Shorebird diet during spring migration stopover on Delaware Bay

During spring migration, thousands of shorebirds gather in Delaware Bay at the same time as horseshoe crabs (Limulus polyphemus) are spawning. During their stopover, the birds store enough fuel in the form of fat and muscle protein to complete their migration to the Canadian breeding grounds. We documented the changes in body mass of shorebirds migrating through Delaware Bay and determined how much of the shorebird diet during this period consisted of horseshoe crab eggs. Migrating shorebirds were captured, morphometric measurements taken, and gut samples collected by stomach flushing. Red Knots (Calidris canutus), Ruddy Turnstones (Arenaria interpres), Sanderlings (C alba), and Semi-palmated Sandpipers (C. pusilla) increased their body mass up to 70-80% while staging on Delaware Bay. Horseshoe crab egg membranes constituted the bulk of the gut contents for all species at all collection sites. Polychaete and oligochaete worms were found in substantial concentrations in gut samples collected from shorebirds in certain beaches. Sand and unidentified decomposed material were found in varying amounts in gut samples of all species and locations. Apparent declines in spawning horseshoe crab populations may adversely affect migratory shorebirds.