Zilvermeeuw <i>Larus argentus</i> en Kleine Mantelmeeuw<i> Larus fuscus</i> als broedvogels in Vlaanderen

During the second half of the twentieth century, the number of Herring Gulls Larus argentatus in Belgium strongly increased from one pair in 1960 to 1703 pairs in 2001. The number of Lesser Black-backed Gulls L. fuscus increased from one pair in 1985 to 2863 pairs in 2001. For both species, the colonisation of Belgium started in the Zwin nature reserve, Knokke, but here the number of pairs never exceeded 74 and 40 for Herring and Lesser Black-backed, respectively. A strong increase was found at newly created land in the outer harbour of Zeebrugge (from 1987 onwards), where 1184 and 2695 pairs of Herring and Lesser Black-backed Gull, respectively, were counted in 2001. Other important colonies were situated in the inner harbour of Zeebrugge (225 and 125 pairs of Herring and Lesser Black-backed respectively) and in Oostende (205 and 8 pairs nesting on roofs of buildings). Except for some occasional pairs nesting inland, the only non-coastal colony of Herring Gull is found in Gent (30 pairs in 2001). Ring recoveries suggest that Herring Gulls nesting in Zeebrugge mainly originate from colonies in the Dutch Delta area, while Lesser Black-backed Gulls originate from colonies along the eastern and western coast of the southern North Sea. The rapid increase of the Belgian populations (26.9 % and 99.3 % growth per annum, respectively for Herring and Lesser Black-backed Gull) seems to be primarily the result of the occupation of new nesting habitat in the harbour of Zeebrugge by immigrants from foreign colonies in addition to a high reproductive output in Zeebrugge. In 2001, 20 and 82 pairs of Herring and Lesser Black-backed Gull (selected for the presence of a colour ring) produced on average 2.0 and 1.9 fledglings per pair. The high reproductive output as well as high proportions of fish in the diet of the young point towards a good food situation for the gulls nesting in Zeebrugge. Given the good breeding results and the thriving population, a further increase is expected in the near future. On the other hand, further industrial development in the harbour of Zeebrugge might in the near future result in the loss of large areas of nesting habitat