Foraging ecology of migrant waterbirds, Lake Abert, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rv042w96k

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  • Seven species of water-dependent birds (northern phalarope, Phalaropus lobatus; American avocet, Recurvirostra americana; Wilson's phalarope, P. tricolor; eared grebe, Podiceps nigricollis; ring-billed gull Larus delawarensis; California gull, L. californicus; and northern shoveler, Anas clypeata) were studied during the fall migrations of 1982 and 1983 to determine the diets and foraging strategies of mixed-species feeding assemblages at an alkaline lake in southcentral Oregon. Information on density and distribution of birds and their prey resources was collected at permanent sampling points along the eastern shore of Lake Abert, Lake Co., OR. Determination of diet composition was based on analysis of stomach samples from collected specimens. Between-species comparisons of diet composition were made using classification analyses. Diet samples were compared to prey availability samples to discern whether birds fed preferentially on particular prey types. Timed observations were used to develop time budgets. Multivariate ordination procedures were used to determine the extent of niche overlap and resource partitioning. Structural measurements were taken from collected specimens to aid in assessment of physiological condition of migrants. Total numbers of migrants using Lake Abert varied from year to year. Numbers of all species except eared grebes and gulls were lower in 1983 than in 1982. Total sample biomass of prey also declined between 1982 and 1983. Fifteen potential prey types were identified within the lake and along the lakeshore. Of these, the alkali fly (Ephydra hians) was the principal prey of all birds except the northern shoveler, whose diet included large percentages of brine shrimp (Artemia salina) and water fleas (Moina sp.). Bivariate correlation analysis revealed no significant relationships between concentrations of birds and aggregations of prey organisms. Foraging was the predominant activity of all bird species. Northern shovelers foraged for a larger percentage of the day than did other species (71%) while eared grebes foraged least (14% of daylight hours). Morning and afternoon foraging peaks were noted for shovelers and avocets. Classification and preference analyses indicated substantial overlap in diets of migrant birds, particularly among shorebirds and between gulls. When other dimensions of resource use (i.e. time allocation, foraging substrate, and technique) were examined along with diet, using a reciprocal averaging technique, some partitioning was evident in the foraging time allocation and techniques of shorebirds. Time allocation, diets and foraging techniques of the two gull species were strikingly similar. Mean weights and/or mean fat scores of all species tended to be lower in 1983 than in 1982. Changes in relative abundance of prey types, and availability of foraging macrohabitats, resulting from rising water levels, may have affected foraging patterns and condition of migrants.
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