If a nightingale sang in Berkeley Square now, it might need to change its tune. Birds are having to adapt their songs to noisy city life so their communications can be heard above the urban hubbub.
City bird songs are becoming shorter, louder, and with longer pauses. They are also sung at higher pitch to rise above the low frequency noise of traffic.
Birds are increasingly singing at night, when noise levels are lower, and there's evidence of an ability to adjust songs by leaving out lower pitch notes which would be drowned by traffic noise.
Some researchers believe that these change are adaptions that will lead to urban and rural birds of the same species becoming reproductively isolated. It's also been suggested that birds and species which fail to adapt will quit city life, reducing urban biodiversity.
While cites and urban areas are attractive, food-rich environments for birds, there is a price to be paid. And the downside of living in a noisy environment can be significant for birds that use acoustic signals to attract mates, defend territories, warn of dangers, and deter competitors.Many human beings find urban noise uncomfortable, but for birds, having vital communications drowned or muffled, threatens their breeding and survival.
Evidence is emerging that birds are rising to the challenge by adapting their behaviour and tailoring their acoustic signals so they can be heard above the urban din. The latest research shows that the blackbird significantly changes its tune as it moved further into a city.
The findings, based on blackbirds in the city of Salamanca in western Spain, show that the frequency of the bird song increased with the background noise. The average maximum frequency of the inner city bird where background noise levels averaged 66 decibels, was 3165 Hz, compared to 2799 Hz in the outer city and 2657 Hz in the rural surrounding area where noise levels averaged 37 decibels.
The blackbird can shift its entire vocalisation to avoid masking by ambient noise, which occurs mainly at low frequencies, according to scientists at the University of Salamanca.
A University of Aberystwyth study has found the same kind of effect with great tits. The study, based on birds in 20 British cities, found that great tits sing at a significantly higher pitch in noisy urban areas compared to the countryside.
Great tits have been one of the most successful species at adapting to urban life, and research at Leiden University in The Netherlands has also shown how different the calls of the city dwellers have become.
Songs that are important to mate attraction and territory defence have significantly diverged in great tits. Urban songs were shorter and sung faster than songs in forests, and often concerned atypical song types. Anthropogenic or man-made noise is most likely a dominant factor driving these dramatic changes.
One of the reasons why great tits have been so successful in adapting to life in the city is that they seem to have a relatively wide frequency range in their singing. Possessing this kind of behavioural plasticity may be key to urban success, while a lack of it may lead to detrimental effects.
Evidence of these effects is already appearing. One German study has found that only high-pitched species now live next to main roads. The low-pitched species have all moved on.
Even for birds that are adapting or trying to adapt, there are problems looming. A study at the University of Mexico, for example, found that while the females of some species, including the blackbird, liked higher-pitched male songs, others, including canaries, prefer a lower pitch. That suggests male who increase their pitch, may be heard, but not heeded.
Males who heighten the pitch of their songs may also be sending out the wrong signal to other males, Lower notes are associated with heavier and bigger birds, more able to defend themselves and their mates. Higher pitches in some species signal vulnerability.
"The frequency of songs could convey information on the singer's ability to fight,'' say the researchers. "It becomes clear that if birds sing at a high pitch to avoid noise masking, they may be sending the wrong message to male competitors. This could result in escalated fights that translates into lower reproductive success.''
Dr Rupert Marshall, animal behavioural scientist at Aberystwyth University, who has carried out a number of studies into birds and urban noise, says the changes being seen could have profound effects. Some of his research suggests that birds from noisy areas respond less strongly to the song of birds from quieter areas, which suggests that young males may have difficulty establishing a territory or attracting a mate if they move to an area with more or less noise than they are used to.
"If they cannot communicate efficiently, it may disrupt their breeding. It may reduce a male's ability to attract a female, or his ability to defend his territory,'' he says. "It also has implications for how urban and rural birds interact. Will they eventually stop recognising each other? Will gene flow be reduced between the urban and rural populations? What will happen in small cities with small populations – will they suffer from genetic bottlenecks?"
He adds: "Communication is important between different species, too. For example, not hearing an approaching predator, such as a sparrowhawk, could have disastrous consequences. Noise may also disrupt a predator's ability to locate prey, and no food may mean a reduced ability to survive. "While some species, the urban survivors, may be able to adapt to man-made noise, the decline of those species that cannot may decrease biodiversity around human settlements. And we would all be the worse off for that.''
There is now evidence that around 15 species have changed their tunes in cities from London to Melbourne. And there are also signs of other behaviour changes among city dwellers.
Research at Sheffield University has shown that urban robins, highly territorial birds that rely on vocal communication, reduce the chances of noise interference by singing during the night in areas that are noisy during the day. They found 18 sites where robins were heard singing nocturnally. At one site, a bird was heard at night but not during the day.
"Urban noise levels continue to increase, through the use of more powerful sources of noise, greater geographical spread and mobility of noise sources, and a greater proportion of the day being exposed,'' say the researchers.
"Our data suggest that such developments will further increase acoustic interference with animal communication, with important consequences for behavioural patterns in urban species.''
A British Trust for Ornithology study has found that city birds have later breakfasts than their country cousins. Small birds such as robins can use up a large proportion of their energy reserves just keeping warm, so they tend to stock up as soon as possible in the morning to replace the energy reserves lost during the night.
But the heat that envelops cities and urban environments can increase temperatures by as much as 8C, and that helps urban birds get through the long winter nights using far fewer energy reserves than those in rural areas. And that means they can forage at their leisure rather than at the crack of dawn.
It has been shown too, that in cities that lie in the snowbelt, birds seek out underground heating ducts over which plants – and food – grow. According to a report from Stanford University, California, urban birds have also learnt to feed in areas lit by artificial light and where they can forage for much longer.
And while rural pigeons usually eat twice a day, filling their crops at each session and digesting the food between feeding bouts, city pigeons are much more opportunistic, and have adopted a snack-like feeding habit.
Swedish city great tits are much paler than those in the surrounding countryside. It's thought to be cause of reduced levels of nutrition in urban areas (especially the lack of carotenoids that make them yellow).
And then there is the curious case of the northern mockingbird. Researchers have shown that these birds, nesting on the campus of a large American university, rapidly learnt to recognise individual humans, and to assess the level of threat posed by them.
In the research, the same person approached and threatened a nest of the bird on four successive days. Each day, the mockingbirds fled from their nest at increasingly greater distances.
On the fifth day, a different person approached and threatened the nest in an identical way. The response of the birds was the same as it was to the first human on the first day. Alarm calls and attack flights also increased from days one to four with the first human being, and decreased on day five with the second person.
"These results demonstrate a remarkable ability to distinguish one human being from thousands of others,'' say the University of Florida biologists. "These are: traits that may predispose mockingbirds and other species of urban wildlife to successful exploitation of human-dominated environments.''
Listen and compare rural and urban great tit songs at: users.aber.ac.uk/rmm/ruralsong.htm