18 September 1982 to 18 March 1983

Waterbird Censuses at Richardson Bay Audubon Sanctuary, 1982 to 2006 Photo by Kerry Wilcox February 2008 Report to Audubon California and the Richardson Bay Audubon Center and Sanctuary W. David Shuford PRBO Conservation Science 3820 Cypress Drive # 11 Petaluma, CA 94954 707-781-2555 www.prbo.org ACKNOWLEDGMENTS The following individuals participated in waterbird censuses at Richardson Bay Audubon Sanctuary and thereby made this report possible: Jean Amidon, Bruce Bajema, Seth Benz, Al Brumter, Carol Farker, Karen Hamblett, Paul Harper, Tony Harrow, Robert Hinz, Toshi Hiraoka, Bob Hogan, John Howard, Ken Howard, Lee Howard, Susana Ives, Richard Jenkins, Anna Kahn, Cindy Kamm, Cindy Lamb, Marti Leland, Bill Lindqvist, Helen Lindqvist, Ricardo Mahler, Maggie Moore, Joe Northrup, Lisa Nostadt, Ed Nute, Marcia Nute, Jolen Osselo, Joel Pagel, Eric Palm, Virginia Plambeck, Lynda Polard, Rachel Pray, Jean Price, Gwen Rino, Clive Saunders, Delanna Schneider, Mike Simms, Hillary Smith, Linda Stevens, Michael Stevenson, Meryl Sundove, Lowell Sykes, Jim White, Karen Wikler, Ricky Ylagan, and several unnamed observers. Thanks also to those who counted birds in the Tiburon subarea of the Marin County (southern) Christmas Bird Count from 1978 to 2005; the subarea data were kindly supplied by Harrison Karr. Bob Hirt and Ed Nute answered questions about coverage boundaries, respectively, for the Strawberry and Tiburon subareas of that CBC. Funding for this report was provided by Audubon California. Gary Langham and Nils Warnock were supportive of this project. Kerry Wilcox kindly provided data from censuses of the Richardson Bay Audubon Sanctuary, maps of the area, the cover photo, and answers to numerous questions relevant to past and current waterbird censuses. Lynne Stenzel provided shorebird data for Richardson Bay from PRBO’s Pacific Flyway Project. Mike Wolder, Joelle Buffa, and Dan Yparraguirre provided perspective on census methods of the midwinter waterfowl surveys of San Francisco Bay. Julie Howar kindly prepared the two maps of the Richardson Bay study area. Jennifer Roth supplied some important references on herring trends in San Francisco Bay. This is Contribution No. 1621 of PRBO Conservation Science. EXECUTIVE SUMMARY Richardson Bay is recognized as an Important Bird Area in California, yet little prior information has been compiled on the abundance of birds in this bay. A summary of waterbird censuses conducted at the Richardson Bay Audubon Sanctuary irregularly from 1982–83 to 2006–07 and in the Tiburon subarea of the Marin County (southern) Christmas Bird Count (CBC) from 1978 to 2005 documents thousands to tens of thousands of wintering waterbirds in Richardson Bay. Numbers of waterbirds in this bay are typically dominated by a few species or species groups of diving birds, mainly scaup, Ruddy Ducks, large Aechmophorus grebes, Double-crested Cormorants, Buffleheads, and gulls. Numbers of the most numerous waterbirds in the Tiburon subarea of the local CBC were particularly high during the late 1970s and early 1980s, declined in the mid- 1980s, and generally have remained at lower levels since. Although numerous in the late 1970s, American Wigeons, Canvasbacks, and American Coots currently are uncommon to rare in Richardson Bay. Weaknesses in prior census methodology make interpretation of the patterns in waterbird abundance difficult. Still, it appears that fluctuations in numbers of spawning herring and changes in the density and extent of subtidal plants may be the most important local factors influencing waterbird numbers in Richardson Bay in winter. Additionally, wintering numbers of waterbirds on Richardson Bay may be strongly affected by factors elsewhere in their range at other seasons. Trends in waterbird numbers at Richardson Bay vary among species in their consistency with trends at a nearby estuary and at the continental scale. Because movements of waterbirds in and out of the Richardson Bay sanctuary with irregular spawning runs of herring increase the difficulty of interpreting census results, it would be valuable to consider expanding the survey area to include additional portions of Richardson Bay to encompass more of the areas among which birds are moving. Without supplemental surveys to obtain accurate ratios of species within species groups typically lumped on censuses (particularly scaup and Aechmophorus grebes), it will be difficult to track the trends of some of the most numerous species on Richardson Bay if trends for component species differ. Conducting additional opportunistic surveys of key waterbirds species during runs of spawning herring likely would provide insight on the short-term fluctuations of waterbirds in response to these events. It also would be valuable to