Photograph by Richard Olsenius, Nat Geo Image Collection
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Places such as Glacier National Park in Montana have poured rubberized asphalt, which reduces road noise that can affect both birds and humans.

Photograph by Richard Olsenius, Nat Geo Image Collection

Why Louder Electric Cars Could Be Bad News for Birds

Pending U.S. rules would make quiet cars louder, but new research shows that the hum of engines and tires can ruin the avian dining experience.

Nobody likes traffic—including birds. Even moderate road noise drives many  from their habitat, and new evidence suggests bad health effects for those that stick around.

Weight loss, for example. Birds exposed to traffic sounds lost body mass during fall migration, a period when they need to gain fat to fuel their long flights, according to a study from Idaho’s Boise State University.

"Birds are at their physiological limits" when migrating, says study co-author Heidi Ware, education and outreach director of the university's Intermountain Bird Observatory, and the noise distracts them from feeding. Beyond weakening the birds for their long trip south, Ware says, the lower weight can hurt breeding chances in the spring.

The research, published Monday in the Proceedings of the National Academy of Sciences, comes as U.S. highway travel is on the rise again after a period of declines. At the same time, the quieter cars on the road stand to get louder. The Obama administration will soon finalize rules that would require electric and hybrid cars—often nearly inaudible at low speeds—to make more noise so pedestrians hear them coming.

The Boise State researchers created a "phantom road" by playing the sound of cars traveling at two sites in the foothills of southwestern Idaho. This road didn't blare car honks, but it did mimic the noise from 12 cars passing by every minute at 45 miles per hour.

"It sounded like a relatively busy city street or highway," Ware says. The noise was enough to drive away 31 percent of the birds, the researchers found. Of those that remained in the controlled study, all but one of 21 species analyzed suffered reductions in body mass. One species, MacGillivray's warbler, gained only a tenth of the body mass it would have gained in quieter conditions.

The outlier, Cassin's finch, actually gained fat better when the traffic noise was turned on. "That's something we're really intrigued by," Ware says. While it isn't clear why exactly the finches alone did well with the noise—Ware theorized that perhaps the din reduced numbers of competing species—the data points to an effect that reaches beyond just one type of bird.

"I think it's more that it's throwing the whole community off balance, that's the bigger concern," she says.

Ware notes that simple strategies, such as pouring quieter rubberized asphalt, reducing speed limits, and restricting cars in general, can help reduce road noise, particularly in national parks. As for electric cars, which can sneak up on pedestrians when they travel at low speeds: "I think you need to have minimum sound levels that cars need to make, but you also need to have maximums," Ware says. "Right now car companies can add any frequencies they want, as loud as they want."

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The white-crowned sparrow decreased its feeding time when exposed to traffic noise, leading to a drop in body mass seen across most species in the Boise State study.

Some hybrid and plug-in models, such as the Chevy Volt, have special pedestrian alert noises that drivers can activate. The National Highway Traffic Safety Administration is proposing that all quiet cars play simulated engine noise when traveling below 18 miles per hour. While sales of hybrid and electric cars have suffered hiccups in the U.S. thanks to low gas prices, among other factors, global sales are estimated to more than double by 2023.

Robert J. Dooling, a University of Maryland scientist who co-wrote an often-cited 2007 report on highway noise and birds, which he says he is currently updating, called the Boise State paper "an extremely well-designed and executed study" that zeroes in on the impact of road sounds in a way that previous studies have not.

However, he pointed out that the Boise team produced just the sounds of traffic without the visual cues that usually accompany it in the real world. "Presenting noise alone without the other sensory effects that always occur with it may actually be more stressful for animals," Dooling says, "since they cannot identify the expected features in the environment."

Ware says her team also plans to look at the impact of noise from oil and gas operations. She points out that road noise affects people, too: It's been linked to higher rates of hypertension and obesity.

"For the average citizen who doesn't care about birds," Ware says, "This is just another sign that we need to start paying attention to this as people as well."

The story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

On Twitter: Follow Christina Nunez and get more environment and energy coverage at NatGeoEnergy.