Effects of mercury on health and first-year survival of free-ranging great egrets (Ardea albus) from southern Florida.

The objectives of this study were to determine whether elevated mercury (Hg) concentrations have a negative impact on the health and survival of nestling and juvenile free-ranging great egrets (Ardea albus) from southern Florida. During 1994, when health and survival was monitored in a cohort of young birds with naturally variable concentrations of Hg, packed cell volume was positively correlated with blood Hg concentrations, and high Hg concentration in blood was not related to the probability of surviving during the first 10.5 months of life. During 1995, 70 first-hatched great egret chicks were included in a Hg field-dosing experiment to compare the effects of elevated Hg on health and survival. Birds were dosed while in the nest orally every 2.5 days for 15 days with 0.5 mg of methyl mercury chloride (MeHgCl) for an estimated intake of 1.54 mg MeHgCl/kg food intake. These birds were compared with controls, which received an estimated 0.41 mg MeHgCl/kg food. No differences were observed in health parameters or in the probability of surviving during the first 8 months of age between egrets that were dosed with Hg and those that were not. A likely explanation for the lack of any effects on health and survival between both groups could be that chicks at this age were eliminating most of the dietary Hg through the production of new feathers.