Noise pollution drives away some birds, but benefits those that stay behind

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.

The team found that fewer species of birds lived near the louder gas wells. Some species like the mourning dove and the black-headed grosbeak were almost exclusively found near the silent sites. It's no coincidence that these birds have very low-pitched calls, with frequencies that overlap with most man-made noise. Theirs are the tunes most likely to be masked by the compressors. And other species nested further away from the wells at the loud locations than at the quieter ones, including flycatchers, vireos and warblers.

As expected, noise was off-putting to several species of birds and it has a negative impact on the richness of local species. But it didn't seem to impact the total number of feathered residents. Francis found the same number of nests near the two types of wells, so clearly, some species were thriving in the noisy neighbourhoods and compensating for those that were driven away.

Noisy woodlands weren't bereft of birds, but they were represented by different species. In particular, black-chinned hummingbirds and house finches positively preferred real estate near noisier places and the vast majority of both species were found near sites with compressors. The hummingbirds' calls have some low-pitched components but they mostly stay within high registers that rise above man-made dins. Likewise, we know that house finches are capable to raising the pitch of their songs in response to urban noise. That certainly explains why they can survive in noisy areas, but not why they seem to actively prefer such spots.

The draw of the noisier venues was a lack of predators. For smaller birds, egg-eating predators, like the western scrub jay, are the major cause of nest failure. But noise affects the jays too, for they are another species with easily masked, low-frequency calls. Their intolerance means that they are less common at noisy sites than at quieter ones.

This means that at loud sites have both drawbacks and benefits - it's harder to be heard, but the broods of small birds are less likely to suffer at the beaks of egg thieves. For birds that managed to tolerate the din, their chicks were more likely to hatch. Even species like the gray flycatcher that prefer to nest away from the noise, nests were more likely to escape the attentions of predators at louder locations.

Francis's study illustrates the complex effects that a single factor - in this case, man-made noise - can have on an ecosystem. The scrub jay may take the eggs of small birds but it is also important for the pinyon pine trees that make up the bulk of the woodlands. By dispersing seeds, it contributes to the overall health of the forest so if it's being driven away by noise, small birds might benefit in the short-term but the entire community might be harmed in the long-term. Understanding these effects will become ever more crucial as the spread of human activity makes the natural world a noisier one.

Images Finch by Matthew Field, hummingbird by MDF; gas wells by Clinton Francis

More on conservation:

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• Human hunters unwittingly shrink their prey species at incredible rates

Reference: Francis, C., Ortega, C., & Cruz, A. (2009). Noise Pollution Changes Avian Communities and Species Interactions Current Biology DOI: 10.1016/j.cub.2009.06.052