Waterbird Responses to Drawdown of Two East Tennessee River Valley Reservoirs

Waterbirds rely on stopover sites in the interior United States to meet the energetic demands associated with migration. Mudflats exposed during annual drawdowns of reservoirs in the Tennessee River Valley (TRV) provide stopover habitat for thousands of migratory waterbirds. Timing of drawdowns may significantly affect waterbird use of TRV mudflats. Thus, I quantified the impacts of drawdown date for Douglas (1 August) and Chickamauga (1 October) Reservoirs on mudflat acreage and characteristics, food availability, and waterbird use. I also quantified waterbird activities on TRV mudflats to determine their functional role to migratory waterbirds. From August 2005/06 – January 2006/07, I conducted waterbird surveys twice weekly at four mudflats each in Douglas and Chickamauga Reservoirs. I sampled mudflat acreage weekly; vegetation, seeds and aquatic invertebrates twice monthly; water depth and quality twice monthly; and soil moisture, compaction and temperature twice monthly. I documented 68 waterbird species using east TRV mudflats. Shorebirds were more abundant at Douglas Reservoir in August – September and at Chickamauga Reservoir from October – January. Total shorebird abundance was twice as great at Chickamauga Reservoir, and the community was composed of short-distance migrants. Shorebird richness was twice as great at Douglas Reservoir, and most species were longer-distance migrants of greater conservation concern. Waterfowl abundance and richness were greater at Chickamauga Reservoir, peaking November – December. Exposed mudflat acreage was greater at Douglas Reservoir from August – October; no mudflats were exposed in Chickamauga Reservoir during August – September either year. All vegetation parameters were greater on Douglas mudflats. Differences in seed biomass, soil characteristics, and water depth and quality were not detected between reservoirs. Invertebrate analyses were not included in the thesis because they are ongoing. Shorebird abundance was positively associated with mudflat acreage and negatively associated with horizontal cover of vegetation and water depth. Primary waterbird activities included foraging, locomotion and resting. My results suggest that early and late drawdowns of TRV reservoirs benefit waterbirds. Thus, I recommend sequential drawdowns of reservoirs, such that new mudflats are continuously exposed from late July – November in the TRV. When possible, reservoir drawdown rate should be as slow as possible (e.g., ≤1 cm/day).