Birds hit the highest notes in fight for survival amid traffic

Songbirds in the city sing at higher pitches to make themselves heard over the roar of traffic and other machinery.

Songbirds in the city are singing at higher pitches to make themselves heard over the roar of road traffic and other machinery.

Urban great tits in the noisiest areas sing at the highest frequencies, almost certainly to counteract the almost continuous rumble of aircraft, roadworks and cars that surrounds them. But while to humans the chirping might sound cute, to the birds it is a matter of life or death. Like other songbirds, male great tits mark out their breeding territories and attract mates by singing. If they cannot be heard, they are less likely to breed. Adapting to the city din is thus essential.

Hans Slabbekoorn and Margriet Peet, Dutch researchers, recorded 32 male great tits in the city of Lieden. In the science journal Nature they report that each bird had a repertoire of between three and nine different song types.

The researchers found the average minimum song frequency was "significantly correlated with ambient noise" and ranged between 2.82 kilohertz and 3.77kHz. Humans normally range from 300 hertz to 2kHz, while machinery noise is typically around 2kHz. "Noisy territories were home to great tit males whose songs had a high average minimum frequency," noted the two scientists, from Lieden University. "Birds in quiet territories sang more notes that reached the lowest frequencies measured for the population."

The birds were not simply choosing territories that suited their singing styles. Great tits learnt their songs and were known to adjust them during interaction with their neighbours. They appeared to be "adjusting song to territory, instead of territory to song".

The scientists added: "Our findings show, to our knowledge for the first time, that human-altered environments might change the communication signals of a wild bird species."

The ability to avoid being drowned out by background noise could be part of a general mechanism that helped bird species to breed in noisy environments, they said. Birds unable to adjust in this way could suffer a decline in numbers. Human noise might therefore affect species diversity in cities.

"All sorts of machinery create a new selection pressure on wildlife species that use acoustic signals to achieve reproductive success. This might create two groups of species: one that can adapt their signals to the competing noise and another that cannot."