Urban birds have taken to singing at night because it is too noisy for their melodies to be heard during the daytime, researchers claim today.
A study of robins in and around Sheffield found that birds nesting in areas that are noisy by day were more likely to sing at night than birds in quieter neighbourhoods. The researchers believe the birds have adapted the times at which they sing to be heard above the din of the city.
The report adds to growing evidence that birds are altering their behaviour to cope with modern life. Last year, scientist revealed that city-dwelling great tits had developed a high-pitched staccato song to attract their mates, while their country cousins adopted a slower and longer twitter. The same year, another group found that American robins began their dawn chorus earlier in areas with lots of streetlamps and other artificial lighting.
In the latest study, researchers at Sheffield University recorded sound levels at 121 sites across the city and beyond between April and June in 2005 and 2006. They heard robins singing during the day at 67 sites, and nocturnally at 18.
Analysis showed that in areas where robins sang at night, the noise levels during the day were 10 decibels louder than at other sites, equivalent to a doubling in volume.
At one site, where daytime traffic raised sound levels to 69 decibels, the researchers discovered a single robin that sang only at night. The study appears in the journal Biology Letters today.
"This shows that there are subtle effects of urbanisation, and that birds are adapting their behaviour to overcome some of these problems," said Richard Fuller at Sheffield University.
Light levels at the different sites were checked, and found to be similar, suggesting urban noise has a greater effect on the birds' behaviour than lighting.
Robins sing to attract mates and ward off rivals, and are among the earliest to join the dawn chorus and the last to fall silent at dusk. The robins' songs are slightly higher pitched than the noise from traffic but scientists say increasing urban noise will affect their song being heard.
The birds may also become more stressed if the sound of the city keeps them awake longer. "If they are singing at night, it is going to take more energy than sleeping, so this may not be good news for them," Dr Fuller added.