But how about singing at night, when cities are quieter? Many birds that
normally sing during the day have been observed singing at night. The culprit
has usually been thought to be light, cities being so bright at night that the
birds stop chirping later or start earlier.
Now a study of European robins in Sheffield, England, suggests that it is
noise, not light, that drives these birds to sing at night.
The study, by Richard A. Fuller and colleagues at the University of
Sheffield, measured noise levels and singing at 67 sites around the city, where
on average ambient noise was an order of magnitude lower at night than during
the day. They found that birds sang only during the day at 49 of the sites, and
both day and night at 18. Daytime noise levels at these 18 sites were
significantly higher than those at the others.
The researchers, whose study is published in Biology
Letters, also measured nighttime light levels and found that increased light was
only a weak predictor of nocturnal singing. Noise was by far the dominant
effect.
http://www.nytimes.com/2007/05/08/science/08observ.html
