It's a grand plan for public art - 24 searchlights along the Ben Franklin Parkway, aimed into the night sky and moving in response to the sound of human voices.

It would form a canopy of light, visible for 10 miles.

But when they scheduled it, city officials had no idea that the dates - Sept. 20 to Oct. 14 - coincided with the peak of the fall bird migration.

Or that as people gaped at the show, songbirds winging overhead might become disoriented by the lights and start dropping from the sky, as has happened before.

Audubon Pennsylvania has stepped in with expertise. The artist - who said his environmentally "radical" daughter, Julieta, 8, would be outraged if she learned he had killed birds - is tweaking the show.

And the new collaboration could result in cutting-edge ornithological research.

"All that has happened is all for the good," said Penny Balkin Bach, executive director of the Association for Public Art, which is sponsoring Open Air by the Mexican-Canadian media artist Rafael Lozano-Hemmer.

That was not the case in June, when staff at Pennsylvania Audubon learned of the show.

"This is when we have huge, huge numbers of birds migrating through," said Keith Russell, Audubon's science and outreach coordinator.

Many of the hundreds of thousands of birds - more than 250 species - migrate at night. The show is planned for 8 to 11 p.m. daily.

Migration is fraught with peril. The route is long, food and shelter are scarce, predators plentiful. Bird mortality rates are 15 times higher during migration.

Lights add to the danger, studies have shown. Researchers have found dead birds around the bases of communications towers, lighthouses, and tall buildings.

Birds use stars as navigational tools, and they are attracted to strong light, especially on cloudy nights when the stars are not visible. Once in the beam, they become disoriented and do not leave it, flying in circles until they slam into something or drop from exhaustion.

A case in point was an ornithological event during the 9/11 tribute in New York City in 2010.

About 88 searchlights were aimed to form two columns of light shooting up into the air.

Some thought the small white streaks they saw in the light were scraps of paper or plastic caught in an updraft. It was like gazing into a snowstorm, one observer wrote on "It was hard not to think of souls."

Actually, the streaks were wood thrushes, warblers, redstarts, and Baltimore orioles. Officials turned off the lights five times that night.

The public art association's Balk said the event was planned for earlier, but that the earlier date conflicted with the Made in America concert. Meanwhile, the association had also promised to overlap with the city's Live Arts Festival. So organizers stuck with the dates.

Lozano-Hemmer was dismayed by the prospect of harming birds. "I'm not pretending this is an environmentally friendly show," he said. But he said he takes "every reasonable, rational step to make sure that every impact is as small as possible."

His generators run on biodiesel. He purchases carbon offsets to make up for the energy his show uses.

The artist has done similar shows worldwide, including near a bird sanctuary in Tokyo and a bald eagle nesting area near Vancouver, British Columbia. By consulting with ornithologists, he said, he avoided impact.

But never had he taken on the Atlantic Flyway - a migration superhighway.

Now, Lozano-Hemmer has agreed to keep the lights moving and to incorporate blackout periods to give disoriented birds a chance to recover. He'll program the beams away from buildings the birds could crash into.

He'll also put filters on the lights to change their color and quality. Birds have receptors that are more sensitive to - and more likely to be disrupted by - warm colors and the UV spectrum.

The scientists will be setting up a network of watchers with binoculars to detect any problems, and Lozano-Hemmer is giving them a telephone number for a technician who can shut down the display immediately.

"This is not a situation of my ego wanting to do this no matter what," he said. "I want to observe what is happening in the field and to react."

Chris Sheppard, an expert with the American Bird Conservancy, and Fordham University's J. Alan Clark, who is studying how migrating birds move through urban landscapes, visited Philadelphia not long ago to scout locations.

He and Audubon scientists hope to get a sophisticated radar - used by the military, the space program and some airports - that is fine-tuned enough to detect single birds.

They plan to set it up north of the city to count the birds and watch what happens when they see the lights. Will they swerve? Increase altitude? Fly on through?

Similar units have been used only twice before, Clark said - in the Bronx and at Governors Island in New York Harbor. This will be the first time they'll use it to track the fall migration through an urban area.

Audubon officials also hope to deploy audio equipment. Migrating birds communicate with each other by producing short calls, and difference species make different sounds.

All this costs money - $15,000 for the radar - so Audubon Pennsylvania's director of urban conservation, Janet Starwood, has been seeking funds.

If she succeeds, "we're going to collect a really amazing amount of scientific data," she said.

Whatever they learn could be applied to what may be an increasing number of such lighting events - from shows at amusement parks to corporate promotions.

Still unhappy is the International Dark-Sky Association, an Arizona-based group that advocates protecting "our heritage of dark skies."

"Lighting really only should be where it needs to be, in the amount it needs to be, and when it needs to be," said Scott Kardell, managing director of the association.

Lozano-Hemmer contended the lights will not be as bright as critics say.

Existing light pollution from corporate advertising "is orders of magnitude stronger than what I'm bringing in," he said. "I honestly feel there are better places where they can put that activism."

In the end, officials hope to have a teachable moment. None too soon, said Nate Rice, an ornithologist with the Academy of Natural Sciences at Drexel University. It dismays him that more people don't know when birds migrate, or that lights make them collide with buildings.

To that end, Audubon folks have visited a bird sound library at Cornell University.

Joining the human voices contributing to the Open Air project will be the melodic warbles of songbirds.