Baby Birds' Efforts to Outshout City Noise May Take Toll

James Owen
for National Geographic News
April 14, 2005
Screeching brakes, wailing sirens, blaring music, roaring jets—the
constant din of city noise is enough to drive some of us to distraction.
But what of the birds that must share our increasingly motorized world?
Can they make themselves heard?

It's a question scientists are now trying to answer, with recent studies indicating that some birds, to avoid being drowned out completely, are making more of a racket themselves.

Nightingales in Berlin, Germany, have been found to call louder during the weekday rush hour than on weekends. Similarly, great tits living in Leiden, Netherlands, sing more shrilly in noisy neighborhoods than birds that live in quieter city precincts.

Meanwhile, a new Canadian study shows that nestlings also turn up the volume so that their parents can hear their begging above the background ruckus.

It shouldn't be a surprise that birds are so sensitive to environmental noise, says Henrik Brumm, a biologist at the University of St. Andrews, Scotland. "Few animals use acoustic communication to the same extent that birds do," he said. "That's why they are especially prone to noise interference."

Brumm, who specializes in animal communication, says such interference can potentially lead to serious difficulties for birds as they try to defend territories or attract mates.

The same pressures may apply to hungry chicks in the nest, according to a study published last week in science journal Proceedings of the Royal Society (Series B).

Canadian biologists at Dalhousie University in Halifax found that tree swallow chicks in Nova Scotia react to increased background noise, including the sound of nearby traffic, by calling louder than nestlings in quieter areas.

Ambient Noise

The researchers say it's the first study to show that ambient noise can affect bird-begging signals in this way. They add that their findings raise concerns about how chicks cope amid urban surroundings.

"Some studies suggest that begging takes a lot of energy," said the study's lead author, Marty Leonard. "If calling loudly adds to that cost, then when nestlings are forced to shout above urban noise, they might have less energy for growth. If so, they could leave the nest in poor condition."

Leonard says a chick's begging signals relay important information to its parents. "Nestling calls increase in rate and length as they get hungrier and decrease in frequency and rate as they get chilled," she said. "So calls appear to encode information on the need for food and warmth."

Louder begging calls may not only consume more energy, but they may also attract unwelcome visitors to the nest. "The louder you call, the more likely you are to be detected by unwanted [listeners], like predators," said Brumm, the St. Andrews biologist.

Brumm has studied how nightingales—famed for their melodious song—respond to the hustle and bustle in the German capital city of Berlin.

He found that male nightingales boost the amplitude of their song in line with levels of background noise, most noticeably during the morning rush hour.

"When traffic noise levels rise, they pump up the volume and sing louder," he said. "At weekends, when there was less traffic, the birds sang quieter."

The biologist says the findings were a major surprise, because songbirds weren't thought to show much variation in the loudness of their calls. "People thought they always sang at the top of their lungs," Brumm said. The fact that they don't suggests songbirds have to balance song output with energy expenditure.

European Starlings

A 2002 study on the metabolic costs of singing in European starlings showed that a 16 decibel-increase in song level led to a 16 percent increase in oxygen consumption.

Researchers trying to gauge the effects of human sounds on wild birds face a significant hurdle: How do they isolate the influence of these human sounds from other factors, such as air pollution and urban development?

"One of the difficulties has been to tease apart the effect of noise, per se, from impacts of the urban environment, such as obvious changes in habitat," Leonard, the Dalhousie University researcher, said. "Our next step is to create experiments that will allow us to disentangle these effects."

Another complication for scientists is that birds also have to deal with natural background noise produced, for instance, by wind, rain, and other birds. Some species have even evolved eardrum-piercing calls to combat the masking effects of such sounds.

Take the European dipper, an upland river bird that emits a shrill zit designed to be heard above rocky, fast-flowing waters.

While recent studies suggest some city songsters might be able to adapt to increased noise pollution, scientists still aren't sure which bird species best tolerate noise pollution.

"There are limitations; birds can't cope with [every] level of noise," Brumm warned.

"Data from humans show that chronic noise levels can affect human physiology and well-being quite considerably," he added. "Perhaps birds also suffer from stress syndromes."

For birds, as for many of us, life is getting noisier. Let's just hope their vocal cords can cope.

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