A bird chirping from outside your window can often wake you up in the morning, whether you want it to or not. But new research shows that the exchange is not entirely one-sided. Early birds are rising sooner in the day because of anthropogenic noise, a study has shown.

Researchers from the University of Seville in Spain report that artificial light and traffic noise are rousing birds earlier in the morning.

"Previously there had been studies examining the effect of artificial light on birdsong, but our study analyzed the effect of noise for the first time. We observed that birds like the spotless starling and the house sparrow start singing earlier for this reason," said University of Seville researcher Aída Arroyo.

Arroyo and her colleagues set up an experiment on the streets of Seville wherein they observed the traffic noise throughout the morning in areas where birds habitually congregated to see how levels of human activity altered the time of the bird song.

"Around three hours before dawn we went to these streets, where we had previously recorded the traffic noise at rush hour. We played these recordings using loudspeakers, increasing the environmental noise to approximately 65 decibels. In this way we could see the effect on the time the birds awoke, comparing it to their usual start time," Arroyo explained, adding that 12 streets were included in the study. On what the researchers regarded as the nosiest streets (ones with high basal noise levels from the early hours), the noise began earlier in the day. The birds living on these noisy streets also rose earlier in the morning, singing 20 to 30 minutes earlier.

"In the birds whose dawn chorus was more precisely timed - like the sparrow, one to two hours earlier, and the starling, one hour before dawn --  we have detected this move forward. Other birds have a more variable and wide-ranging start time," Arroyo concluded.

On quieter streets, where short bursts of noise are caused by an intermittent traffic flow, leading to more significant increases in noise levels throughout the morning, this move forward was also observed, "which seems to reveal that species highly adapted to city life, like the sparrow, are very sensitive to fluctuations in anthropogenic environmental factors, in this case noise," Arroyo said.

The research is published in the Journal of Avian Biology.