In nature a female left a mercury-contaminated area and began feeding in an uncontaminated area, the concentration of mercury would be expected to rapidly decrease in successive eggs.
But, birds in the wild would lay fewer eggs than our mallards laid.
Consequently, if a species laying a few eggs carried high body burdens of mercury, it probably could not eliminate enough mercury to decrease mercury levels in eggs to those believed to be safe and the possible harm from mercury in eggs would depend on how much mercury was stored in her body and how long she was away from the contaminated area.