Humboldt Bay, California, USA hosts a globally important shorebird community year-round

Conservation relies on accurate descriptions of wildlife diversity and abundance over the annual cycle, coupled with a spatial component. The Western Hemisphere Shorebird Reserve Network recently upgraded Humboldt Bay, California to a site of Hemispheric Importance. Our objective was to provide further insight into the importance of the bay to shorebirds along the Pacific Americas Flyway (PAF). We conducted surveys of nonbreeding shorebirds over a complete calendar year (Jun–May, 2018–2019) at Humboldt Bay. In total, observers detected 31 species (range: 12–26 on 14 separate surveys), which constituted ~60% of the 52 species recorded for the area over the past ~60 years. An estimated 176,000, 54,000 and 622,000 shorebirds used the bay in summer/fall, winter, and spring, respectively. Species’ abundances varied over six orders of magnitude, with calidridine sandpipers numerically dominant, especially during spring migration. For several taxa (e.g., Marbled Godwit Limosa fedoa beringiae; Western Sandpiper Calidris mauri), abundance estimates surpass flyway population thresholds, justifying additional recognition under Ramsar Convention. We also reviewed eBird data for sites at comparable latitudes in Canada, USA and Mexico along the PAF and the Atlantic Americas Flyway (AAF). The relatively high winter diversity of the PAF (average 25 species per site, compared to 17 at AAF sites) helps explain the high diversity at Humboldt Bay.