Scientists aren’t sure why Tomales Bay has seen a 65 percent decline in wintering shorebirds over the last 30 years, and a 70 percent decline in dunlins, a common brown wader with a long, drooping bill. They say it’s likely due to a host of human-induced factors, but to aid in further research, the Board of Supervisors last week allotted $3,000 for dunlin monitoring by scientists with Audubon Canyon Ranch. “We’ve lost billions of birds [nationally] since 1970, and shorebird populations specifically have declined by over 30 percent,” said Nils Warnock, the director of conservation science at A.C.R. “As our human population is growing, we whittle away at their habitat and that just has a cumulative negative effect.” Mr. Warnock noted that despite declines, dunlins are still the most abundant wintering shorebird in Tomales Bay. Scientists hypothesize that their decline is due in part to warming temperatures that cause the birds to spend the winter farther north, leading to fewer traveling to the bay each year. The waders’ habitat ranges from Mexico to Alaska, where they breed before returning south to Tomales Bay and other regions along the West Coast of North and Central America to feed in coastal estuaries. In April, they migrate back to western Alaska. The species’ broad range means identifying the factors impacting the population is more difficult. “When you have a species that ranges from Alaska all the way down to Mexico, there could be any number of problem areas for these species,” Mr. Warnock said. “The more we can learn about how they move around the landscape, the more insight we have on potential problems.” Audubon Canyon Ranch is also collaborating with Point Blue Conservation Science, Audubon California and the Nature Conservancy to study the impact of drought on dunlins. Mr. Warnock noted that during dry years, wetland habitat in the Central Valley declines, causing more birds to move to Tomales Bay. Blake Barbaree, a senior waterbird ecologist with Point Blue, said development has also likely played a role in species loss. “Their wintering habitat has really been focused on the West Coast of British Columbia down to Baha and western Mexico,” Mr. Barbaree said. “Those are also, on a broad scale, some of the most impacted regions as far as habitat and wetland development over the last 50 years.” Aquaculture may also be a contributing factor, as areas where shellfish are cultivated tend to also be key habitat areas for dunlin and other shorebirds. Past research at the Walker Creek delta in Tomales Bay has shown that dunlin may avoid structures used for oyster farming due to disturbances. The newly allotted money will fund 20 new radio tags to track migration patterns and movements. This November, scientists will begin tagging dunlins at the Walker Creek delta. The tags, which weigh about a gram and are attached either with glue or a leg harness, are linked to automatic receiving stations called Motus stations. The stations listen for signals and record if a bird comes within range; there are two stations in Tomales Bay and one at the Bolinas Lagoon. Scientists from Point Blue and a consortium of nonprofits and public agencies are using Motus stations to track dunlin and other shorebirds from Canada to Mexico. The technology allows scientists to view data from the entire network to compare research and draw conclusions. Considering the abundance of dunlin in Marin, Mr. Warnock said there’s good reason to pay attention to their movements and changes. “I think we should obviously care if we have a species in our backyard that’s very, very common,” he said. “The fact that they’re declining, you know, that’s alarming.”