It is the sound synonymous with the start of a new day. But because of increased urbanisation and man-made noise levels, birds are having to begin their dawn chorus long before sunrise to make themselves heard.
Robins, blackbirds and nightingales are among species which are altering the time of theirmorning song so their efforts are not in vain, a study has found.
In some cases, birds are starting their dawn chorus two hours before sunrise, potentially putting themselves at risk from predators. A study conducted at five airports found that birds were anticipating the morning rush of planes, which start taking off at 6am, and changing their song times to avoid it.
The study builds on research showing that many species are singing louder and at a higher pitch to increase their chances of being heard in noisy cities - although much of the time they are still unable to compete, especially if trying to drown out a jet.
"Our results suggest that birds may anticipate aircraft noise, and show that birds change their behaviour in anticipation of the increase in noise," said Dr Diego Gil, who led the research. "An earlier dawn chorus is being seen in our cities too because of the joint effects of high ambient noise levels at dawn and continuous artificial lighting overnight."
Birds use song for attracting mates, defending territories or warning against predators. Songbirds are able to vary the pitch, intensity and content of their calls.
Man-made sounds, the authors said, mask signals between birds, hampering their ability to communicate.
In the research, carried out by the Museo Nacional de Ciencias Naturales in Madrid and other institutions, scientists investigated the timing of the bird chorus at five European airports - Berlin, Barcelona, Madrid, Valencia and Malaga. The results, published in the journal Behavioural Ecology, showed that all species sang earlier at the sites where the aircraft noise overlapped the most with their natural singing time.
