Like many birds, male white-throated sparrows belt out songs to defend their territories and attract mates.

And until the year 2000, one particular song stood out as the most popular white-throat tune in forests across Canada.

"The end part of the song is a three-syllable repeat.

That's supposed to sound like 'Oh my sweet Canada,Canada,Canada.

'" University of Northern British Columbia behavioral ecologist Ken Otter.

"And if you look in most books that describe what the song sounds like, it always has a three-syllable phrase at the end of it."But when Otter moved to Prince George,a wooded city in western Canada, 20 years ago,he noticed that its sparrows were singing a different tune.

"The males have dropped one of the notes,so instead of having three syllables repeating, it's actually two syllables repeating ...""'Oh my sweet Cana,Cana,Canada.

'" Back in the 1950s,though, the Prince George population was still singing the standard triplet song.

"So sometime during that 50-year period,the song had transitioned to this new dialect, and all the males had adopted it."Over the next few years, Otter's team discovered that the doublet song variant was spreading eastward.

"And it's replacing the old song as it goes."By 2010 the song had invaded Ontario, thousands of kilometers from Prince George.