A small crown jewel of a coastal estuary exists near Point Blue Conservation Science’s first home, the Bolinas Lagoon is a geologic basin astride the San Andreas Fault, where cold Pacific waters from the Gulf of the Farallones mix with stream flow from Mount Tamalpais. The rich mix of nutrients in the water and mud supports a myriad of invertebrates (tiny crustaceans, bivalves, worms, and more), attracting throngs of migrating and overwintering birds. Statuesque Long-billed Curlews forage for pink ghost shrimp in their curved burrows; tiny Least Sandpipers and Dunlin flock in mesmerizing patterns to evade a hungry Merlin or Peregrine Falcon.
Point Blue’s first shorebird biologists, led by Gary Page, have sought to document the ecology and trends for birds using Bolinas Lagoon, and we have gathered extraordinarily long-term data, dating back to 1971. Today our scientific insights support the efforts of regional agencies charged with protecting and possibly renewing this precious natural system. Bolinas Lagoon represents a microcosm of our work on behalf of other estuaries that are key links in the Pacific Flyway.
Change is at work in Bolinas Lagoon, and some of it threatens the estuary’s well-being. Sedimentation, caused in large part by human activities since the late 19th century, hinders the estuary’s circulation more dramatically than natural processes might. An extra surge of fill entered the lagoon during high-rainfall winter flooding in the early 1980s. In response, the County of Marin and U.S. Army Corps of Engineers have considered a design for restoration that would involve dredging portions of the lagoon. Today, sea level rise represents an unprecedented cycle of change on the horizon for this and other sensitive coastal habitats.
Point Blue has lately concentrated on winter waterbird numbers on Bolinas Lagoon, providing a historical perspective on the avian community and a benchmark with which to compare birds’ numbers after a proposed restoration of the lagoon’s habitat.
Volunteer citizen scientists and Point Blue interns—many dozens of people over the years—have helped census birds on Bolinas Lagoon. We focus on shorebirds, waterfowl, gulls, terns, coots, herons, egrets, grebes, pelicans, cormorants, loons, and kingfishers. In the early years we conducted censuses year-round; subsequently we have concentrated on monitoring winter numbers.
Winter is the period of greatest stability in numbers for most of the species we survey. Spring and fall periods often encompass several waves of birds, sometimes of different age or sex class, and the timing of peak numbers may vary between years, making it difficult to monitor migratory numbers with few censuses.
Among our key findings: shorebird census results over four-plus decades show that, with the accelerated sedimentation in the lagoon, there have been both losses and gains for shorebirds. For the many species dependent on tidal flats uncovered by the lowest tides, numbers of birds have generally declined. Other species have increased since 1971, among them Long-billed Curlews, Marbled Godwits, and American Avocets.
What does a waterbird census measure?
When we conduct a census we obtain an estimate of the number of birds using the lagoon on a particular day, and using counts from several days in winter, we obtain an estimate of the average number of birds on the lagoon for that winter. The actual number of birds using the lagoon over the course of a winter may change (or vary) for two reasons: movement and mortality. Although many species show a strong degree of fidelity to their wintering areas, some birds move in some years: out of the lagoon to other wetlands or from other wetlands to the lagoon. Some birds using the lagoon die. We found that as many as 21% of some shorebird species succumb to just one source of mortality alone, predation by hawks and owls, during their winter stay on the Bolinas Lagoon.
The number of birds that are counted on censuses may vary due to change in the actual number of birds using the lagoon—but also due to census error. The latter can occur because birds move: within a census area, between census areas, or temporarily out of the lagoon to nearby habitat. We designed a protocol for our census efforts, to minimize, identify, and account for error. An example of a method in our protocol is to census birds on moderately high tides; mudflat habitat then is limited, making counting birds relatively easy (before higher tides makes them retreat to other areas). We also design census routes so that different teams counting opposite each other at the same time can be aware of and correct for bird movements between the areas.
The Bolinas Lagoon winter census is currently part of our Pacific Flyway Shorebird Survey (PFSS). To find out about joining this effort as a volunteer citizen scientist visit the volunteer page on our PFSS website.
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- County of Marin
- Marin County Open Space District
- Marin Audubon Society
- Audubon Canyon Ranch
- College of Marin
- Marin Community Foundation
- Greater Farallones National Marine Sanctuary
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