Age-dependent trait variation: the relative contribution of within-individual change, selective appearance and disappearance in a long-lived seabird

1. Within populations, the expression of phenotypic traits typically varies with age. Such age-dependent trait variation can be caused by within-individual change (improvement, senescence, terminal effects) and/or selective (dis)appearance of certain phenotypes among older age classes. 2. In this study we applied two methods (decomposition and mixed-modelling) to attribute age-dependent variation in seven phenological and reproductive traits to within-individual change and selective (dis)appearance, in a long-lived seabird, the common tern (Sterna hirundo). 3. At the population level, all traits, except the probability to breed, improved with age (i.e., phenology advanced and reproductive output increased). Both methods identified within-individual change as the main responsible process, and within individuals, performance improved until age 6-13, before levelling off. In contrast, within individuals, breeding probability decreased to age 10, then levelled off. 4. Effects of selective a...