KEY POINTS:
Songbirds in cities are damaging their health, exposing themselves to predators and weakening their gene pool by trying to be heard above the din of urban life.
New research shows that male birds trying to compete with traffic and city sounds are singing louder and at increasingly higher frequencies, which could harm their vocal cords and hearing. As a result their songs are becoming more chaotic and less diverse, which makes them less attractive to female birds and damages their mating opportunities.
Some birds, including robins, are choosing to sing at night instead of during the day. This not only makes them more vulnerable to attack but, because the birds need to be awake in daylight hours to feed, creates stress and exhaustion.
"The difference between urban and rural birdsong is becoming so great that the two groups could now be unable to communicate, leading to inbreeding and a weakened gene pool," said Dr Sue Anne Zollinger, of the University of St Andrews in Scotland, who has studied the impact of environmental noise on birds' song learning and development.
A group sharing a small gene pool, said Zollinger, was less likely to adapt quickly to new diseases or environmental pressures, putting them at risk of being wiped out.
Birds sing to warn of danger, attract a mate and mark out their territory. A study of nightingales found that birds in Berlin sang up to 14 decibels louder than their counterparts in the forest. The highest volume occurred on weekday mornings.
A further group found that great tits in European cities sang at a higher frequency than birds in the country, so that they could be heard above cars, lorries and industry.
"By trying to sing over the sound of the city, birds are risking vocal injury because they're using more pressure to sing loudly, while also singing at higher frequencies to try to counteract the low rumble of traffic noise," said Zollinger.
Singing under such pressure means birds have less control over the sound they make.
"Their songs may lose quality and become more rough-sounding," said Zollinger.
"This could have serious implications on how fit and attractive they're perceived to be by females. Even when they manage to mate, however, their female offspring will prefer males who sing in the same lower-quality way, which will eventually lead to complete isolation between different groups and inbreeding."
Henrik Brumm, from the Max Planck Institute for Ornithology in Germany, agreed. "There are many different strategies that birds deploy to counter the noise of the urban environment. These strategies could lead to us seeing a completely separate urban species of bird which can't breed with its country cousin."
Andre Farrar, spokesman for the Royal Society for the Protection of Birds in London, said: "If birds start simplifying their songs so they can be heard against background noise, they might find themselves attacked by other birds who think they're sick. This could have serious implications for conservation."
Mark Constantine, author of The Sound Approach to Birding said birdsong had been proved to reduce people's blood pressure and the impact on humans of birdsong was huge.
"It harms us, as well as the birds, if their songs become simpler, shriller and louder."
OUT OF TUNE
* Birds sing to warn of danger, attract a mate and mark out their territory.
* Male birds in cities are singing louder and at increasingly higher frequencies as they try to compete with traffic and other noise.
* This is making them less attractive to female birds and damages their mating opportunities.
* To beat the noise, some birds are singing at night, making them more vulnerable to attack and more tired and stressed during the day.
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