After dozens of nesting birds died this spring when they got trapped in netting placed at construction sites under two Petaluma River bridges, state and federal transportation officials are investigating whether they should make changes for next year's nesting season to avoid more deaths.

Local wildlife advocates have provided Caltrans with a list of alternatives they say could prevent more birds from dying next year.

"It's highly likely (the birds) will return next year," said Animal Legal Defense Fund lawyer Danny Lutz. "Our concern is that, if no changes are made, we'll just go through this show all over again."

Caltrans contractors started construction on the Petaluma River and Lakeville Highway overpass bridges this spring, as part of an effort upgrade and widen the structures in anticipation of the Highway 101 widening. The project is expected to last through 2016. When beginning construction, contractors installed netting under the bridges to prevent the federally protected cliff swallows from building their mud nests there.

But local wildlife advocates say the netting wasn't properly installed and that it trapped, rather than deterred the birds.

They demanded that the netting be taken down completely. Caltrans kept the netting up after taking steps to better secure it, and bird deaths declined as a result. But wildlife advocates continued to call for the netting's removal, documenting continued deaths at the site.

In May, the Cotati-based ALDF, on behalf of Sebastopol's Native Songbird Care & Conservation and the Madrone Audubon Society, among other groups, filed a lawsuit, asking that Caltrans be required to study how the construction was affecting the birds.

That suit is ongoing, although in June a federal judge denied a motion by the ALDF to halt the project.

In late July, Caltrans and the Federal Highway Administration sent a letter to the ALDF announcing that it would be reevaluating how it excludes the cliff swallows from the sites.

The agency is trying to determine whether or not it should perform an additional environmental impact study about how the highway work impacts the birds, prompted by "a need to protect wildlife near our construction projects," said Caltrans spokesman Allyn Amsk. Such a study would determine whether there are things the agencies could do differently to avoid future harm to the birds.

The ALDF has suggested a range of alternatives. One is the costly option of halting construction during nesting season, which lasts about six months starting in mid-February. The agency could also exclude the birds using other methods or provide alternative nesting sites, Lutz said.

"Caltrans now has a chance to get this project right and should immediately supplement the environmental review so bird protection measures can be evaluated and in place before swallow nesting season," said Veronica Bowers of Native Songbird Care and Conservation.

(Contact Jamie Hansen at jamie.hansen@ar