SONOMA  (AP) — State transportation officials' inability keep swarms of swallows off a Sonoma County bridge is threatening to delay an $82 million project to replace the bridge structure.

Dozens of the birds have become caught and have died in netting on the Petaluma River Bridge that's intended to maintain a 50-foot buffer between workers and the swallows, the San Francisco Chronicle reported Monday.

State and federal laws require Caltrans to protect the birds and their nests.

Caltrans officials said they have counted 50 dead birds since March 27, when the problem was discovered, and they have been trying to fix it.

"The fact that all these birds came to the same location was totally unexpected," said Brigetta Smith, Caltrans' deputy for external affairs. "It was definitely more birds than we anticipated."

The 5-inch-long, brownish birds form colonies that seek out overhanging structures near water. The Sonoma County swallows have for years used mud from the Petaluma River to build nests on the underside of the bridge, where they raise eggs, the Chronicle reported.

The birds returned to the bridge in March from South America to nest. Some found wind-blown gaps in the net but couldn't get out of them. Contractors have tried to close the gaps with limited success.

Wildlife advocates, who protested at the bridge Friday, are demanding that the nets come down.

Caltrans is considering using Teflon sheeting or silicone-based paint.

The bridgework is part of a long-term effort to widen Highway 101 along the Marin-Sonoma narrows, an area that often experiences traffic bottlenecks.