I just can’t understand what those country birds are on about (Image: Neil Bowman/FLPA)

Call them the urban new breed. We know birds raise their voices to make themselves heard in the noisy big city, but for the first time there is evidence that they may even be evolving as a result of city living.

“Urban birds might be becoming genetically distinct, which is the first step towards becoming a new ‘urban’ species,” says Dominique Potvin of the University of Melbourne, Australia.

Unlike rural areas, towns have constant background noise, as well as buildings that reflect and distort sound. Since male birds use songs to defend their territories as well as to attract females, the urban din can cramp their style. “Whether a species can survive in urban habitats depends on whether or not it can adapt acoustically,” says Potvin.


Last year, researchers found that great tits (Parus major) spontaneously change their tune depending on background noise levels. Potvin and colleagues wondered if urban birds might be evolving, not just learning.

Jamming songs

To test their idea, the team recorded 14 silvereye (Zosterops lateralis) populations over 1 million square kilometres of east-coast Australia, making this the largest study, in terms of area, of its kind.

Silvereyes are common native Australian songbirds found in both town and country. “Their natural sounds would be masked by urban noise,” says Potvin. Such noise, generated mostly by road and air traffic, is at pitches between 1 and 4 kilohertz – bad news for silvereyes, which sing in the 2 to 6-kilohertz range.

Potvin recorded male birds in seven Australian cities and corresponding rural areas: for example, in cosmopolitan Melbourne and the nearby Lerderderg State Park. In total, 81 birds were recorded, and each one tagged to ensure it was not sampled twice.

She analysed both songs and contact calls. Contact calls mark danger or food, are shorter than songs, and are innate – unlike songs, which birds learn from their fathers. “If you raise a baby bird in a box it wouldn’t know to sing, but it would know how to contact call,” Potvin says.

High and slow

At all sites, on average, urban birds sang and called at higher frequencies than their country cousins, with city hipster birds singing 195 hertz higher and calling 90 hertz higher. “It’s a bit of a shock,” says Potvin. “The city is pushing these birds to evolve.”

The city birds also sang slower songs, singing fewer syllables per second. According to Potvin, this could be because sound-reflecting buildings make songs less clear and longer pauses between syllables make them easier to understand.

Potvin speculates that birds who don’t adapt cannot be heard by prospective mates and therefore cannot breed. It’s also possible, she says, that these birds have difficulty protecting their young, or surviving up to a reproductive age.

“We cannot exclude adaptation,” says Hans Slabbekoorn, a behavioural biologist at Leiden University, Netherlands, who specialises in birdsong. But it is also possible, he says, that bird calls are more flexible than previously thought: silvereyes might be just calling louder under noisier conditions.

Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2010.2296