Mercury contamination of the Belgian Avifauna 1970–1981

Two hundred birds found dead in Belgium between 1970 and 1981, and belonging to 30 species, were analysed for total mercury contamination. The contamination of aquatic birds ranged between 0·11 and 35 μg g⁻¹ wet weight. For terrestrial birds, the extreme values were: not detectable and 14 μg g⁻¹. In both cases, differences in diet can explain the differences in contamination. The order of diets associated with increasing mercury contamination for aquatic birds was invertebrates, zooplankton and garbage, and fish; and for terrestrial birds this consisted of plants, invertebrates, mammals and birds. For raptors and owls, this effect of diet includes geographical variations within species. A higher mercury contamination level in the winter and early spring was noted for two species of owls. For aquatic birds, the contamination of liver was higher than that of kidney, with ratios varying between 1·2 and 2·5. For terrestrial birds, the ratio was closer to 1. A few determinations were also made for muscle and heart, giving respectively 0·25 and 0·6 of the liver contamination. Among the birds analysed for their liver contamination, 15% showed levels higher than 3 μg g⁻¹ and could have been affected in their reproduction; 3% had levels higher than 10 μg g⁻¹, and could have died from mercury poisoning; and 6% showed an abnormally high liver: kidney ratio, which could reflect an acute intoxication.

There exists a striking parallelism between the levels of mercury and of organochlorine residues (DDT) in birds of prey, suggesting the existence of common ecotoxicological mechanisms.