In the mid-nineties the German Wadden Sea island Mellum (53 degrees N 43'E 08 degrees 09'E) harboured about 39% of the Herring Gull (Larus argentatus) population breeding along the German North Sea coast. In 1995 we estimated the actual population size as well as reproductive success and chick growth in relation to population density and food choice. Furthermore, the methods of monitoring population size and breeding success recommended for the trilateral Joint Monitoring Project for Breeding Birds in the Wadden Sea were evaluated. To survey population size gull nests were counted on 50 randomly selected plots of 0.25 ha each ('stratified sampling'). For 1995 the estimation indicated a colony size of 13,449 Herring Gull and 1846 Lesser Black-backed Gull (Larus fuscus) pairs settling on an area of about 245 ha. Highest densities were recorded in the northern dune area, where mean density was 126 Herring Gull nests/ha. Breeding biology was studied on two plots with different population density. One plot with 134 pairs/ha was located in the north of the island, and a second with only 65 pairs/ha in the south. Nests were visited every 3 days. In the southern dune area clutch initiation started significantly later than in the northern part of the island (p < 0.01; median: 11.-15.5. vs. 6.5.-10.5.). Whereas mean clutch size (2.8 eggs/clutch) was similar in the two sites, reproductive success was different. On the southern plot reproductive success was 1.5 fledged young/breeding pair, compared to 1.1 on the northern plot. The difference was mainly caused by greater predation by conspecifics, in particular predation on chicks in the more densely populated north of the island. During the first 4 weeks after hatching, the growth rate of chicks was nearly identical in the 3 years of investigation (1994-1996). Between day 5 and day 25 (linear phase of growth)body mass increased by 26.3 g/d. From day 30 onwards, the body mass of most chicks decreased, thus causing different body masses of the fledglings. At the age of 45 days the highest body masses were recorded on the northern plot in 1995, on average 781.7 g, the lowest masses in the southern dune area 1994 (552.5 g). The methods recommended for monitoring population size and breeding success in the international Wadden Sea seem also to be appropriate for recording spatial differences as well as temporal changes. Diet composition was examined by pellet analyses. The birds of both study plots fed predominantly on mussels. Whereas Macoma balthica was the main prey species of birds breeding in the north, Mytilus edulis dominated in pellets of birds breeding in the south of the island. Interestingly, birds of both plots, but in particular from the southern plot fed on terrestrial food, too. About 25% of the pellets from birds breeding in the south contained earthworms and/or miscellaneous refuse. Direct observations and radio tracking studies showed that birds from both plots regularly fed at the mainland coast, especially on refuse tips. A further increase of the Herring Gull population seems to be possible. Though reproductive success decreased with increasing population density, even a density of greater than or equal to 100 pairs/ha resulted in a high breeding success. In 1995 mean fledging success was estimated at 1.3 fledged chicks/pair. Only about 0.6 chicks/pair are necessary to maintain population size. On only about 20% of the colonized area has a density of greater than or equal to 100 pairs/ha been reached as yet. Furthermore, there are some potential breeding grounds on Mellum nor yet colonized. Though feeding conditions were good, it can be assumed for the future that feeding conditions can became one of the main factors limiting population size. Additionally, interspecific competition with the ever increasing population of Lesser Black-backed Gulls has to be taken into consideration.
