Trends in abundance of wintering waterbirds relative to rainfall patterns at a central California estuary, 1972–2015

236 Abstract: During winter, habitat for waterbirds in California comprises primarily coastal estuaries, whose water resources are consistent, and inland wetlands and agricultural lands, both of which are highly dependent on or influenced by annual precipitation. At Bolinas Lagoon, a shallow central California estuary, we monitored the numbers of waterbirds in winter during two periods, 1972–1993 and 1998–2015, and used generalized linear models to investigate the relationship between levels of annual precipitation and patterns of abundance for 42 taxa. The regional extent of rice fields flooded in the Central Valley in the late fall increased substantially in the 1990s. Anticipating that waterbird distributions might shift in response to this increase in habitat, we structured models to detect changes between the two census periods in trends at Bolinas Lagoon. We found a significant relationship between abundance and rainfall within the winter that abundance was measured for 19 taxa (14 negative, 5 positive). Abundance was related to annual rainfall during the previous 1–4 years for 28 taxa (18 negative, 10 positive), and only 7 taxa showed no relationship between abundance and rainfall. We found a significant linear trend in the abundance of 39 of the 42 taxa during one (12 taxa, 29%) or both (27 taxa, 64%) census periods. Thus a positive or negative trajectory, adjusted for rainfall, was more frequent than a lack of trend. In 14 taxa the trend in the two survey periods differed; 12 of these increased from 1972 to 1993 but then decreased after 1997, a pattern consistent with a distributional shift from the lagoon to newly created inland habitat. Among 22 taxa that regularly used rice fields flooded after harvest, the pattern of 12 was consistent with a distributional shift away from Bolinas Lagoon, whereas that of only 6 was inconsistent with such a shift. Almost twice as many species increased at Bolinas Lagoon from 1972 to 1993 than decreased, whereas the opposite was true from 1998 to 2015. In a comparison of species foraging intertidally versus subtidally, the proportion increasing or decreasing at Bolinas Lagoon did not differ significantly, despite pronounced changes in the lagoon’s bathymetry over the 43 years of study. In only 8 of the 26 taxa for which regional data were available was the regional trend consistent with the local trend at Bolinas Lagoon. Our study underscores the importance to monitoring and conservation programs of identifying key drivers of species’ abundance at individual sites and understanding how waterbirds respond to local and regional habitat changes.