One night in 1998, Australian park ranger Graeme Burgan was patrolling Phillip Island Nature Parks in Victoria, Australia, when he came across an awful sight: a bridge covered in dead baby seabirds that had been run over by cars.

“There were literally hundreds of them,” Burgan recalled. “The bridge was so slick with blood and guts that it wasn’t safe to drive on. It was heart-wrenching to see the mortality on the bridge that night.”

From then on, Burgan kept an annual death tally of fledgling short-tailed shearwaters during the three-week period when they leave their nests.

Now, armed with 15 years of data, he and other colleagues believe they have homed in on the culprit: streetlights.

Why? Burgan has a theory: Moonlight reflects off the ocean, and that helps baby seabirds navigate to the water, where they forage for food. But when the moon is not out, the illuminated surface of the road looks a lot like a moonlit ocean to young birds, so instead of diving into the ocean, they plunge into asphalt.

“When we analyzed the results and compared them to the moon phases, it appeared there were more birds grounded when there was no moon than when there was a moon out,” Burgan said.

The 15-year study was published in the journal PLOS ONE on Wednesday. Burgan and fellow researchers found that 39 percent of 8,871 fledglings found on the ground over the course of the study were dead or dying. That’s a mortality rate four to eight times higher than reported elsewhere for shearwater populations.

Scientists and park researchers at Phillip Island Nature Parks looked at a number of factors that could be responsible for grounded birds. Those included date, moon phase, wind direction and speed, number of visitors to the park, and holiday periods. They also turned the lights off in a section of the road to test whether those were what the birds were attracted to.

Adult birds didn’t seem to have the same problem with the lights, so it is confined to fledglings leaving the nest.

Seabirds’ schedules make them susceptible to confusing flat, dark roadways with flat, dark oceans.

“Seabirds spend most of their lives in the ocean, but they have to come to land to nest,” Burgan said. “And while land-dwelling birds roost at night, seabirds are semidiurnal, so they’re active at night.”

“It seems that birds in urban areas have adapted to living in these other environments—wetland or bushland birds don’t get run over in the same numbers,” said Burgan.

Issues with artificial lights plague seabirds around the world. In Hawaii, homeowners are asked to turn off all unneeded lights during the two-month season when baby endangered Newell’s shearwaters and Hawaiian petrels emerge from their nests. In the Azores, residents are collecting birds, mapping the locations of their falls, and reporting information about artificial lights.

As for the bridge where Burgan witnessed so much death in 1998, the park has decided to keep the lights off, and that seems to have stopped the carnage.