A coalition of wildlife advocacy groups is asking a federal judge to halt work on a major freeway project in Petaluma to protect migratory birds that nest in the area.

Advocates say that Caltrans and the Federal Highway Administration failed to consider the effect of $130 million construction at the Highway 101 bridges over the Petaluma River on the federally-protected cliff swallows that like to build their mud nests on the bridge girders. Caltrans later tried to keep the birds away using netting tacked underneath the bridge, but those nets wound up entangling and trapping the birds.

"The agencies are well aware that the netting is killing the birds ... and that the bird deaths are unlawful," said Carter Dillard, director of litigation for the Cotati-based Animal Legal Defense Fund, one of the organizations that filed the suit.

The advocates are asking a court in San Francisco to force the agencies to do additional environmental study before resuming work on the project, which will add HOV lanes to the congested highway. They say the agencies failed to study the population of cliff swallows, which is well-established on the bridge and well-known to bird watchers, and failed to study effects of and alternatives to netting.

"There have been problems with netting at other Caltrans sites, so this should have been a known event," said Jeff Miller, conservation advocate for the Center for Biological Diversity, also a plaintiff in the case.

No date has been set for a hearing, though the advocacy groups say they will press for quick judicial action if the agencies don't respond promptly.

The Federal Highway Administration declined to comment because the situation is part of an active lawsuit. Caltrans also wouldn't comment on the lawsuit, but repeated its previous statement that it had long-since fixed any problems with the netting.

"Caltrans cares about wildlife and takes environmental protection very seriously," spokesman Bob Haus said in a written statement. "Fixing this material has improved bird welfare dramatically, which is why we installed it in the first place."

Advocates agree that the situation has improved since March and early April, but they say it is far from fixed. Veronica Bowers, founder of the Sebastopol-based Native Songbird Care and Conservation and also a party to the lawsuit, has been monitoring the bridge almost daily since April. She said it appears about 100 birds, mostly swallows, but a handful of starlings and other birds as well, have died in the nets and a handful continue to become entangled every day.

She also said she has seen Caltrans crews knocking down the mud nests, though she has not observed any eggs or birds being harmed in the process so far, though the lawsuit mentions such harm as a likelihood.

The cliff swallows are protected under a century-old treaty that sets international rules for regulating migratory birds. The swallows breed in Northern California, but they winter 6,000 miles away in South America. Advocates say they are highly motivated to nest in the same areas year after year and will go to great lengths to build their mud nests in the same areas, so it is difficult to keep them away from the worksite.

Bowers and others have said that Caltrans should have applied a non-stick coating on the bridge, which prevents the mud from sticking but does not harm the birds.

The federal Fish and Wildlife Service continues to investigate the bird deaths and has met with Caltrans and the contractor onsite, spokesman Michael Woodbridge said, though he declined to offer details of the investigation. He said the agency has also offered Caltrans advise and material on non-lethal ways to prevent cliff swallows from nesting on the bridge.