The mystery surrounding 100 dead birds between Healdsburg and Geyserville appears to be solved.

The European starlings were found dead on the roadway and shoulder at Highway 101 and Independence Lane, just south of Geyserville Saturday afternoon.

State wildlife officials Tuesday said the birds were hit by a truck, a conclusion drawn from a witness statement and the condition of the birds at the scene.

"All the signs point to blunt force trauma," said Andrew Hughan, spokesman for the state Department of Fish and Game. "It looks like they were hit by a truck."

The roads are flanked by vineyards which are an attractive place for starlings to soar. They fly in large numbers, grouping closely together and are distinctive for their darting flight.

Wildlife officials were at the scene Tuesday, and brief necropsies showed the birds suffered broken necks, Hughan said. No signs of gunshots or shotgun pellets were found, he said.

Some of the birds will be taken to a department lab in Sacramento for further tests to formally rule out other causes, he said.

"It will take three days," he said.

It appears a swarm of the birds flew directly in front of a big rig, he said. The big rig didn't stop, but a witness later called authorities to report seeing the bird strike, Hughan said.

"She said it looked like they were committing suicide," he said.

A Highway Patrol officer was on the scene shortly after the incident, but didn't witness it.

European starlings are an invasive species that feeds in fields and woodlands, Hughan said. Though they often fly in thick swarms, it's unusual for so many to be killed at once, he said.

The black birds are slightly smaller than robins.