Preliminary analysis of bivalve prey available to spring migrant Red Knots at Grays Harbor and Willapa Bay, Washington, USA

Grays Harbor and Willapa Bay, Washington, are key stopover sites used by Red Knots Calidris canutus roselaari during the northbound migration along the Pacific Americas Flyway. Quantitative information about the availability of food resources at these sites is lacking. We conducted a study in April 2017, designed primarily to assess sample-size requirements to inform more comprehensive investigations of invertebrate food resources. Our samples, collected at two sites used by Red Knots early in migration in previous years, contained three species of bivalves in the 6–18 mm size range, of which Macoma balthica was by far the most abundant at both sites. The density of Macoma balthica, Mya arenaria, and of all bivalves combined, was significantly greater at Grays Harbor than Willapa Bay. Future field investigations should evaluate spatial and temporal aspects of prey availability and use during northbound migration at these two estuaries.