A volley of cannon fire greeted afternoon commuters on the Interstate 5 Bridge on Monday, the first day of a semi-annual campaign to scatter a pooping horde of non-native birds.

At 3:30 p.m., cued by an automated timer, four propane orchard cannons began popping off in random fashion. Flocks of pigeons swirled around with each report, a twilight serenade that will continue through early March.

The Oregon Department of Transportation, which maintains the bridge, has used the air cannons off and on since 2003 to scatter as many as 30,000 starlings that perch on the steel lattice work. The birds typically congregate together during winter nights. Since bridge workers first noticed them in the early 1980s, the starlings have steadily turned the twin freeway spans into the Studio 54 of the avian world.

“It’s a big bird cage,” said Marc Gross, ODOT bridge supervisor. “It’s got a lot of edges, and they love edges.”

So many starlings in such a condensed space equates to a massive abundance of slippery bird poop. Gross said the fecal matter not only creates a slip and trip hazard, it presents a disease risk to pedestrians, bicyclists and bridge workers.