Mortality of ducklings of the black-bellied whistling duck (Dendrocygna autumnalis) during their premier swim in a hypersaline lake in south Texas

Adult black-bellied whistling ducks (Dendrocygna autumnalis) were observed leaving islands with recently hatched young in a South Texas hypersaline lake during summer 2003. We counted 216 ducklings from 20 broods attempting to make their first swim to the mainland, presumably heading for suitable rearing habitat. Of these, 144 (67%) either died in the water or on the salt-encrusted shoreline within minutes to a few hours of first entering the water. An additional 35 carcasses from unobserved broods were found. Nine carcasses of ducklings were necropsied and they had levels of sodium in the brain above the lethal threshold concentration for sodium poisoning. From our surveillance efforts, we documented a decrease in maximum number of adult black-bellied whistling ducks observed on the islands from 489 in 2002 to 138 in 2003.