Abstract
Shorebirds migrate over long distances from breeding to wintering grounds, stopping at a few bays and estuaries to refuel. Most information on migration of shorebirds concentrates on population dynamics and foraging behavior on intertidal habitats. We studied the behavior of shorebirds on mudflats, beaches and marshes on Delaware Bay to understand how they use different habitats. Dense flocks of shorebirds concentrated on a tidal mudflat, but shorebirds used all the habitats, including several marshes. The overall percent of shorebirds feeding ranged from 34% (open beach), and 59–63% (tidal and non-tidal marshes), to 80% (tidal mudflat). Variations in the percentage of shorebirds engaged in feeding, resting and other behaviors depended on location, date, time, tide and species. A higher percentage of shorebirds fed during the middle of migration, in early to mid-morning, and during low and rising tides than at other times. Some shorebirds fed on the marshes and mudflats during all tidal states, but none fed on beaches at high tide (beaches were too narrow). Within each habitat, the highest percentage of shorebirds engaged in foraging during low tide (marshes) or rising tides (mudflats and beaches). Using the percentage of shorebirds engaged in foraging as an indication of foraging value for each habitat type within the landscape, we concluded that a mosaic of habitat types ranging from mudflats to high marshes is essential to sustain the high populations of shorebirds that use Delaware Bay during spring migration.