In a study published in 2003, Slabbekoorn and another colleague showed that great tits sang at higher frequencies in noisier parts of cities.
Other groups later found the same effect with two other species of city-dwelling birds: house finches and song sparrows.
But the new study, published this week in the journal Current Biology, is the first to compare city birds with those of the same species living in natural settings.
Across all ten cities, the birds deleted the lower-pitched parts of their songs. Those are the parts competing the most with traffic noise, so singing low notes is often a waste of time and energy, the researchers explained.
Great tits usually sing two or three notes and then repeat this several times.
The city birds sped up their songs by shortening the first note of these sets, as well as the pauses between them.
The great tits also sang more varied songs with only one note, or with as many as 16 notes strung together, patterns that were unusual in the forest, according to the scientists.
The study makes "a convincing case that this is a broad phenomenon, at least in this species," said Gail Patricelli, a biologist at the University of California, Davis.
"It didn't just happen once. The same phenomenon emerged several times" in the widespread cities, Patricelli said.
Studies like this could eventually help researchers predict how urbanization will affect birds, she added.
Using this knowledge could help urban planners design more pleasant city environments, study co-author Slabbekoorn said.
"You can make a city much more livable if you pay attention to where the noise goes and where the people go," he said. "It would be easy to fit the birds into those models."
Then cities wouldn't be strictly for the people, but also for the birds.
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