BOISE - Nearly 6,000 feet up on an Idaho mountain, nature's soundtrack generally includes little more than the wind whistling through the pines and the warble of some 60 species of songbirds that use this high-altitude pit stop to fatten up on berries and bugs before continuing their migration south.

Last summer, however, Idaho scientists blasted the sounds of cars and trucks barreling past from a half-mile string of treetop loud speakers high on a ridgeline above Snake River Plain.

The researchers called it their "phantom road" and say the experiment shows that traffic noise alone - separate from pollution or the sight of cars - can have a dramatic effect on songbird migration. The work could eventually guide wildlife managers trying to balance the interests of visitors driving through America's preserves and the wildlife they have come to see.

Boise State University biology professor Jesse Barber says that when the speakers blared, songbird numbers declined by more than a quarter and some species disappeared almost completely. He says the work may be the first to isolate the effect of sound, eliminating other traffic-related disturbances from the equation of what scares birds away.

The work, funded by the National Park Service and published in Britain's Proceedings of the Royal Society B, has already led Barber toward some early suggestions to minimize harm to migrating birds.

"Probably the simplest thing to do is to reduce traffic during important times of day, and important times of year, in good habitat," Barber said. "So when we have a road going through protected natural areas, maybe you shouldn't be allowed to drive on it in the early morning, when songbirds are trying to communicate and mate."

Nearly 300 million annual visitors go through the National Park Service's 401 units annually, mostly arriving in automobiles. Though studies have concluded these parks are generally quiet, developed areas and roadways can be quite noisy, especially in high season, said Karen Trevino, who heads up the agency's Natural Sounds and Night Skies division.

Visitors don't always realize the potential problems traffic noise can cause for wildlife, she said.

"You'd be surprised at how many people would say, 'It's just a car going by,' or 'It's just a truck, and there's no longer any impact,'" Trevino said from her Fort Collins, Colo., office.

So the service partnered with Boise State University and Barber's team on the $267,000 project, using traffic sounds recorded on Glacier National Park's famed Going to the Sun Road in Montana to fill in knowledge gaps.

Data collected in Idaho will buttress the scientific record that park managers use, not just to help federal agencies determine where and when people should be driving, but also to give parks options such as selecting quiet pavement technologies.

"Research by Dr. Barber and his team gives us some new information that we can factor in with all other existing information," Trevino said.

Barber's research was done from July until October in 2012 on 5,900-foot Lucky Peak just east of Boise, home of the Idaho Bird Observatory and a key spot where songbirds take what's generally a weeklong pause on their journey from Canada to Central America.

"The number of birds present along the phantom road during noise-on periods was 28 percent lower," according to the team's published conclusions. "Cedar waxwing and yellow warbler almost completely avoided the phantom road during noise-on periods."

"It might drown out their communication more than other species," said Christopher J. W. McClure, a post-doctoral researcher, who was involved in the study.

He added, "whether it's airplanes, or cars, or weed eaters - if wildlife are being managed for a purpose, we need to think of noise."

Barber's team has moved on to the next phase research by examining the effect of noise on the birds that used the route in spite of the noise.

Heidi Ware, a 24-year-old Boise State biology student, says preliminary results show those along the phantom road didn't pack on as much fat as their undisturbed counterparts, possibly making survival more precarious on their migration south.

"My main curiosity is why it happens," Ware said. "Are they distracted or stressed out? I'm looking at the cost of staying."