Habitat Preferences of Wintering Shorebirds in a Temporally Changing Environment: Western Sandpipers in the San Francisco Bay Estuary

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-We examined habitat preferences of 106 radio-marked Western Sandpipers (Calidris mauri) in the San Francisco Bay estuary during winter and spring at two scales: comparing proportions of habitats in their home range with habitats available in the study area (second-order selection), and comparing proportions of radio locations in different habitats with their availability in the home range (third-order selection). Daily and seasonal habitat preferences differed significantly as habitat availability changed temporally. Under second-order selection, Western Sandpipers preferred tidal sloughs and mud flats on winter low tides, and salt-pond levees at high tides. They preferred salt-pond levees and mud flats at low tides, and salt-pond levees at high tides under third-order selection. During the spring, their preferred habitats were drained and tidal salt ponds, and seasonal wetlands at high tide. At low tide, their preferred habitats were tidal sloughs and tidal salt ponds. Salt-marsh plains were the least preferred habitats during both seasons. Adults were more selective than juveniles in use of low tide habitats, but salt-pond levees were the most preferred habitats for both. Habitat preferences varied considerably when different estimates of habitat availability and use were used. If mud-flat habitats were measured as linear foraging areas along the tide line, the preference for those habitats increased from second to first. When secondorder selection was estimated from radio locations rather than home ranges, the resulting composition was similar to third-order selection. Our results suggest that regional conservation plans that restore salt marshes for the benefit of endangered species must consider the effects of losing artificial salt-pond habitats, which are locally important for sandpipers. Received 14 October 1994, accepted 22 February 1995. SAN FRANCISCO BAY is one of the largest estuaries on the Pacific coast of North America (Conomos 1979), but beginning in 1850 most of the natural wetlands of the bay were diked and altered for agricultural, urban, and industrial purposes (Ver Planck 1958). Although much of the natural habitat has been lost over the past 150 years, the estuary still supports many species of wildlife, including the largest and most diverse community of wintering and migrating shorebirds on the western coast of the United States (J. Kjelmyr, G. W. Page, W. D. Shuford, and L. E. Stenzel, unpubl. 1991 report) Despite the importance of the San Francisco I Present address: EECB/186, 1000 Valley Road, University of Nevada, Reno, Nevada 89512, USA. 2 Present address: U.S. National Biological Service, California Pacific Science Center, San Francisco Bay Estuary Field Station, P.O. Box 2012, Vallejo, California 94592, USA. Bay estuary for shorebirds, few studies have been conducted on their wintering ecology (Recher 1966, Kelly and Cogswell 1979, Holway 1990). Little is known about selection of habitats by shorebirds during the winter, or how their habitat preferences change temporally. Thus, the extent to which further modification of wetlands may change the distribution and abundance of shorebirds in the estuary is not known. We describe habitat preferences of radiomarked Western Sandpipers (Calidris mauri) in South San Francisco Bay during winter and spring, 1991-1992. We examine preferences at two levels of selection (Johnson 1980): (1) use of habitats within home ranges of Western Sandpipers compared to available habitat in the overall study area (second-order selection); and (2) comparison of habitat use determined from radio locations to habitats available in home ranges (third-order selection). In addition, we address the inherent difficulties of measuring habitat availability in dynamic ecosystems, and