Demographics of Piscivorous Colonial Waterbirds and Management Implications for ESA-Listed Salmonids on the Columbia Plateau
Jessica Y. Adkins, Donald E. Lyons, Peter J. Loschl, Department of Fisheries and Wildlife, 104 Nash Hall, Oregon State University, Corvallis, Oregon 97331 Daniel D. Roby, U.S. Geological Survey-Oregon Cooperative Fish and Wildlife Research Unit1, Department of Fisheries and Wildlife, 104 Nash Hall, Oregon State University, Corvallis, Oregon 97331 Ken Collis2, Allen F. Evans, Nathan J. Hostetter, Real Time Research, Inc., 52 S.W. Roosevelt Avenue, Bend, Oregon 97702 Demographics of Piscivorous Colonial Waterbirds and Management Implications for ESA-listed Salmonids on the Columbia Plateau Abstract We investigated colony size, productivity, and limiting factors for five piscivorous waterbird species nesting at 18 loca- tions on the Columbia Plateau (Washington) during 2004–2010 with emphasis on species with a history of salmonid (Oncorhynchus spp.) depredation. Numbers of nesting Caspian terns (Hydroprogne caspia) and double-crested cormorants (Phalacrocorax auritus) were stable at about 700–1,000 breeding pairs at five colonies and about 1,200–1,500 breed- ing pairs at four colonies, respectively. Numbers of American white pelicans (Pelecanus erythrorhynchos) increased at Badger Island, the sole breeding colony for the species on the Columbia Plateau, from about 900 individuals in 2007 to over 2,000 individuals in 2010. Overall numbers of breeding California gulls (Larus californicus) and ring-billed gulls (L. delawarensis) declined during the study, mostly because of the abandonment of a large colony in the mid-Columbia River. Three gull colonies below the confluence of the Snake and Columbia rivers increased substantially, however. Factors that may limit colony size and productivity for piscivorous waterbirds nesting on the Columbia Plateau included availability of suitable nesting habitat, interspecific competition for nest sites, predation, gull kleptoparasitism, food availability, and human disturbance.
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