SITE SELECTION BY MIGRATORY SHOREBIRDS IN DELAWARE BAY, AND ITS RELATIONSHIP TO BEACH CHARACTERISTICS AND ABUNDANCE OF HORSESHOE-CRAB (LIMULUS-POLYPHEMUS) EGGS

The distribution of migratory shorebirds within Delaware Bay, New Jers ey, was examined relative to prey abundance and the physical character istics of the intertidal beaches. Red Knots (Calidris canutus), Sander lings (C. alba), Ruddy Turnstones (Arenaria interpres), and ''peeps'' comprised nearly 100% of the shorebirds on seven study beaches from mi d-May to early June in both 1990 and 1991. The most abundant food item on these beaches was horseshoe crab (Limulus polyphemus) eggs, and th ere were few other available macroinvertebrates. Beaches preferred by shorebirds had higher numbers of crab eggs; the density of eggs in som e surface (0-5 cm) sediments exceeded 10(5)/m2. In general, both horse shoe crab eggs and shorebirds increased along the bay shore from Higbe e's Beach (near Cape May Point) to Moore's Beach, 32 km up-bay. Howeve r, shorebirds were widely distributed within the bay, possibly because eggs were sufficiently abundant on most beaches to support foraging b y at least four birds per meter of shoreline. Shorebirds aggregated ne ar shoreline discontinuities, such as salt-marsh creeks and jetties, t hat acted as concentrating mechanisms for passively drifting eggs. Sed iment grain size and heterogeneity were probably not a primary determi nant of shorebird distribution within Delaware Bay. Intertidal sand fl ats were not extensively used by foraging shorebirds, but the potentia l importance of nearby salt marshes as foraging sites requires further investigation.