Mercury exposure to passerine birds in areas close to local emission sources in Slovakia and Norway

Mercury concentrations in vegetation, insects and insectivorous birds were studied in the vicinity of a mercury production plant at Rudnany, Slovakia and a zinc smelter at Odda, Norway. Comparison was made with pristine reference sites in both countries. Sampling of eggs and juvenile birds was facilitated through the use of nest boxes. In Norway the migrating species Ficedula hypoleuca was studied, whereas in Slovakia both migrating and resident birds were represented. While the Hg levels in vegetation and insects corresponded well to the expectations based on previous knowledge of contamination levels, the levels in passerine bird eggs seemed to depend mainly on whether the bird was migratory or resident. In chicks liver Hg levels at different sites in Norway showed no clear correspondence with egg levels at the same sites. At the background site an initial Hg burden provided through the egg was rapidly reduced, probably through excretion during feather development, whereas at the most contaminated site intake of Hg through contaminated food outweighed the excretion effect.