A Parus major: New score

New Delhi, Dec. 6: Cities are forcing birds to change their songs.

Biologists have for the first time observed striking differences between bird songs in cities and forests, a signal that birds are adapting their songs in response to urban noise.

Researchers at the University of Leiden in the Netherlands have found that city birds use low notes less frequently, avoid the use of long notes, and appear to end their songs faster than forest birds.

The researchers recorded the songs of the great tit (Parus major) — a bird the size of a sparrow found across Eurasia, including India — from 20 sites — 10 cities in Europe and 10 sites in neighbouring forests.

“These birds are adapting to the background noise of cities,” Hans Slabbekoorn, a behavioural biologist at the University of Leiden, told The Telegraph.

“The reduced use of low notes in the songs by urban birds, for instance, is a response to the presence of constant traffic noise which consists of low-frequency sounds,” Slabbekoorn said.

The findings, published on Tuesday in the journal Current Biology, provide strong support to the theory of acoustic-adaptation, according to which some components of animal and bird sounds may be influenced by the environment.

“The birds can adapt through a process called behavioural flexibility,” Slabbekoorn said.

The capacity of the great tit to sing across a relatively wide range of frequencies and its ability to tune its songs by leaving out lower frequencies appears to be critical to the bird's ability to survive in cities despite urban noise, according to the biologists.

“Birds that do not have such abilities are likely to vanish from cities,” Slabbekoorn said.

The Dutch biologists recorded the songs of the tits in ten cities, including Amsterdam, London, Paris, and Prague, and ten sites in forests. “We found a consistent pattern in all ten out of ten city and forest areas,” he said.

The comparisons showed that for songs important for both mate attraction as well as territory defence, songs by city birds were shorter and sung faster than forest songs. The urban songs also displayed an upward shift in frequency that, the researchers said, fits in with the need to compete with low frequency noise of traffic such as the sounds of cars and trucks.

Slabbekoorn said the findings are not surprising. Longer notes travel better in air with dense vegetation. “In cities, longer notes degrade fast and have less chance of being copied by other birds. The urban birds thus ‘learn’ to use shorter notes,” he said.

The scientists have also found that the first note of a song in urban birds is much shorter in cities than in forests. This is the first observation of differences in a song note than in a full song.

In a previous study, Slabbekoorn had shown that individual birds appeared to have adjusted their songs to traffic conditions.

That earlier study had predicted the possibility that great tit songs may undergo an environment-dependent shift in acoustic features.