Winter abundance of shorebirds on Humboldt Bay, California, USA

Estimates of population size are the foundation of conservation actions; they facilitate identification of important habitats and provide evidence of population growth. We conducted four high-tide, winter (4 Nov 2018–17 Feb 2019) surveys of Humboldt Bay, California, a site of Hemispheric importance under the Western Hemisphere Shorebird Reserve Network (WHSRN), to provide estimates of shorebird abundance. Observers detected 25 species totaling approximately 42,500–72,500 individuals. Five species that foraged on expansive unconsolidated sediments of tidal flats dominated the assemblage (Dunlin Calidris alpina, 40%; Western Sandpiper C. mauri, 26%; Least Sandpiper C. minutilla, 13%; Marbled Godwit Limosa fedoa, 11%); by contrast, species frequenting rocky intertidal habitats or ocean-fronting beaches comprised <10% of observations. Ranked abundances of species correlated highly across surveys. For most species, abundance estimates are similar to those reported 25 years ago. Our findings confirm the importance of Humboldt Bay as an estuary that hosts a diverse and abundant winter shorebird assemblage along the Pacific Americas Flyway.