WANDERS R (1985) Breeding success of Lesser Black-backed Gull Larus fuscus and Herring Gull L. argentatus near Wassenaar (ZH). LIMOSA 58 (2): 57-60.

Lesser Black-backed Gull Larus fuscus European Herring Gull Larus argentatus In 1980, the breeding success of Lesser Black-backed and Herring Gull was studied in four plots, totalling 25784 m , in the dunes near Wassenaar, Netherlands. Onset of egg-laying and the density of breeding gulls around the nests were also investigated. On average, Lesser Black-backed Gulls started eggl~ ying on 14 May, two days earlier than Herring Gulls dId (16 May). In the Herring Gull, clutch size was smaller and hatching success lower than in the Lesser Black-backed Gull. Nest density did not influence clutch size and hatching success. However, for both species it was found that the later the clutch was started, the smaller the average number of eggs produced and hatched. Chick mortality was highest during the first 20 days after hatching. The percentage of young that survived was highest in the Herring Gull, although the difference between species was statistically not significant. In the Lesser Black-backed Gull 1.77 young per pair reached the age of 20 days, in the Herring Gull 1.57 young. The ratio in fledging success between species (1.13) is about the same as the ratio in the annual increase of the population size (1.30/1.20 = 1.08). In Wassenaar, breeding success is high compared to other gulleries.

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