City Song: Birds sing higher near urban traffic

City songbirds that stake out territories near loud traffic tend to pitch their songs at higher frequencies than do birds in quieter neighborhoods, Dutch researchers have found.

Recordings of a common European species, the great tit (Parus major), showed a higher minimum frequency in the noisier parts of Leiden, says Hans Slabbekoorn of Leiden University. In the loudest places, engine roars overlapped the lower frequencies of the tits’ songs, Slabbekoorn and his Leiden colleague Margriet Peet report in the July 17 Nature.

“We don’t know enough about the effects of noise pollution,” says Slabbekoorn. “This hints at a difference between birds that adapt to the city and those that can’t.”

Great tits, relatives of North America’s chickadees, sing several songs, including one that Dutch bird-watchers compare to the “tee-tah, tee-tah” of a bicycle pump.

A classic study in 1979 demonstrated that great tits living in dense woods tend toward songs simpler than the more ornamented vocalizations of birds living in areas with more open ground. Researchers have also shown that birds such as nightingales sing louder in a laboratory when there’s background noise.

For their study of urban birds, Slabbekoorn and Peet turned to great tits, which abound in European cities. “I’ve recorded tits under the Eiffel Tower; I’ve recorded tits in Buckingham Palace,” says Slabbekoorn.

The data the team has analyzed so far come from recordings of 32 male tits in various parts of Leiden. The researchers also took a series of recordings and background-sound measurements in each location before, during, and after rush hour.

The average minimum frequency of the males’ songs, ranging from 2.82 to 3.77 kilohertz, was lower in quieter neighborhoods than in noisier ones. Urban noise, mostly from engines in cars, trucks, boats, and modern conveniences such as leaf blowers, encroached on birds’ lower frequencies in the loud neighborhoods, the researchers say.

Slabbekoorn points out that young great tits learn their songs in large part when they establish a territory and have song duels with neighboring males. He speculates that in noisy spots, the higher-pitched songs may be more effective in deterring rivals, and it’s these songs that the young males are more likely to copy.

André Dhondt of Cornell University, who has spent 25 years studying great tits, says he’s not surprised by the finding, considering the previous work on the effects of natural background noise. His own work showed that male great tits with longer, more precise songs tend to live longer and father more offspring than less-vocal males do. He emphasizes that males have a strong incentive to make themselves heard, no matter what the environment.

Haven Wiley from the University of North Carolina at Chapel Hill, whose research team has studied the effects of natural background noise on animal communication, points out that urbanization has many effects on birds, and it remains to be seen how noise ranks among them.

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