Waterbird Census at Bolinas Lagoon
Marin County, CA
PRBO is deeply grateful to the Marin Community Foundation for their on-going support of our research on Bolinas Lagoon.
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Project Start Date
Background
Objectives
Focal Species
Habitat Type
Survey protocol
Results
References
Volunteer Opportunities
Partners
Project Start Date: 1971
Background
Census results confirm that with the accelerated sedimentation in the lagoon, numbers of many bird species that depend on subtidal habitat have declined on the lagoon since 1971. Some species of shorebirds have increased, particularly Long-billed Curlews, Marbled Godwits, and American Avocets.
Objectives
To monitor winter waterbird numbers on Bolinas Lagoon, providing a historical perspective on the lagoon's avian community and a benchmark with which to compare waterbird numbers after a proposed restoration of the lagoon's habitat.
Waterbird flock over Bolinas Lagoon
Focal Species Groups
Shorebirds, Waterfowl, Gulls, Terns, Coots, Herons, Egrets, Grebes, Pelicans, Cormorants, Loons, Kingfishers
Habitat Type
Seasonal estuary
Survey Protocol
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 variability in the actual number of birds using the lagoon, but also due to census error. Census errors may occur because birds move: within a census area, between census areas, or temporarily out of the lagoon to nearby habitat. Our census protocol is designed to minimize, identify, and account for census error. Censuses are taken on moderately high tides, when mudflat habitat is sufficiently concentrated to make counting birds easiest but before it is so restricted that species that retreat to nearby areas (e.g. the outer coast, pastures on the Bolinas mesa) at high tides are forced to leave. All censuses are conducted on weekdays to minimize bird movement caused by human disturbance. The lagoon is divided into non-overlapping census areas (Kent Island, Pine Gulch Creek delta, the east side, Seadrift spit) and birds in each area are counted by a team of experienced observers. Censusers both count the birds in their areas and note flock movements between their areas and other areas. The census routes are designed to have different teams counting opposite each other at the same time so that counters are aware of and can correct for bird movements between the areas.
Results
Example waterbird graph of Black-crowned Night-Herons on Bolinas Lagoon. Bird artwork by Jerry Buzzell. Please visit the waterbird trends graphs on the Bolinas Lagoon sidebar menu or by clicking a link below.
Shorebird trends
Diving Duck trends
Dabbling Duck trends
Grebe trends
Cormorant trends
Heron & Egret trends
Tern trends
Other waterbird trends
About the Graphs
The graphs presented here represent the numbers of waterbirds counted on Bolinas Lagoon since 1971-1972. (Since most species nest in the arctic or the interior and arrive at Bolinas Lagoon after the breeding season, “bird years” begin and end in the summer.) The birds are counted on comprehensive censuses of the entire lagoon, including the inner lagoon on the Seadrift spit. Please visit the waterbird trends pages from the sidebar menu under Science / Wetlands / Bolinas Lagoon.
The census program began in the summer of 1971 for shorebirds (plovers, avocets, sandpipers, etc.) and 1972 for other waterbird species (geese, ducks, loons, grebes, pelicans, cormorants, herons & egrets, coots, gulls, and terns). For one shorebird, Wilson’s Snipe (now "Common" Snipe), we did not discover the best habitat (a wet, vegetated field in the Pine Gulch Creek delta) until late in the 1971-72 season, so its graph begins in 1972 with the other waterbirds.
In the first years we conducted censuses year round, but in subsequent years we concentrated on monitoring winter numbers. Winter is the period of greatest stability in numbers for most of these species. 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. Consequently, most of these graphs display winter results from November through February. A few species do not reach winter population levels until December and other species may depart rather abruptly in January, particularly in years of high winter rainfall. For these species, we have displayed December through February or November through December results, respectively.
We have included one graph of spring migratory peaks. Although numbers of most species slowly diminish over late winter and into spring, a spring migratory peak is apparent for a few species. On Bolinas Lagoon, the Western Sandpiper is primarily a migrant and numbers have exceeded 10,000 birds in 12 years.
Technical notes on the graphs
Sample sizes (number of censuses used to create the curves) depend on which months are used in calculations. Winter estimates determined for November through February censuses are based on 211 censuses for the shorebirds and 179 censuses for other species. Winter estimates determined for November though December censuses are based on 104 censuses for shorebirds and 88 censuses for other species. Estimates determined for December through February censuses are based on 157 censuses for shorebirds and 134 censuses for other species. We used the maximum count in April or May of 25 years from 1972- 2004 for the spring Western Sandpiper graph.
The graphs display “smoothed” census data. Smoothing is a useful graphical technique to identify non-linear trends from “noisy” data - that is, data with a lot of variability. The technique used to create these graphs is called a Loess Smooth, which estimates a smooth curve using a locally-weighted regression. (Regression is a standard statistical method to fit a line or curve to data points.) In a locally-weighted regression, the estimate at any point on the curve is more strongly influenced by nearby data points than by more distant ones. A span parameter between 0 and 1 is provided to the smooth that determines how much distant data points influence a point on the curve, with lower values decreasing the influence of distant points more rapidly (we use 0.3 on winter counts, 0.5 for the spring Western Sandpiper). We also specified that the regression include a quadratic term.
References
For more information about birds on Bolinas Lagoon:
Aspects of the occurrence of shorebirds on a central California estuary. Authors: G. W. Page, L. E. Stenzel, & C. M. Wolfe. 1979. Studies in Avian Biology 2:15-35.
Seasonal abundance of waterbirds at Point Reyes: a coastal perspective. Authors: W. D. Shuford, G. W. Page, J. G. Evens, & L. E. Stenzel. 1989. Western Birds 20:137-265.
Volunteer Opportunities
Contact: Lynne Stenzel
Partners
County of Marin
Marin County Open Space District
Marin Audubon Society
Audubon Canyon Ranch
College of Marin
Marin Community Foundation
Gulf of the Farallones National Marine Sanctuary
Bird artwork by Jerry Buzzell
