Tuesday, May 08, 2007

Song Birds After Dark

Stuart and I have noticed lots of chirping birds during the dog's ~11pm walk. We thought it might be due to the city lights. It turns out they might be adjusting to urban noise:

Being the divas that they are, songbirds don’t like noise. Urban birds have been known to alter their songs to be heard above the racket — making them shorter, higher pitched or
louder, or changing their patterns.

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.