We report nesting success for a population of ground-nesting black-bellied whistling ducks (Dendrocygna autumnalis) for the summers of 2003 and 2004 at a hypersaline lake in southern Texas, and a population decline of 71% between 2002 and 2003. We attribute the population decline to a change in ecology of the lake, specifically, an increase in salinity from cessation of inflow of fresh water in 2000. The increased salinity led to high mortality in ducklings and brood failure due to sodium toxicity. We compared nest success, nest habitat, and nest predation to other reported populations of black-bellied whistling ducks in the northern area of their range.