| Summary: | I investigated the spatial distribution of spring staging black brant Branta bernicla nigricans on South Humboldt Bay, California, to determine how brant selected intertidal feeding sites. From January through May 2000, I mapped the locations of brant throughout the bay during low tides. I compared brant distributions to 2 indices of food availability. First, I estimated changes in food availability through space and time directly using a spatially explicit, discrete model that incorporated growth and depletion rates of the brants' food source in the bay, eelgrass Zostera marina. These temporally and spatially explicit estimates were derived from an initial condition of eelgrass biomass, which I mapped for the entire bay. I created this map using kriging to interpolate a continuous surface from samples collected throughout the bay in December 1999 and January 2000. The second index of food availability consisted of a surface map of summer eelgrass biomass based eelgrass samples collected in July 2000. Brant distribution during the spring staging season was best described by eelgrass biomass estimated during the summer; brant use was highest in areas of high food biomass. Failure of modeled food availability estimates to predict brant use was probably due inaccurate parameter estimates in the model. Correlations between summer eelgrass biomass and brant distributions suggest either that heavy brant grazing resulted in strong compensatory growth of eelgrass, or that brant fed in areas where eelgrass growth rates were greatest. Additionally, brant distributions were negatively correlated with the intertidal elevation of eelgrass beds, distance from eelgrass beds to nearest grit sites, and the distance from eelgrass beds to nearest large channels during many tide conditions throughout spring. All results were consistent with predictions derived from optimality theory. Feeding in areas of higher food biomass may have helped brant increase their food intake rates. Feeding in deeper water and closer to large channels ...
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