Mallard ducks were fed a control diet or a diet containing 0.5 or 3 ppm mercury (as methylmercury dicyandiamide) based on the dry feed. These mercury diets are approximately equivalent to 0.1 and 0.6 ppm mercury in a natural succulent diet. I measured for the ducklings the approach behavior in response to a tape-recorded maternal call and the avoidance of a frightening stimulus....There were no significant differences among controls and ducklings from mercury-treated parents in the percentage of ducklings that approached the tape-recorded call. Control ducklings, however, moved back and forth toward the call more than ducklings from mercury-treated parents and also spent more time in the end of the runway near the loudspeaker than ducklings whose parents were fed a diet containing 0.5 ppm mercury....Compared to control ducklings, ducklings from parents fed a diet containing 0.5 or 3 ppm mercury were hyper-responsive in the test of avoidance of a frightening stimulus....Mallard eggs collected in the wild have been found to contain levels of mercury exceeding the 1 ppm (wet-weight) found in the eggs of hens fed a diet containing 0.5 ppm, but there are no reports of mallard eggs collected in the wild that were found to contain as much mercury (6 to 9 ppm) as eggs from hens fed a diet containing 3 ppm mercury. On a dry-weight basis, the concentration of mercury in the eggs was about 6 times as great as that in the feed for ducks fed the 0.5 ppm mercury diet and about 6 to 9 times as great for ducks fed the 3 ppm mercury diet.