Experiment of the month

Natural History, Nov, 2002 by Stephan Reebs

Anyone who has ever taken a walk in the woods knows that songbirds, despite their size, can make themselves heard a long way off. But that raises an intriguing question. Do the birds, like opera stars trying to project to the nosebleed seats at the Met, always sing at the top of their lungs? To find out, Henrik Brumm and Dietmar Todt of the Free University in Berlin ingeniously exploited a well-recognized human behavior for modifying speech, known as the Lombard effect: as the noise level rises, people tend to speak more loudly (think of the last party you attended). If songbirds, unlike people, always vocalize as strenuously as they can, the investigators reasoned, the birds' singing should not exhibit the Lombard effect.

Brumm and Todt played white noise to nightingales--ardent European songsters--and measured the amplitude, or loudness, of the birds' vocal performance. Result: the territorial birds would fit right in at a human party. The louder the white noise, the louder the birds sang. Average sound levels of bird-song rose from about 74 decibels to 84, which human listeners would perceive as a doubling in loudness. The results also make good sense energetically--by adjusting their output to the level of environmental interference, the songbirds may be limiting the metabolic costs of belting out a tune. ("Noise-dependent song amplitude regulation in a territorial songbird," Animal Behaviour 63:891-97, May 2002)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).