has covered a distance of over 3,300 km, from British Columbia to Ontario since it started spreading after 2000.
The research paper by Ken A Otter, Alexandra Mckenna, Stefanie E LaZerte of University of Northern British Columbia and Scott M Ramsay of the Wilfrid Laurier University, Waterloo, ON N2L 3C5, Canada was published in the journal Current Biology on July 2.
What have the researchers found?
Researchers studied the cultural evolution of songs sung by 1,785 male white-throated sparrows over a period of two decades and found that doublet-ending songs spread from east to west, replacing the traditional triplet-ending songs in Canada.
According to surveys done in the 1960s across Canada, the white-throated sparrows traditionally sang a whistled song terminating in a repeated triplet of notes. Between the 1960s and the 2000s, however, doublet-ending songs emerged and replaced the triplet-ending songs west of the Rocky Mountains. The song had reached the east of the Rockies by the 2000s.
Essentially, from recordings that were collected over two decades across North-America, the researchers note that the novel doublet-ending song that originated in western Canada, has now spread at a continental scale. In 2004, when researchers recorded birds across Alberta, they found that half of the sampled males were singing the traditional triplet-ending songs. Within the next 10 years, they found that all the males in the area had switched to the new doublet-ending song.
How did this happen?
Potentially, the novel song’s spread could be attributed to birds from western Canada spending the winter with birds in central Canada, where the song initially spread, possibly through song tutoring as birds from large portions of the breeding ranges intermingled.
What are the key findings?
What is unprecedented is that the novel song, which was initially a rare variant, emerged to be the “sole, regional song type”. It is uncommon for a novel song to “invade” and “replace” an established regional variant since birds rarely change their songs and even if they do the change is limited to a region, unlike what researchers have observed in the case of the white-throated sparrows, where the song has spread across the continent.
“To our knowledge, this is an unprecedented rate of song-type transition in any species of birds,” the researchers note, adding that in the historic recordings taken prior to 2000, males can be heard singing the triplet-ending song.
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From the new song’s initial detection in 2005, when in the survey researchers found that 1 out of 76 males sang the novel song, the proportion of males singing the song had increased to 47.8 percent of one surveyed population in 2017, suggesting that once a new song emerges, it takes time to build momentum before it becomes established.
“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,” researchers note.
But why would the male birds adopt the new song at all?
It is not entirely clear as to why the males would do this, but as per one hypothesis called the indirect biased transmission hypothesis, certain innovations in songs are non-randomly adopted by the males within a population, which leads to fast population-level transitions to new variants. For instance, juvenile males learn and actively integrate novel elements into their songs thereby furthering the transition. There is also the possibility that males may integrate novelty into their songs to maintain female interest, the research paper states.
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