POPULATION-SIZE AND DENSITY-DEPENDENT EFF ECTS ON BREEDING SUCCESS INHERRING-GULLS (LARUS-ARGENTATUS) ON MELLUM ISLAND

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 a rgentatus) population breeding along the German North Sea coast. In 19 95 we estimated the actual population size as well as reproductive suc cess and chick growth in relation to population density and food choic e. Furthermore, the methods of monitoring population size and breeding success recommended for the trilateral Joint Monitoring Project for B reeding Birds in the Wadden Sea were evaluated. To survey population s ize gull nests were counted on 50 randomly selected plots of 0.25 ha e ach ('stratified sampling'). For 1995 the estimation indicated a colon y 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 He rring Gull nests/ha. Breeding biology was studied on two plots with di fferent 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 sou th. Nests were visited every 3 days. In the southern dune area clutch initiation started significantly later than in the northern part of th e island (p < 0.01; median: 11.-15.5. vs. 6.5.-10.5.). Whereas mean cl utch 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 nort h of the island. During the first 4 weeks after hatching, the growth r ate of chicks was nearly identical in the 3 years of investigation (19 94-1996). Between day 5 and day 25 (linear phase of growth)body mass i ncreased by 26.3 g/d. From day 30 onwards, the body mass of most chick s decreased, thus causing different body masses of the fledglings. At the age of 45 days the highest body masses were recorded on the northe rn 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 popu lation size and breeding success in the international Wadden Sea seem also to be appropriate for recording spatial differences as well as te mporal 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, My tilus edulis dominated in pellets of birds breeding in the south of th e island. Interestingly, birds of both plots, but in particular from t he southern plot fed on terrestrial food, too. About 25% of the pellet s from birds breeding in the south contained earthworms and/or miscell aneous refuse. Direct observations and radio tracking studies showed t hat birds from both plots regularly fed at the mainland coast, especia lly on refuse tips. A further increase of the Herring Gull population seems to be possible. Though reproductive success decreased with incre asing population density, even a density of greater than or equal to 1 00 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 chic ks/pair are necessary to maintain population size. On only about 20% o f the colonized area has a density of greater than or equal to 100 pai rs/ha been reached as yet. Furthermore, there are some potential breed ing grounds on Mellum nor yet colonized. Though feeding conditions wer e good, it can be assumed for the future that feeding conditions can b ecame one of the main factors limiting population size. Additionally, interspecific competition with the ever increasing population of Lesse r Black-backed Gulls has to be taken into consideration.