EXECUTIVE SUMMARY
This report serves as a data summary and coarse-scale assessment of waterbird and water quality monitoring efforts at six pond complexes in the South San Francisco Bay. Coyote Hills, Dumbarton, and Mowry salt ponds are owned by Don Edwards San Francisco Bay National Wildlife Refuge and managed for salt production by Cargill Salt. Alviso and Ravenswood complexes are owned and managed by Don Edwards San Francisco Bay National Wildlife Refuge. Eden Landing Ecological Reserve (Eden Landing) ponds are owned and managed by California Department of Fish and Wildlife (CDFW), with the exception pond CP3C, which is owned by Cargill Salt. This report is based primarily on data collected by the San Francisco Bay Bird Observatory between September 2019 and February 2021.
The purpose of this ongoing study is to describe avian use of ponds to guide regional waterbird conservation, management, and habitat restoration efforts. The South Bay Salt Pond Restoration Project (SBSPRP) is restoring over 15,000 acres of former salt evaporation ponds to a mix of tidal marsh and ponded wetland habitats. As the SBSPRP proceeds, understanding how waterbirds use ponds, identifying key habitat associations, and incorporating features essential to pond-dependent species into restoration plans will be increasingly important to maintain baseline numbers of waterbirds in the South Bay.
From September 2019 to February 2020, we conducted waterbird surveys and water quality sampling at 82 ponds (22 Cargill-managed salt production ponds and 60 SBSPRP managed ponds). Due to site access limitations associated with the COVID-19 pandemic, we surveyed 45 ponds across multiple complexes from March to April 11, 2020 and 25 ponds within Eden Landing Ecological Reserve from April 15 to May 2020 and December 2020 to February 2021. We examined species richness, abundance, and behavior of waterbird assemblages within and among pond complexes. We grouped species into guilds (e.g., dabbling ducks, diving ducks, gulls) based on foraging methods and prey requirements to understand waterbird use of these ponds. We also put these waterbird counts in the context of long-term trends to assess changes in waterbird numbers relative to baseline counts from before marsh restoration. We recorded 1,293,322 waterbird observations of 78 species (all sites combined). The Alviso and Eden Landing pond complexes supported the greatest species diversity and also the highest abundances of all complexes. The abundances of diving ducks, Ruddy ducks, dabbling ducks, and medium shorebirds in winter, as well as small shorebirds in fall and Least Terns in summer have increased in SBSPRP ponds since prior to restoration activities in 2005–2007. Counts of small shorebirds in spring remain at baseline values. Eared Grebes have increased in active salt production ponds (Coyote Hills, Dumbarton, and Mowry complexes), but not in SBSPRP ponds (Alviso, Eden Landing, and Ravenswood complexes), and Bonaparte’s gulls have declined by 18%. Since limited ponds were surveyed in 2020, meaningful comparisons to previous years’ data were restricted to a subset of sites in spring and winter. Further, no fall surveys were completed in 2020. While the results presented in this report are informative, a complete understanding of contemporary waterbird trends in the project area will require resumption of surveys at all sites in fall, winter, and spring of 2021-2022.
Phalarope migration surveys in 2020 found that counts of Wilson’s phalaropes peaked at 767 phalaropes on July 21 and counts of Red-necked phalaropes peaked at 1,700 phalaropes on August 28. The peak count of Red-necked phalaropes was higher than in 2019 (1,447 phalaropes), despite visiting fewer ponds in 2020.
As the SBSPRP progresses, we recommend a precautionary approach to waterbird management and tidal marsh restoration and maintenance of enough of the ponds within the project footprint to provide a variety of salinity and water levels suitable for many different guilds. Special consideration should be given to birds that prefer medium to high salinity ponds, such as phalaropes and Eared Grebes, since restoration activities have already reduced the prevalence of these habitat conditions and the remaining high salinity habitat is managed for salt production rather than waterbird needs. Creating or maintaining islands or undisturbed levees will provide additional nesting and roosting habitat for other guilds. As the restoration advances, continued monitoring of avian use of Cargill-managed and SBSPRP ponds will be valuable in assessing progress toward the management target of maintaining baseline waterbird numbers. However, a regional perspective will be needed to tease apart drivers of waterbird use in the project area. With more than a decade of bird and water quality monitoring data available, a useful next step will be to model bird habitat use and to use the model to predict the impact of future restoration scenarios on bird abundance.
