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 nightingalesfamed for their melodious songrespond 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.
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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