Sparrows in San Francisco's Presidio district changed their tune to soar above the increasing cacophony of car horns and engine rumbles, details new Mason research in the April edition of Animal Behaviour.
"It shows a strong link between the change in song and the change in noise," says David Luther, term assistant professor in Mason's undergraduate biology program. "It's also the first study that I know of to track the songs over time and the responses of birds to historical and current songs."
The study, "Birdsongs Keep Pace with City Life: Changes in Song Over Time in an Urban Songbird Affects Communication," compares birdsongs from as far back as 1969 to today's tweets. Plus, the researchers detail how San Francisco's streets have grown noisier based on studies from 1974 and 2008.
Luther wrote the study with Elizabeth Derryberry, an assistant professor of ecology and evolutionary biology at Tulane University and a research assistant professor at Louisiana State University's Museum of Natural Science. "We've created this artificial world, although one could say it's the real world now, with all this noise -- traffic, leaf blowers, air conditioners," Luther says. "A lot of birds are living in these areas, and what, if anything, is this doing to their songs?"
Turns out, quite a bit.
Just as we raise our voices to be heard when a car speeds past, birds making their homes near busy intersections have to tweet a little louder, Luther says. But it's more than just whistling the same tune and turning up the volume. Most birds stopped singing some old songs because those ditties couldn't cut through the racket.
The bird they studied is the white-crowned sparrow, a small bird that sports a jaunty white cap with black stripes. Only male birds were studied.
Even birds from the same species don't sing the same song. "Some bird species sing in different dialects just like the way people talk differently if they are from Texas or California or New York, even different parts of New York," Luther says.
The sparrows warble in low, medium and high frequencies.
"It's the really low hum where almost all of this human-made noise is -- in this very low bandwidth. The birds can often sing at the top end of that low bandwidth," says Luther, whistling a lively bird tune, "and if there's no traffic around, that's just fine. But if they're singing and there's this," he says, making a low humming noise, "the lowest portion of that song gets lost, and the birds can't hear it."
So the birds changed their tune. Sparrows in the Presidio used to sing in three distinct dialects when famed ornithologist Luis Baptista made his recordings in 1969. When Luther worked with Baptista some 30 years later, those song stylings had dropped to two, with one higher-range dialect clearly on the way to be the only song in town.
"One dialect had basically taken over the city," says Luther, adding that it is officially called the "San Francisco dialect."
Songs need to be heard, not just because they sound pretty -- birds use them to talk to each other, warn away rivals and attract mates.
"If you go into a bird's territory and play a song from the same species, they think a rival competitor has invaded its territory," Luther says. "It's just the same way if you're in your house and you hear strange voices, as if someone broke in."
If the rival bird can't hear the song and vamoose, then it may come to bird fisticuffs. That can lead to injury or death.
To do the study, the researchers found territories of 20 sparrows in the Presidio where there's lots of traffic, especially in the morning rush hour when the birds do most of their singing.
They set up an iPod speaker, shuffled the sparrow songs from 1969 and 2005 and waited for a reaction.
"The birds responded much more strongly to the current song than to the historic song," says Luther, adding that the sparrow flew toward the speaker while chirping a "get out of here" song. "The (current) songs are more of a threat."
Chirps from 1969 didn't raise a feather. "They don't think that bird is as much of a threat," he says.
This study sets up the next one, Derryberry says. The next question is whether the females care about these changes or if any song will do. "We want to understand if the females discriminate between these songs as well," she says.
White-crowned sparrows are interesting birds because their songs changed with the noise environment, Derryberry says. "Here's a bird that's able to hang around," she says. "A lot of species haven't been able to adapt to and live in an urban environment."
- David A. Luther, Elizabeth P. Derryberry. Birdsongs keep pace with city life: changes in song over time in an urban songbird affects communication. Animal Behaviour, 2012; 83 (4): 1059 DOI: 10.1016/j.anbehav.2012.01.034
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