Many effects due to exposure to POPs have been observed in populations of wild bird species in North America and Northern Europe in the 1960s and 1970s and they are essentially of the correlational type. In the region of North American Great Lakes, the species involved were the herring gull (Larus argentatus), the Caspian (Sterna caspia) and the Forster's terns (Sterna forsterii), and double-crested cormorants, all piscivorous birds. In the gull, some colonies showed hatchabilities of less than 20% and a productivity of less then one fledged young every ten nests was found. The authors believe that these effects were due to the presence of a few TCDD nanograms per tissue gram (1—3 ngg~ ) as determined in the herring gull eggs. Similar effects were observed in the above-indicated species, although, in particular, the effects were more pronounced on Caspian terns. On this species, the effects included longer incubation times with smaller individuals due to a wasting syndrome, and growth deficiencies and deformities in the embryos. North European peregrine falcon (Falco peregrinus) and golden eagle (Aquila chrysaetos) populations were deeply influenced in the 1960s following exposures to the pesticide dieldrin and p,p'-DDE, a persistent metabolite of p,p'-DDT. Dieldrin, in particular, was implicated in population reduction, and apart from acute toxicity the effects were to reduce the breeding success. Nesting peregrines lay one clutch of eggs per year, of three or four eggs. The mean clutch size is 3.5 eggs, with an average of 2.5 fledged young and only 4% of clutches with broken eggs. This percentage increased to 39% and appeared to be due to eggshell thinning. Similarly, the breeding success of golden eagle was greatly reduced in the 1960s when adults successfully reared offsprings in only 31% of the nests. Notwithstanding this phenomenon, the population did not decline, and a possible explanation for this trend was suggested some years ago as the population aged. The lifetime of this species spans as much as 30 years, so of this effect in the ecological perspective is that the golden eagle is a long-life ^-strategist organism with a low reproductive rate. The effects on these organisms can be very dramatic, because when their population is reduced to low numbers it takes much more time to recover, and are thus subject to a greater extinction risk compared to an r-strategist. Anyway, the DDE effects were clearly demonstrated in the laboratory, confirming the historical observations made in the field, but with a strong species-specific effect pattern. Galliform species are very resistant to shell thinning whereas birds of prey are particularly affected. Until recently, as a result of the long transportation to Arctic and subarctic areas, hexachlorobenzene (HCB) and DDE levels were found along Norwegian coasts in Larus marinus eggs. These eggs showed a characteristic contamination pattern with highest levels found in samples collected in nearby Arctic areas. In particular, there was a significant positive relationship between blood concentrations of the cited substances in females, egg-laying dates, and probability of nest predation. In females with high levels of PCBs, there was also a decline in egg volume as egg laying progressed, compared to females with low OC levels.
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