DAYBREAK in the city. The brief quiet of the night gives way to the low rumble of cars, trucks and industry, but one sound is notable by its absence. Gone is the familiar dawn chorus, with its rich mix of enchanting melodies and calls. In its place is a strangely depleted music – abrupt, high-pitched and sometimes ear-piercing. Welcome to the urban soundscape of the future.
This is no dystopian vision. It is the prediction of scientists who have been studying the way in which noise pollution affects urban bird life. The growing clamour of cities and roads may be annoying to us, but for many birds it can mean the difference between life and death. Background noise can mask both the sounds of approaching predators and the alarm calls that warn of danger. They can also rob individuals of reproductive success by drowning out the songs that male birds use to attract mates and demarcate their territory.
The impact of this noise is now becoming clear. Some species simply are not able to make themselves heard above the ever-growing racket and are finding themselves squeezed out of the city. Others are beginning to change the way they communicate. In the long term, new species may evolve. If noise levels continue to rise, it seems inevitable that urban bird life will change dramatically.
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You can already hear the changes, if you know what to listen out for. One giveaway is birds unexpectedly singing outside their traditional peak times of morning and evening. At these prime times of day, wind noise and turbulence are at their lowest, so sound carries further – but not if you factor in the impact of rush-hour traffic. Richard Fuller of the University of Sheffield in the UK has found that some local robins have abandoned the traditional dawn chorus and taken to singing at night to avoid the daytime din altogether. This shift had previously been attributed to the confusing effects of light pollution, but Fuller’s analysis reveals that daytime noise has a much stronger effect: the parts of Sheffield with nocturnal singers were an order of magnitude noisier by day than other areas of the city ().
It remains to be seen whether singing at night is a successful way to combat noise pollution. It is not the only option, however. Nightingales, when not singing at night, have opted for an approach that seems at odds with their delicate melodies – they simply shout louder. When Henrik Brumm at the University of St Andrews in the UK recorded nightingales singing between 5 am and 10 am he found that those in Berlin sang up to 14 decibels louder than their forest counterparts, achieving volumes of up to 95 decibels – enough to send humans reaching for ear protection. The loudness of their vocals was proportional to the level of background noise, with birds singing particularly loudly on weekday mornings ().
Hitting the high notes
Changes in the timing or volume of songs are fairly obvious solutions to the problem, but some songbirds have taken a more subtle approach. Urban noise is particularly loud at low frequencies – between about 1 and 3 kilohertz. By avoiding these low notes, birds can make their songs more audible. Blackbirds, song sparrows and house finches have all adapted in this way, but the most well-studied practitioner is the great tit.
For the past five years, Hans Slabbekoorn of Leiden University in the Netherlands has analysed the ways in which great tits deal with noisy cities. He found that those inhabitating noisier parts of Leiden with higher minimum frequencies than those in quieter areas of the city. When he looked at populations of great tits in 10 European cities, including London, Paris and Amsterdam, he found that every one of them sang higher-pitched tunes than their forest-dwelling counterparts, raising the minimum frequency by 200 hertz on average, to around 3500 hertz. Not only do urban great tits sing in a higher key, but they also eschew the standard riffs of their forest peers for more original ones ().
The ability to change one’s tune is a valuable asset in the growing urban hubbub. Unlike some birds that learn their entire repertoire while in the nest, great tits, song sparrows and others regularly modify their songs throughout their lifetime. They have far more tunes than they require and select different songs depending on the context. By monitoring which songs work best in a particular situation, individuals can learn from experience and adapt to local changes. Successful new singing strategies can spread as young birds learn to sing by imitating the performances of their seasoned neighbours. Alternatively, songs may become better adapted by default: if younger birds cannot hear the low-frequency segments of their tutors’ songs, they may never learn tunes containing these lower notes, which could then drop out of local repertoires altogether.
“This plasticity provides adaptive value in natural conditions,” says Slabbekoorn. Forests can vary greatly in how loud they are and those birds that live near noisy areas like waterfalls and river torrents also sing at higher frequencies, in a similar way to urbanites. By chance, their flexible singing has put them in a good position to cope with the artificially noisy conditions caused by humans.
