Development of Aquatic Bird Indicators of Sub-lethal Mercury Exposure and Risk in Wild Populations of Water Birds in the Everglades (Florida, United States of America)

We describe over 25 years of research in mercury (Hg) contamination and sub-lethal effects in the Florida Everglades using local wading bird species as model species. We assessed various potential indicator tissues and used nestling feathers as an indicator of Hg exposure and Hg risk to breeding. Long-term monitoring of wading bird populations and analysis of museum specimens’ Hg revealed temporal and geographical variation in Hg within the study area. We found strong and relevant reduction in the number of nests in colonies exposed to Hg supported by a reduced breeding propensity and increased early failure, before egg-laying, in exposure experiments. We also found a strong influence of variation in food availability on failure associated with Hg. Variation of food availability modulated Hg effects and the same amount of Hg in feathers could be associated with no apparent reduction in breeding success if food availability was high or with a reduction higher than expected from toxicity reference values if food availability was low. The evaluation of nestling feathers and other tissue indicators suggested that there is no unique best indicator; rather, our results suggested specific endpoint–indicator associations. Indicators explained better variation in endpoints closer in time to the development of indicators.