Sing high, sing fast. This is what songbirds have to do to survive in the din of city life, a study has found.
Songbirds living in forests sing more slowly and in lower frequencies than their cousins who have opted for an urban lifestyle, according to scientists.
The study was conducted on great tits living in 10 European cities and in 10 nearby forests. Scientists analysed the way the birds used songs to attract mates and establish territorial boundaries.
Hans Slabbekoorn, of Leiden University in Belgium, said: "Birds sing faster in the cities compared to forests. The forest birds sing low and they sing slow."
A songbird's vocal repertoire is a key feature that can determine its success in finding and keeping a mate, along with a sizeable territory for rearing its offspring. Great tits living in noisy cities have to compete with the low-frequency sounds of heavy traffic, which means their songs go up in pitch to make themselves heard, Dr Slabbekoorn said.
Previous research found the louder the background noise from traffic, the fewer low-frequency notes were included in a great tit's repertoire of songs compared with birds living in quieter territories.
These earlier findings suggested that great tits living in noisy cities were more likely to sing higher notes than great tits living in quieter forests - which turned out to be the case.
The latest study also found city birds sing faster on average compared with their forest-dwelling cousins because they deliberately shorten the important first notes of each song.
Dr Slabbekoorn said this was because of differences in average wind speed between the two types of habitat - cities have higher air turbulence, which interferes with the transmission characteristics of sound waves. In forests, air turbulence is lower on average than in relatively open urban areas.
The lower turbulence means that the birds can afford to sing long, low-frequency first notes because they tend not get distorted in the same way that long first notes do in urban areas, Dr Slabbekoorn said.
The study was conducted in major cities, including London, Nottingham, Amsterdam, Paris, Brussels and Prague, and could help to explain why some wild birds avoid urban life.
The capacity of great tits to sing within a relatively wide frequency range, and the ability to adjust their songs by leaving out lower frequencies, seems to be critical to their ability to thrive in cities, Dr Slabbekoorn said.
Scientists hope to understand why urban sprawl is leading to the disappearance of some birds and the increasing frequency of others. "By doing studies of this type we may get important insights into why some birds survive well in cities and why others disappear from them," he said.