Robins in urban areas are singing at night because it is too noisy during the day, researchers suggest.
Scientists from the University of Sheffield say there is a link between an area's daytime noise levels and the number of birds singing at night.
Until now, light pollution had been blamed because it was thought that street lights tricked the birds into thinking it was still daytime.
The findings are published in the Royal Society journal Biology Letters.
"You generally only seem to hear nocturnal singing in cities," explained Richard Fuller, one of the study's co-authors.
"So this led us to think that there was some aspect of the urban environment that was driving this phenomenon."
Light pollution had been widely held as the prime suspect. It was thought to prevent the birds from roosting, leading to them remaining active through the hours of darkness.
"That was the stock answer you would get," Dr Fuller said, "that it was basically tricking the birds into thinking it was daylight and tripping some sort of physiological threshold.
"But we thought that was pretty unlikely because birds are much more complex than that."
He said that there had never been a scientific study to measure the impact of light pollution on the behaviour of urban robins.
"So we went out and measured both noctural light and daytime noise levels and we found that daytime noise had a far stronger effect.
"We found that night-time light had a small effect, but very much smaller than the impact of noise levels."
This led the team to conclude that it was an active decision by the birds to sing at night rather than passively responding to light levels.
"The birds appear to be singing at night to avoid competition with high noise levels caused by our cities during the day," Dr Fuller suggested.
"Noise levels were 10 times higher in places where birds were singing at night."
The findings form a part of a seven-year research programme by the university's Department of Animal and Plant Sciences to measure the impact of urbanisation on biodiversity.