Tweet Tweet? TWEEEET!!

The other day I was talking to my parents via Skype when we were rudely interrupted by a very loud bird. It was a starling, and it was hanging out on the chimney, its (very loud) song echoing down into my parents’ living room.

While we commented on the prettiness of the bird’s song, he was probably caught up in something more serious. Birds generally sing to attract a mate or to warn other birds of an intruder, not to entertain human beings. However while birdsong generally only has a positive impact on our day-to-day lives (except for the odd incident of an aggressive dawn chorus exacerbating a nasty hangover), humans have made the lives of urban birds a little tricky.

A recent study published in Animal Behaviour by David Luther and Elizabeth Derryberry looked at how human noise, particularly traffic noise, has affected the song of white-crowned sparrow in San Francisco. Male white-crowned sparrow generally learn one song, which they use throughout their whole life, and the song they learn depends on where they grew up. Much like a New Yorker will struggle to understand someone from the Deep South, a sparrow from the country will have trouble deciphering the warbling of a city sparrow. But what’s the difference?

Sound waves are defined both by their frequency, which determines pitch, and their amplitude, which determines volume. Anthropogenic noise, such as the sound of a car engine, tends to have a low frequency. Bird song is more varied, and utilizes sounds of both low and high frequency. In order to be heard above the surrounding cacophony of cars, a male urban sparrow would be therefore expected to either sing louder, or change his song so that it uses higher frequency sounds.

Luther and Derryberry used an archive of white-crowned sparrow recordings from different areas of San Francisco, spanning over 30 years, to address how these birds have adapted to life in the big city. Using these data, combined with an assessment of how noise in San Francisco has changed over the same time period, they found that the frequency of the sparrow song had indeed increased.

In their second experiment, the scientists took recordings of white-crowned sparrow from the 1970’s and played them back to sparrow in various parts of the city. The sparrow were far less interested in these songs than they were in songs from their contemporaries, suggesting they either couldn’t hear them as well over the ambient city sounds, or that they were less able to recognize them as local dialects.

In a similar study, Derryberry has looked at rural populations of the same birds and found that the frequency of the sparrow song had actually dropped, likely due to the preference of female sparrow for these more sultry tones.

And this is what Luther and Derryberry plan to address next, but when they’re done perhaps they could assess the acoustic advantage of singing atop a chimney by starlings in rural areas of northern England…

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Katie Pratt is a graduate student in Molecular Biology at Brown University. She has a passion for science communication, and in an attempt to bring hardcore biology and medicine to everyone, she blogs jargon-free at www.katiephd.com. Follow her escapades in the lab and online on Twitter.

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