The importance of freshwater flows over estuarine mudflats for wintering waders and wildfowl Original Research Article

The attraction of wintering waterbirds to freshwater flowing over intertidal mudflats of estuaries was studied between 1996 and 1998. This was a response to the increasing levels of freshwater abstraction around internationally protected estuaries in the UK, as its relevance to waterbirds was largely undocumented. The numbers and densities of waterbirds in corridors around freshwater flows were consistently greater than those on the remaining mudflats at all estuaries studied. Large proportions of the estuary populations of several species sometimes occurred around flows. The attraction of freshwater on mudflats to waterbirds, especially ducks and geese, is probably attributable to the presence of water for preening and drinking close to their feeding grounds. Microclimate and shelter may be enhanced, and prey density or availability may be increased for foraging birds on wet mudflats. The amount of freshwater in flows was important to several species and greater densities occurred around larger discharges although very large discharges across narrow mudflats created deep, incised creeks that were unsuitable for most waterbirds. Thresholds of discharge attractive to birds are likely to be estuary-specific and depend on the width of mudflat if freshwater is to reach mid and lower shores at low tide. Surrounding landscapes also influenced the variability of discharge and therefore the predictability of flows. The impact on birds from freshwater abstraction may be similar to effects of displacement from other intertidal grounds and could affect large numbers of birds. The potential impact on bird populations should be included in assessments wherever there are proposals to alter or remove discharges within estuaries.