Modelling responses of herring gull and lesser black-backed gull populations to reduction of reproductive output: implications for control measures.

1. Between 1984 and 1988 breeding output of herring gulls Larus argentatus and lesser black-backed gulls L. fuscus on the Isle of May, Scotland was severely restricted by the repeated destruction of clutches of eggs. 2. Annual censuses showed that the breeding population of herring gulls declined at a rate of 6% per year 1983-92, and then increased dramatically by 41% in 1993. The number of lesser black-backed gulls decreased rapidly 1984-87, but then increased gradually before also rising sharply by 68% in 1993. 3. During the period of control, breeding output was limited to 150-390 herring gull chicks per year and 15-20 lesser black-backed gull chicks. After control measures were relaxed, output increased to 1917-2709 herring gulls and 331-1018 lesser black backed gulls. 4. Estimated mean adult annual survival was 88% for herring gulls and 91% for lesser black-backed gulls. There were no significant sex or year effects, the 3.1% difference between the species was significant at the 10% level. 5. A simple demographic model is developed which closely matches the observed changes in numbers of herring gulls (range of mean absolute percentage error of stepwise predictions 5.4-6.1%), but which does not provide a good fit to the counts of lesser black-backed gulls (19.7-20.9%). 6. This difference appears to be due to interspecific differences in recruitment patterns. In management terms it indicates that: (i) limiting reproductive output can be an effective method of control for herring gulls, but not for lesser black-backed gulls; and (ii) on the basis of current information on vital rates it is possible to predict future population trends of herring gulls, but not lesser black-backed gulls