Migrant Shorebird Habitat Use and the Influence of Water Level at American Falls Reservoir, Idaho

Abstract

AssTRAcr-Shorebirds migrating at American Falls Reservoir, Snake River, Idaho, were counted in fall from 1986 to 1988 to determine their habitat use and the influence of water level changes. Reservoir water levels changed within and between seasons, sometimes changing the habitat type at specific sites. Shorebirds concentrated on mudflats, whereas sandy, clay, and boulder beaches, and bedrock supported very low numbers of individuals and species. Shorebirds were excluded from some important mudflats in early migration in 1986 because of flooding by high water, and densities drastically dropped after a rise in water flooded some mudflats later in the season. Shorebird numbers were correlated with water drawdown in 1986. There were equal or higher numbers of shorebirds at the large mudflat site in 1987 and 1988 when water drawdown caused the shoreline to recede 15,000 to 20,000 m versus 1986 when the shoreline receded 3200 m. This study shows that man-made reservoirs can provide important staging areas for migrating shorebirds. Mudflats should be made available on reservoirs in the intermountain western United States from late June through October.