Birds Struggle to Tweet Over Traffic Din

July 10, 2008 -- Planes, machinery and car traffic are so noisy now in many cities that some birds must tweet louder and alter their songs in an attempt to be heard over the near-deafening urban din, new research has determined.

Since birds rely on their songs for attracting mates and defending their territories, the battle could mean the difference between bird survival and death, with some species possibly not having the ability to literally change their tune, or others being forced to forever sing in a different way.

"Traffic noise may change bird songs in a particular area permanently as cultural transmission may result in a continuation of current songs," biologist Hans Slabbekoorn, who conducted the study, told Discovery News.

"Even when it gets quiet again, typical urban singing styles may remain," he added.

Slabbekoorn, of Leiden University, has been studying the situation for years, but recently focused his work on one of the more adaptable birds, the chiffchaff, a common and widespread leaf warbler.

Under "normal" forest conditions without traffic, the birds still have to contend with fluctuating noise levels from other bird species, wind and rustling leaves, so it's likely they developed a more flexible singing approach.

To test this theory out, Slabbekoorn observed and recorded the birds alongside a busy stretch of highway in the Netherlands. He determined they use a few different strategies to beat the noise pollution.

"They may sing louder when it is more noisy," he said. "They may also shift upward in frequency (because) if they avoid using notes with low frequencies they will be less masked by predominantly low-frequency traffic noise."

Just as a shouting human's voice rises in frequency when it goes up in amplitude, so too does the bird's tweet. Since, like many birds, chiffchaffs sing most activity at dawn and just before dusk, they really had to blast out their songs, as these times also coincide with rush hour traffic.

Yet another coping strategy utilized by this species is to make time adjustments, by singing longer notes or by inserting new song passages into relatively silent breaks that occur every so often when traffic subsides.

The findings were recently presented at the Acoustics '08 meeting in Paris.