Wintering Waterfowl Habitat Use and Food Dynamics in Moist-soil Wetlands of the Mississippi Alluvial Valley

In some regions used by nonbreeding waterfowl, conservation planners assume that food may be a limiting factor for waterfowl populations, and carrying capacity estimates are based on food availability. Conservation planners require precise estimates of parameters used in carrying capacity models, including characterizations of waterfowl diets, temporal trends in food availability in response to management actions, and estimation of a food availability threshold (FAT; i.e., food density when foraging becomes unprofitable because energy expended during or risk of continued searching exceed potential benefits gained from obtaining foods). We experimentally estimated FAT and identified factors that likely affect foraging thresholds and food depletion in moist-soil wetlands. During winters 2006-2009, we estimated abundances of waterbirds and potential foods following autumn, pre-flooding treatments of disking, mowing, and no manipulation (control) of vegetation in 26 moist-soil wetlands in and near the Mississippi Alluvial Valley. By late winter, residual seed, tuber, and invertebrate abundances were similar among treated plots (~260 kg[dry]/ha), but were greater than observed in other waterfowl habitats (i.e., 50 kg[dry]/ha in flooded rice). To identify factors that may influence foraging thresholds other than vegetation structure, we added Japanese millet (Echinochloa frumentacea) in plots of 50, 250, and 550 kg/ha to nine wetlands with limited natural plant foods during winters 2007-2009. Dabbling ducks (Anatini) apparently reduced supplemental millet to ~10 kg/ha and other natural seeds and tubers combined to ~170 kg/ha and foraged similarly among experimental plots. Total dabbling duck use of experimental plots was not related to initial seed density. In both experiments, residual seed densities varied among plant taxa and wetlands, but not by initial seed densities or vegetation treatments. Residual seed abundances after waterfowl stopped reducing foods suggest not using an appropriate FAT value could overestimate food availability and carrying capacity of moist-soil wetlands by 40%. Furthermore, removal of seed taxa identified by this study and other literature that are not commonly consumed by dabbling ducks resulted in a 30% reduction in estimates of food availability. Conservation planners in nonbreeding regions should consider reducing food availability estimates in managed moist-soil wetlands by as much as 70%.

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