A group of environmentalists Thursday reached a settlement with Caltrans in a lawsuit over the killing of federally protected birds during a highway construction project at the Petaluma River bridge.

The settlement requires Caltrans to remove nets that contractor C.C. Myers installed under the bridge to keep migratory cliff swallows out of the construction zone. Conservationists said the nets entangled and killed dozens of cliff swallows during last year's nesting season.

Last September, C.C. Myers paid a $3,525 fine for violating the Migratory Bird Treaty Act, which prevents the killing of cliff swallows, according to Michael Woodbridge, a spokesman with the U.S. Fish and Wildlife Service. The fine was not part of Thursday's settlement.

The Federal Transit Authority, another defendant in the federal lawsuit, is expected to sign the settlement in the coming weeks, conservationists said.

As part of the settlement, Caltrans is required to meet with the conservation organizations four times per year to review measures to protect the birds. The agency will pay environmental groups $4,000 to educate the public about cliff swallow protection.

Construction crews will be allowed only to demolish bridges outside of the nesting season.

The cliff swallow colony spends winters in Argentina and returns each year to build nests under the Petaluma River bridge and Highway 116 overpass from about Feb. 15 to Aug. 15.

Caltrans has agreed to use hard plastic sheeting to keep birds out of the construction zone, and it will remove nests that birds begin to build with a scraper or high-pressure water hose.

"This agreement creates concrete protective measures for cliff swallows," said Danny Lutz, a lawyer representing Veronica Bowers and her Sebastopol-based Native Songbird Care and Conservation, the Center for Biological Diversity and the Madrone, Marin and Golden Gate Audubon societies.

Caltrans announced some of the protection measures at a meeting of bird advocates last week that was mandated by a law written by Assemblyman Marc Levine, D-San Rafael.

The bridge construction, which is expected to last two more years, is part of the $130 million highway widening project to alleviate traffic through Petaluma. The project eventually will complete carpool lanes from Windsor to Novato.

Wildlife advocates hailed the agreement with Caltrans.

"We're pleased that Caltrans is removing the ineffective and deadly netting from these important swallow nesting locations, and will use safer measures to keep swallows from nesting in construction zones," Bowers said. "It's important that Caltrans continues to meet with wildlife agencies, conservation groups and our swallow expert before each nesting season during the project to assess the effectiveness of bird exclusion measures at the bridges."

Susan Kirks, president of the Madrone Audubon Society, said she would monitor the construction site daily to make sure construction crews complied with the agreement.

"We will closely monitor the bird exclusion measures at both bridges to ensure that the settlement protects the swallows and make sure the bird kills of last spring do not ever occur again," she said.

Allyn Amsk, Caltrans spokesman, declined to comment on the settlement, and a message left with contractor C.C. Myers was not returned Thursday.