Berlin - Big-city noise levels prompt birds to sing louder in order to be heard by other birds over the din, according to research by German ornithologists.
The study of free-ranging nightingales in and around Berlin showed that chocolate male birds "singing in the dead of night" have to raise their little voices in order for females of their species to hear them.
An analysis of sound pressure levels revealed that males at noisier locations sang with higher sound levels than birds in territories less affected by background sounds, according to research headed by Dr Henrik Brumm of the Institute for Biology at Berlin's Freie University.
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This is the first evidence of a noise-dependent vocal amplitude regulation in the natural environment of an animal.
'Some birds clearly developed hoarseness and coughing' The results, published in the May issue of Journal of Animal Ecology, demonstrate that the birds tried to mitigate the impairments on their communication caused by masking noise.
This behaviour may help to maintain a given transmission distance of songs, which are used in territory defence and mate attraction.
Dr Brumm's team of researchers studied 15 male nightingales in various densely-populated districts of the German capital. In one test, along busy Potsdamer Chaussee avenue, the birds had to compete with traffic noise levels of up to 89 decibels - as loud as a high-power motorcycle.
But at weekends, when traffic along the same road was greatly reduced, the birds sang much softer, according to Brumm's findings.
At the same time, birds forced to sing with higher amplitudes have to bear the increased costs of singing, Dr Brumm wrote. He said some birds clearly developed hoarseness and coughing.
'During the night only those males who have no partner sing' This suggests that in songbirds the level of environmental noise in a territory will contribute to its quality and thus considerably affect the behavioural ecology of singing males.
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