Bird surveys were carried out from 1973 to 2010 in Mar Chiquita, an extensive saline lake and associated Dulce River marshes in central Argentina. The wetland has had marked fluctuations in water level and salinity over the past four decades. At the beginning of the 1970s, Mar Chiquita experienced a large drop in level and there were no waterbirds on the Segundo River mouth. By 1974 the lake had recovered its level and the Segundo River estuary had formed, harboring large concentrations of local waterbirds and Nearctic shorebirds. From mid-1977, a rise in water level led to the disappearance of the Segundo River estuary and by 2003 the lake area had expanded by about 3.5 times and covered half of the Dulce River marshes. Consequently, the wetland, especially the river estuaries, became less important for local waterbirds and Nearctic shorebirds. Since 2003 Mar Chiquita has fallen 4 m, beaches have formed and salinity has increased, but currently there are no Nearctic shorebird concentrations. For the site to recover its importance for birds, Mar Chiquita should continue dropping and salinity continue increasing.