Effects of Tide Cycles on Habitat Selection and Habitat Partitioning by Migrating Shorebirds

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Abstract
Assemblages of feeding shorebirds were studied in 3 intertidal habitats on the coast of New Jersey [USA] during Aug. to document how species segregate spatially both among and within habitats and to determine the effects of tidal cycles on these patterns. The habitats were a sandy beach facing the ocean proper (outer beach), a sandy beach on the mainland side of a barrier island (inner beach), and a small mudflat adjacent to a Spartina alterniflora salt marsh. Several microhabitats on the outer beach and mudflat were identified. Most species fed in more than 1 habitat, but only 2, Charadrius semipalmatus and Calidris canutus, used all 3 habitats regularly. Within habitats, most species exhibited strong preferences for the wettest areas, but there were differences among species in degrees of preference. The least amount of partitioning occurred on the inner beach, where birds crowded into a small zone near the water''s edge and had frequent agonistic encounters suggesting intense competition. Shorebird feeding activity was partly a function of tide time, each habitat had a characteristic temporal pattern of use by shorebirds related to tide time rather than diel time. Within habitats species-characteristic feeding activity rhythms that were also a function of tide time were found. Feeding by most species peaked during the 1st 2 h after low tide on the outer beach and mudflat. The results are discussed in terms of feeding strategies and interspecific competition.