Following restriction on PCB use, PCB levels in herring gull (L. argentatus) eggs declined until the early 1980s, when PCB decline rate slowed down except on Lake Erie where egg levels have continued to decline rapidly. The herring gull is a piscivorous species, but it also consumes a variety of other food types including garbage, small mammals, invertebrates, songbirds, amphibians, and vegetation. With the exclusion of Lake Erie colonies, in all the Great Lakes colonies the reduction of rates of PCB decline in eggs was c. 60%, while in the two Lake Erie colonies, the rate of decline increased by c. 20%. Feed values of the stable nitrogen and carbon isotopes in Great Lakes herring gull eggs changed through time in particular in eggs from Lake Erie, where the examination of stable isotope fingerprint suggested that the shifts in herring gull egg values were probably the result of a shift in the herring gull diet from fish to terrestrial prey. In Northern Europe, a similar effect has been observed in Sweden on the peregrine falcon due to different feeding behaviors distributed along the country's north-south axis and in the migration pattern. The same may be said of the Mediterranean area for the gull species Lams cachinnans and L. argentatus. The first has a more variable diet including garbage and small mammals, whereas the second is essentially piscivorous. The TCDD-equivalent concentration levels in the eggs from L. cachinnans were 2 orders of magnitude below the concentration determined in the eggs from L. argentatus.
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