Do Ocean Productivity and Freshwater Inflow Affect Decadal Trends of Wintering Waterbirds at a California Estuary?

North American bird populations have declined significantly since 1970 due to a variety of causes. We estimated long term changes in abundance of wintering waterbirds at Tomales Bay, Marin Co., CA, from 1992-2022. Average total waterbird numbers decreased 11.8%. The most concerning declines for taxa whose numbers in 1992 accounted for at least 5% of the combined numbers of waterbirds were for Surf Scoter (Melanitta perspicillata), Brant (Branta bernicla), Ruddy Duck (Oxyura jamaicensis), American Coot (Fulica americana), and Western (Aechmophorus occidentalis) and Clark's (Aechmophorus clarkii) Grebe. Of 29 taxa that we estimated trends for, 13 declined across our study period. Fourteen taxa had negative trends of ≥2% year–1 for at least 10 consecutive years, meeting the IUCN's Red List criteria for Near Threatened or higher. We accounted for two influencers of primary productivity in the bay: the Northern California Multivariate Oceanic and Climatic (MOCI) index and winter freshwater inflow. MOCI was included in the best model for six taxa while freshwater inflow was informative for eight taxa. A local wetland restoration project led to increases in three of the seven species we modeled. The dynamics of waterbird numbers on Tomales Bay are linked partly to herring but other local to global factors are important.