Mercury and selenium have adverse effects on health, and in the past their individual levels in tissues have been used as biomarkers of environmental contamination. These selenium:mercury molar ratios has been proposed as an alternative way to anticipate possible health risks to organisms. We examine selenium and mercury levels, their molar ratios, and variability in the ratios in the brain, liver, muscle, and feathers of a common waterbird, the eared grebe (Podiceps nigricollis), at several locations and phases of their annual cycle. We found: (1) Mean total mercury, for any site or tissue, ranged from 0.15ppm in the brain to 29.2ppm in breast feathers; In any tissue, mean mercury levels varied by as much as 10-fold while selenium varied by 3-fold; Mercury and selenium levels were correlated only in liver; Selenium:mercury molar ratios varied significantly in regular patterns among tissues (less than 1 in feathers, up to 23 in brain), sites, and stages of annual cycle; Molar ratios were affected by body weight (but not age), and the heaviest birds had the lowest ratios; and Molar ratios varied more for brain than in other tissues. Low molar ratios are generally considered harmful, although no threshold ratio has been identified. Despite wide variation of molar ratios, field studies of eared grebes have not detected overt adverse effects. Before being adopted as a biomarkers, we suggest that selenium:mercury molar ratios be used in conjunction with studies of individual metal levels, and in accordance with detailed studies of selected species, to provide a baseline of variation in different organisms and tissues.