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

-We studied assemblages of feeding shorebirds in three intertidal habitats on the coast of New Jersey during August 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. We were able to identify several microhabitats on the outer beach and mudflat. Most species fed in more than one habitat, but only two, Charadrius semipalmatus and Calidris canutus, used all three habitats regularly. Within habitats, most species exhibited strong preferences for the wettest areas, but we found 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, we found species-characteristic feeding activity rhythms that were also a function of tide time. Feeding by most species peaked during the first 2 hours after low tide on the outer beach and mudflat. The results are discussed in terms of feeding strategies and interspecific competition. -Department of Biology, Livingston College, Rutgers University, New Brunswick, New Jersey 08903 (Burger, Chase); National Fish and Wildlife Laboratory, National Museum of Natural History, Washington, D.C. 20560 (Howe); and Institute of Animal Behavior, Rutgers University, Newark, New Jersey 07102 (Hahn). Present address of Chase: Department of Biology, Barnard College, New York, N.Y. 10027. Accepted 11 March 1977. (This paper was subsidized by the Migratory Bird Program, U.S. Fish and Wildlife Service.) ALTHOUGH usually solitary on their breeding grounds, migrating shorebirds (Charadrii) often form large, mixed-species aggregations that feed along coastal beaches, mudflats, and marshes. These concentrations of shorebirds result in high population densities in prime feeding areas (Recher and Recher 1969). As these habitats are effectively two-dimensional, spatial segregation of flock members can only occur in a horizontal plane. Increased density may lead to more intense competition for space, resulting in more frequent aggressive interactions and/or niche partitioning. Moreover the cyclic tidal inundation of mudflats and beaches causes changes both in the available feeding space and in the diversity and availability of prey items. Shorebirds respond to these changes by moving continually from one foraging area to another. Studies of ecology of migrating and wintering shorebirds have dealt primarily with feeding methods and rates (Goss-Custard 1969, Ashmole 1970, Burton 1972, Baker and Baker 1973, Baker 1974), feeding dispersions (Goss-Custard 1970a), prey densities (Bengtson and Svensson 1968, Brooks 1967, Goss-Custard 1970b), feeding ecology (Recher 1966, Thomas and Dartnall 1971, Holmes 1966, Holmes and Pitelka 1968), and aggressive behavior (Hamilton 1959, Recher and Recher 1969). Most of these studies stress behavior and diet as related to prey availability or broader ecological considerations. Collectively, most data suggest that shorebird species overlap broadly in their diets and feeding habitats. Competitive exclusion may be avoided because species move from point to point without fully exploiting the available food reserves (Recher 1966). Some studies have also identified patterns of niche partitioning with respect to foods, feeding methods, 743 The Auk 94: 743-758. October 1977 This content downloaded from 157.55. 39.211 on Tue, 09 Aug 2016 05:06:27 UTC All use subject to http://about.jstor. org/terms 744 BURGER ET AL. [Auk, Vol. 94