Cities are noisy places. If you ever get annoyed by the constant din of traffic, machinery and increasingly belligerent inhabitants, think about what songbirds must think. Many birds rely on songs to demarcate their territories and make their advances known to mates. They listen out not just for the sounds of seduction or rivalry, but for approaching predators and alarm calls that signify danger. Hearing these vital notes may be the different between life and death.
Last year, I wrote a feature for New Scientist about the effect that urban noise has on songbirds. Those that can’t make themselves heard are being pushed out of cities; others have developed strategies to rise above the clamour. British robins have avoided the traditional dawn chorus, when rush hour is at its peak, in favour of night-time singing when their tunes can stand out. German nightingales take the more straightforward approach of singing very loudly, belting out their songs at 95 decibels, enough to damage human hearing if sustained. And some species – great tits, house finches and blackbirds – have opted for higher notes, which are less easily masked by the typically low frequencies of urban noise.
So some species are adaptable enough to thrive in a cacophonous environment that would drive out those that can’t change their tune. And if the species that are driven away include predators and thieves, the birds that remain fare even better. That scenario is playing out in the cities of America. Clinton Francis from the University of Colorado at Boulder has found that noise reduces the diversity of bird communities but it actually helps those that remain.
Previous studies have linked the presence of noisy roads and industries with sparser populations of local birds, but never conclusively. Noise is also associated with habitat changes or visual disturbances, and it makes it harder for scientists themselves to spot birds – all of these factors could explain any disappearances.
To get around these problems, Francis relied on a unique natural experiment, taking place in the woodlands of New Mexico. Here, natural gas is pumped out of the ground and at some sites, it is then pushed along pipelines by compressors that are very noisy and that operate constantly. Other sites that lack compressors are much quieter but essentially the same in terms of environment and the surrounding trees. By comparing woods near noisy and quieter gas wells, Francis could isolate the effects of noise from those of the mere presence of industry. He even managed to get the compressors turned off for short windows while his team took stock of the local birdlife.