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Most city dwellers have been woken up at some point by the chirping of birds outside their apartment window, but a in the Journal of Avian Biology suggests that it might be the other way around: Humans—or, more specifically, their vehicles—are making so much morning noise that they rousing these urban birds earlier than normal.

Typically, birds begin singing at dawn. Scientists offer many possible explanations for this timing. Some believe the early hour increases the birds’ chances of attracting a mate; others believe that the morning air is better at carrying sound. Still others believe that low light makes dawn an inopportune time for foraging. Another explanation is that birds, at least urban ones, need to get their singing in before it’s drowned out by rush-hour traffic.

“Traffic noise is a growing and common problem,” said Aida Arroyo, who explained that the noise can interfere with or “mask” bird songs and calls. “[It] might have negative consequences on those animals living in affected habitats, especially if they depend on acoustic communication for defending resources, feeding, or breeding.”

Some recent studies have shown a shift toward an earlier start time for morning birdsong, known as the “dawn chorus.” Researcher Arroyo and her colleagues at Spain’s University of Seville were interested in why this shift was happening. Previous studies had examined the effect of artificial city lights on birdsong. Arroyo chose to analyze the effect of noise.  

The researchers conducted their experiment in Seville, on 12 tree-lined streets known to be home to six common bird species. These urban streets ranged from relatively noisy to quiet. They recorded the changes in traffic noise from dawn to rush hour. They then tested whether playing pre-recorded traffic sounds over loudspeakers three hours before dawn could cause a shift in the timing of the birds’ songs. 

They found that two of the six species, the spotless starling and the house sparrow, started singing earlier when they played the recorded traffic sounds. Without the recording, the house sparrow would start singing between one and two hours before dawn; the starling would start singing about a half hour before. On days the scientists used the loudspeakers, the birds’ start time was, on average, 20 minutes earlier. 

They also found that house sparrows would wake earlier on quiet streets—even without hearing the recorded sounds—when they experienced sudden loud noises or brief bursts of traffic. This suggested that even species highly adapted to city life are very sensitive to sound fluctuations in their environment. 

 “I hope that our findings can help improve the [awareness] about the effects of noise on animal life,” Arroyo said. She added that she hopes measures will be taken to decrease noise pollution, such as decreasing traffic or using plant noise barriers. She said her future research will likely be aimed at examining such measures.