Wintering waterbird use of two aquatic plant habitats in a southern reservoir

Increasing use of grass carp (Ctenopharyngodon idella) to control aquatic plants in large reservoirs has been a concern of waterbird managers because grass carp reduce biomass and diversity of native aquatic macrophytes and potentially lower habitat quality for birds. Native macrophytes were virtually eliminated from Guntersville Reservoir, Alabama after the Tennessee Valley Authority (TVA) released grass carp, but Eurasian watermilfoil (Myriophyllum spicatum) was unaffected. We revegetated 0.1-ha areas at Guntersville Reservoir with native plants, and compared waterbird use and activity in native plant and milfoil habitats during October-January to better evaluate effects of these habitat changes on migrating and wintering waterbirds. We also determined the relative availability of these 2 plant types during fall and early winter. Milfoil and native plant habitats were used predominately by dabbling ducks and American coot (Fulica americana). Waterbirds were absent more often from native vegetation plots (36%) than from milfoil plots (1%). Density of waterbirds was high in native plant habitats in October, but was greater in milfoil habitats from November to January as native plant biomass declined and milfoil remained relatively abundant. Feeding was the predominant activity in most months, and time spent foraging did not differ between plant habitats in 1994-95. In 1993-94, waterfowl foraged more in native plant habitats in October and November, but foraging was greater in milfoil habitats in December and January. Submersed native plants and milfoil both were important to ducks and coot using Guntersville Reservoir. Results suggest that maintaining a diverse aquatic plant community at southern reservoirs is beneficial to migrating and wintering waterbirds.