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.
This is the first evidence of a noise-dependent vocal amplitude regulation in the natural environment of an animal.
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.
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.
A migratory bird, the common nightingale (Luscinia megarhynchos Brehm) arrives in Berlin in late April from its winter quarters in the African tropics. From then until early June the male birds sing at dusk and dawn to attract females and also to mark their territories.
Nightingales leave in the autumn between the end of July and September, and they tend to move through Europe on a broadly south-west basis, with birds occurring throughout the Mediterranean region though commonest in west.
The are relatively scarce in much of North Africa and also the Middle East in autumn which tends to suggest that the Mediterranean and Sahara are normally crossed in one continuous flight.
When it is winter in Europe, nightingales can be found in sunny climes between the Sahara and the rain forest in West Africa east to Uganda.
They are present in their wintering quarters from early November through to early April.
Some stay on in the Afrotropics until early May, but spring passage through Nigeria is concentrated in late March and early April with arrivals in North Africa and southern Europe at this time.
Nightingales do not just sing at dawn and dusk, but sometimes also as the song says, "at the dead of night".
However, Valentin Amrhein from the University of Basle, Pius Korner of the ETH Zurich and Marc Naguib from Bielefeld University have ascertained that not all of them sing at night.
Males who have captivated a partner go quiet when it gets dark.
The three researchers have been investigating the behaviour of nightingales for many years and their emphasis is on the bird's elaborate songs, of which the scientists say there is an amazing variety.
During the day both female and male run through their repertoire of 260 "verses", which usually mutate after a few seconds to the nightingale's well-known song. But during the night, some of the birds are silent.
"During the night only those males who have no partner sing" the scientists say.
"Nightingale males return from their winter quarters in the south earlier than the females," explains Korner. On their arrival the males occupy a space and begin to sing during the night.
"But all singing males become silent once they had found a partner," says Korner "only bachelors carry on with their trilling."
But suddenly, shortly before the eggs are laid acoustic activity increases and, for about three nights, successful males also begin to sing again. After that all fall silent again.
Amrhein, Korner and Naguib asked themselves, "why this second phase of nightly singing?" and they came up with three propositions.
It is possible that the male sings to induce the female to come to him, for a so-called "extra-pair copulation".
Or the male sings to ward off his rivals who are also looking for an extra-pair copulation.
It is also possible that the male wants to impress his own partner with his singing and incite her to greater reproductive willingness. Perhaps females lay more or better eggs the more often their partners sing.
Or perhaps, they theorised, "he just enjoys singing". - Sapa-dpa