Ardea
Official journal of the Netherlands Ornithologists' Union

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van der Molen E.J., Blok A.A & de Graaf G.J. (1982) Winter starvation and mercury intoxication in Grey Herons (Ardea cinerea) in The Netherlands. ARDEA 70 (2): 173-184
A short cold spell in January and February 1976 caused a marked mortality among wintering Grey Herons Ardea cinerea in The Netherlands. This resulted in the next breeding season in a 19% decrease of the population. Analysis of 41 specimens from the western part of the country revealed the presence of relatively high residues of mercury and selenium. In at least 5% of the birds (those with more than 400 ppm) liver mercury residues alone were considered lethal. In 20% of the herons (those with mercury concentrations over 160 ppm) sublethal residues combined with stress from undernourishment and cold weather may have been fatal. The levels found varied widely; liver mercury 1.6--773, liver selenium 1.5-23.2, kidney mercury 0.9-700, kidney selenium 2.7-14.4 ppm (all values in ppm dry weight, Table 2 and Fig. 1 and 2). Neither sex, age, condition or breeding experience correlated with residue levels (Tables 3 and 4). It is concluded that the accumulation largely took place between the last autumn moult and the cold period of January/February 1976 and was caused by local pollution. With one exception, all herons with more than 160 ppm mercury in the liver were found in a limited area in the north of Noord-Holland (region A). The residues occurring in other rural areas did not differ from those in industrialised regions. These generally remained below 100 ppm. The residues in herons from the northern part of Noord-Holland were 4 to 10 times higher than hitherto observed in other bird species in The Netherlands. The molar ratios of mercury and selenium varied between 1 to 10 (kidney) and 15 to 1 (liver). The amount of mercury and selenium in the kidney showed linear correlation. However, the relative amount of selenium in the liver declined as the mercury level rose (Fig. 3 and 4). In most herons both mercury and selenium levels were higher in the liver than in the kidneys. In view of the origin of the birds it is suspected that the use of fungicides in agriculture was the primary source of the high mercury residues in the northern part of Noord-Holland.


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