SAN FRANCISCO — A federal judge could rule over the weekend whether to halt construction on the Highway 101 bridge over the Petaluma River after wildlife advocacy groups argued Friday that the work was harming a protected bird species.

Lawyers for the groups said that the $130 million Caltrans project is killing cliff swallows, which are protected under federal law. Caltrans installed nets under the bridges to keep the birds away, but the netting ended up ensnaring the birds.

U.S. District Court Judge Jon S. Tigar ended the hearing Friday afternoon without making a ruling. A crucial phase of the project is set to begin on Monday when crews will work on piles in the river.

"We're concerned that if they fire up the bulldozers on Monday, it will continue to cause more harm to the cliff swallow colony and the plaintiffs' interest in that colony," said Danny Lutz, a lawyer for the Animal Legal Defense Fund. "We would like to see a ruling as soon as possible."

The plaintiffs include Sebastopol-based Native Song Bird Care and Conservation, the Center for Biological Diversity and three Audubon groups. They want Caltrans and the Federal Highway Administration to do more environmental study before resuming work on the project that would eventually widen the freeway to ease traffic congestion.

Caltrans and the Federal Highway Administration declined to comment while the outcome of the case is pending.

Lawyers for Caltrans said that they have corrected the problem with the netting under the bridge and said that there have been no bird deaths in the last three months. They said that halting construction now would delay the project by a year and cost taxpayers $5 million to $10 million.

Lutz said that the agencies did not consider the affects of the construction on the colony of cliff swallows that like to build their mud nests under the Petaluma River bridge and the Lakeville Highway overpass.

"The defendants didn't even take a sidelong glance at this specific cliff swallow colony," Lutz told the judge.

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 place year after year and will go to great lengths to build their mud nests in the same areas, making it difficult to keep them away from the worksite.

Their nesting season lasts from March until the end of July. Lutz said that there is an 80 percent to 90 percent chance that the birds won't return if the construction continues to harm their nests.

Advocates 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.

(You can reach Staff Writer Matt Brown at 521-5206 or