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Piscivorous aquatic birds,being on the final level of the trophic chain,are the most exposed to the toxic influence of mercury which undergoes magnification and is introduced to their systems with food.Mercury elimination takes place mainly through incorporating it into feather and excreting with guano.Females may additionally transfer mercury into eggs.This process is important beacause it can affect the embryos developing inside the eggs.In this study levels of mercury in eggs of three waterbirds species were determined.Furthermore the detoxification ability of the embryo was examined.Samples were collected in years 2010 – 2012,including: 91 eggs of sandwich tern(Sterna sandvicensis,Latham,1787)nesting in the Mewia Lacha Sanctuary at the outlet of the River Vistula; 30 eggs of herring gull(Larus argentatus,Pontoppidan,1763)nesting on the Wloclawek Dam on the River Vistula(situated aproximately 370 km from the river mouth)and 19 eggs of african penguin(Spheniscus demersus,Linneaus,1758)from the colony at the municipal zoo in Gdansk..Breedeing colony situated in Gdynia Harbour provided one more egg and 2 dead two-week-old herring gull chicks that had fallen out of their nests.Sample preparation involved separating the albumen and the yolk.If that was not possible the sample was constituted by whole eggs.In 10 herring gull eggs developing embryos were found.Both the embryos and the 2 two-week-old chicks had their liver,heart and down taken for analysis.In the analysed samples,the concentration of total mercury was assayed using an AMA – 254 atomic absorbtion spectrometer.Owing to the presence of sulfhydryl groups characterised by a high affinity to Hg,the albumen was found to be the egg component with the highest mercury concentrations in all the studied bird species.Median mercury concentrations in yolks were 15 to 24 times lower than in albumen,depending on bird species.The increase in mercury concentration in tissues and internal organs of the embryo,as it develops,is the result of the absorption of supplies accumulated in the egg.Moreover mercury was most effectively removed when developing embryo into down.However,the detoxification route into the first down probably ends before the nutrient supplies in the egg become fully exhausted.Once hatched,a herring gull chick underwent rapid detoxification and,within two weeks of life,Hg concentrations had dropped in the internal organs: by 87%in the heart and by 57%in the liver.