San Francisco Estuary and Watershed Science

Abstract: Monitoring species abundance is a critical tool for identifying trends in wildlife populations. Using data collected in the San Francisco Bay Area between 1995 and 2019, we evaluated trends in nesting abundance of four Ardeid species across the entire study area and in 10 subregions, while accounting for the effect of rainfall. Overall, Great Egret (Ardea alba) nest abundance increased by 27% (95% confidence interval -1%, 54%) from 783 to 993 nests. Great Blue Heron (Ardea herodias) and Snowy Egret (Egretta thula) nesting abundance was similar across the study period, averaging approximately 503 and 509 nests, respectively, but Snowy Egret abundance was highly variable between years. Finally, Black-crowned Night-Heron (Nycticorax nycticorax) abundance declined -22% (95% confidence interval -59%, 15%) from 682 to 535 nests. At the subregional scale, trends were variable within species, and no species had consistent positive or negative trends across all 10 subregions, although it appears the distribution of all species except Great Blue Heron shifted among subregions. Our results suggest conservation action may be warranted to recover the Black-crowned Night-Heron population in our study area, but there is uncertainty on the magnitude and the reasons for their decline. Further investigation of the mechanisms for demographic change is needed to guide effective actions. In the absence of that mechanistic information, protection of the few colony sites occupied by Black-crowned Night-Herons and Snowy Egrets, especially islands in San Francisco Bay and two large urban colonies in Santa Rosa and Fairfield, is a prudent immediate action.