The hatching success of two Common Tern colonies, a highly contaminated one at the Elbe estuary (Hullen) and a less contaminated one at the Jade (Augustgroden, southern North Sea coast, Germany), was similar in 1988; 69% at Hullen and 73% at Augustgroden. Most egg loss was due to predators. The number of eggs failing to hatch from unknown causes was 6.3% higher at Hullen than at Augustgroden. Egg-shell thickness and strength were not correlated with the chemicals' concentrations. At Hullen unhatched eggs were compared with randomly collected fresh eggs; the former containing a significantly higher amount (20%) of PCBs (134 microg) than the eggs collected at random (112 microg). Seven out of 45 PCB-congeners were found in significantly higher amounts in the failed eggs, among them the toxic congener PCB 138. The two samples showed no difference in mercury concentration, which was 6.2 mg kg(-1), nor difference in shell strength. Eggs of Common Terns breeding at the Elbe estuary had concentrations of some contaminants reaching levels endangering breeding success.
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