Mercury in feathers of little egret Egretta garzetta and night heron Nycticorax nycticorax chicks and in theirprey in the Axios Delta, Greece.

Mercury concentrations were measured in feathers oflittle egret and night heron chicks and in their prey in the Axios Delta,Greece. Significantly higher concentrations occurred in night heron than inlittle egret in 1993. In the night heron the mercury content of feathers wasnegatively correlated to the size of chicks, possibly due to inhibition ofgrowth. Mercury concentrations were higher than reported for heron feathersin seriously polluted sites in North America and Japan, but the toxic hazardis unclear. Diets differed considerably between the two species due to use ofdifferent foraging habitats and this seems responsible for different mercurycontents of feathers. Mercury concentrations in the pumpkinseed sunfishLepomis gibbosus, goldfish Carrassius auratus, and indragonfly Odonata larvae were the highest among the prey categories. Frogs and water beetles Dytiscidae had moderate concentrationswhereas saltwater fish and terrestrial prey had very low mercuryconcentrations. The implication is that the deltaic marshes are the habitatmost polluted with mercury. Night heron chick feathers, freshwater fish anddragonfly larvae could be used to monitor mercury contamination in thisregion, but use of bird feathers alone could give misleading results ifchanges in diet occurred.