Life History of the Common Gull (Larus canus) : A Long-Term Individual-Based Study

Abstract: An individual’s life history is a sequence of events which eventually determine its contribution to the next generation, or fitness. These events are affected by environmental factors, genetic make-up and decisions made by an individual and its breeding partner. Recognition of these determinants helps to understand both short-term ecological changes and long-term evolutionary dynamics in a population. In this thesis long-term individual-based data on common gull (Larus canus) is used to study age-dependent reproductive success, survival, pair retention and breeding time. Collected pedigree data enabled to study genetic variation of traits and the selection on them. The reproductive success of common gulls increased steadily until very old age. This increase cannot be explained by selection effects, since individuals with lower breeding success were shown not to have lower survival. Consequently, this gain must be ascribed primarily to an age-related improvements of individual competence and/or increased reproductive effort. Annual survival of adult birds was age- and year-dependent. The latter was partly explained by winter severity. The size of the white spots on five outermost primaries predicted the bird’s future survival and divorce probabilities and hence, pair endurance capability. Gulls with larger spots enjoyed higher survival and lower divorce rates compared to birds with smaller spots. This suggests that the wing tip pattern might function as a condition dependent signal, revealing individual variation in quality. One of the advantages of persistent pair bond was the ability to start breeding early in the season. The timing of breeding of firm pairs advanced with time not only due to mates’ increasing age, but also owing to their experience together. Although both sexes had phenotypic effects on laying date, it was heritable only in females. On the phenotypic and genotypic level, early laying was under positive fecundity and survival selection in females.