coordinate monitoring of waterbirds at Richardson Bay with the long-term census programs at Bolinas Lagoon and Tomales Bay on the outer coast of Marin County. California’s San Francisco Bay estuary is an extremely important area within the Pacific Flyway for migrant and wintering waterfowl, shorebirds, and other waterbirds. Its large size, diverse aquatic habitats, and mild climate make it particularly attractive to these birds despite extensive historic loss and degradation of wetland habitats within the bay. The multiple threats posed by the ongoing activities of the six million people living in proximity to the bay put waterbird populations at risk from contaminants, oil spills, impacts to aquatic food webs from invasive non-native invertebrates and plants, disturbance, lowered water quality, and additional habitat loss and degradation. Planned and ongoing restoration of salt marshes and other wetlands, particularly by conversion of salt ponds in the north and south portions of the bay, may prove beneficial to some species, but the effects of planned restoration remain to be seen. Hence, it will be important to monitor the distribution and abundance of waterbirds in the San Francisco Bay estuary over the long term to assess trends and patterns that my reflect responses to human activities in the bay and to enable remediation of any adverse effects that may occur. Prior research in San Francisco Bay has included comprehensive baywide surveys of waterfowl (Accurso 1992), shorebirds (Stenzel et al. 2002), and some species of rails (Evens et al. 1991, Spautz et al. 2005) and colonial waterbirds (Strong et al. 2004, Kelly et al. 2006). Because of the bay’s large size, most comprehensive surveys have been conducted either infrequently or irregularly. Baywide midwinter waterfowl counts, including waterfowl, American Coots (Fulica americana), and the large Aechmophorus grebes, have been conducted annually in the bay for many years (Joelle Buffa pers. comm.). Baywide shorebird censuses were conducted once during spring and fall migration and twice during winter in the late 1980 and early 1990s, and once in November in 2006 and 2007 (Stenzel et al. 2002, PRBO unpubl. data). Counts of waterbirds have also been conducted in restricted portions or habitats of the bay (e.g., shorebirds, Holway 1990; salt ponds, Warnock et al. 2002) at various intervals and over varying periods of time. Among these are counts of waterbirds conducted irregularly from 1982 to the present within the Richardson Bay Audubon Sanctuary, a portion of that bay in southern Marin County within the larger San Francisco Bay estuary. Although Richardson Bay is recognized as an Important Bird Area in California (Cooper 2004), little information has been compiled on the abundance of birds in this bay. To meet this need, this report summarizes the results of waterbirds censuses at Richardson Bay, describes patterns of waterbird abundance and diversity within the bay, and makes recommendations for continuation and improvement of those surveys. STUDY AREA Richardson Bay, in southern Marin County, California, is a two-pronged embayment just inside and north of the Golden Gate at the mouth of San Francisco Bay (Figure 1). The narrower and shallower arm, including Bothin Marsh and Pickleweed Inlet, extends northwest to the town of Mill Valley. The Richardson Bay Audubon Sanctuary encompasses about 900 acres of the wider and somewhat deeper northeastern arm of this bay. The sanctuary boundary includes the shoreline of the bay from near the tip of Strawberry Point on the west clockwise north to the head of the bay, south along the Tiburon Peninsula to the vicinity of Belvedere, southwesterly out in open water to its 2 southern apex, and back northwesterly to Strawberry Point. The sanctuary includes primarily the open waters of inner Richardson Bay and excludes Strawberry Lagoon and Greenwood Cove, connected to the bay on the west, and Belvedere Lagoon, set back from the bay to the southeast. Richardson Bay is relatively shallow and has extensive eelgrass (Zostera marina) beds both within and outside the sanctuary. Spawning runs of Pacific herring (Clupea pallasi) occur in Richardson Bay at irregular intervals in winter, and the large numbers of these fish and the eggs they attach to eelgrass and other subtidal and tidal substrates periodically attract exceptional numbers of waterbirds to this area. The sanctuary’s waters are closed to boats and other watercraft each winter (1 October to 31 March), and, though occasional violations occur, this greatly reduces disturbance to the large numbers of waterbirds that occur at this season. METHODS The methods described here include those for waterbird censuses of Richardson Bay Audubon Sanctuary organized by sanctuary staff and those

Load more