Behavioural flexibility is what distinguishes species coping with noise pollution from those that are struggling. The relatively recent rise of urban noise means that most of the vocal strategies used by city birds are likely to be learned responses rather than the result of evolution. In the long term, however, genetic changes are likely to occur because of the role that songs play in survival and reproduction.
Songs are primarily sexual traits that influence the mates females choose and so the success of males. If females come to see the ability to avoid acoustic masking as an indicator of mate quality, they will prefer to mate with males that can do this and the trait will be boosted by sexual selection. In addition, individuals whose hearing is attuned to picking out the songs of other birds amid urban noise are also at a selective advantage, which will ultimately increase their proportion of the population.
If singing and hearing diverge enough, urban birds may be less likely to find the vocals of rural birds attractive, or even to recognise them as members of the same species. These changes could serve to eventually split populations into genetically distinct urban and rural species. Alternatively, different populations of the same species might adopt differing strategies to cope with urban noise, leading eventually to a species split occurring in birds living in the same neighbourhood. “It would be absolutely fascinating to see this kind of sympatric speciation,” says Fuller.
This is not mere speculation. Some scientists believe that the European blackbird has already diverged into separate urban and rural subspecies, with different body shapes and life histories. Slabbekoorn and Erwin Ripmeester, also at Leiden University, are now investigating how urban noise is driving this separation. They are using playback recordings to see if urban blackbirds respond more strongly to urban songs than to rural ones. They have also taken blood samples from urban and rural blackbirds and plan to check for genetic divergence. “It’s certain that we are seeing parts of the process of speciation taking place,” says Slabbekoorn, “but we might not be there to see the end result.”
“Some believe the blackbird has diverged into urban and rural subspecies”
While blackbirds, great tits and others are apparently taking advantage of their adaptable songs, not all birds are flexible enough to cope with the urban clamour. Amid the loud and low-pitched noise, the biggest losers are those that rely heavily or exclusively on low-frequency calls and are physically incapable of switching to higher frequencies. Orioles, cuckoos, great reed warblers and even the familiar house sparrow all fall into this category. House sparrows were once frequent visitors to the UK’s parks and gardens, but populations are now falling, as they are in mainland Europe. “We don’t really understand why that is, but noise may be a factor,” says Slabbekoorn. “House sparrows use an important low-frequency component in their calls.”
In the Netherlands, the great reed warbler has suffered a similar decline. Anecdotal evidence suggests that road noise may have contributed to this: the construction of a road near a particular reed bed reduced the number of warbler breeding pairs from around 10 to just two. When the road was closed for repairs for two years, five more pairs moved into the area, although the subsequent return of traffic drove them away again.
Loud noises can also have unexpected effects, including driving otherwise faithful birds to adultery. Zebra finches, for example, maintain monogamous relationships through a series of calls that allow them to recognise and locate their mate. John Swaddle from the College of William and Mary in Williamsburg, Virginia, found that loud environmental noise prevents female finches from hearing these calls. This erodes the otherwise strong bonds between partners and leaves females showing no greater preference for their chosen males than for strangers ().
Even among species that seem to be adapting successfully to noise pollution, there are signs that they are merely making the best of a bad situation and that such flexibility is not without its costs. House finches, for example, expend more energy on louder songs in noisy cities, and their efforts are shorter as a result. Female house finches prefer mates with longer songs, so males who compensate for background noise could be left with fewer mating opportunities. A great tit’s natural inclination is to show off its lower vocal register during the prime moments of the breeding season. These notes take more effort to produce, says Slabbekoorn, so they probably provide a good indicator of the singer’s strength and his potential as a mate. In an urban setting, however, these dulcet tones may be lost in the hoo-ha, so urban males face a compromise between singing a highly rated song, or simply one that is audible (see Graph).
Noise is just one of the challenges facing urban birds. Until recently, concern has centred on other forms of pollution, including chemicals and light. Yet there is no doubt that noise has already contributed to a decline in the diversity of bird species around cities and major roads. Frank Rheindt at the University of WĂĽrzburg, Germany, measured the diversity of different bird species near a busy local highway and found a dearth of birds that sing at lower frequencies.
The very fact that urban birds around the world are coming up with strategies to deal with noise speaks volumes about the gravity of the problem. “There are many factors that affect a bird’s capacity for breeding in cities but noise has been the most neglected one,” says Slabbekoorn. Just how much it will change the familiar dawn chorus remains to be seen.
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