Senescence, the time-dependent loss of function and exponential increase in mortality rate, is a fundamental process found in several biological systems. However, despite several theories to explain senescence, the causes of senescence remain largely unexplored in wild vertebrates. This is mainly because of the difficulties involved in collecting good quality longitudinal data in natural settings, and the problems associated with separating between-, and within-individual effects. Here we propose a research plan to experimentally investigate the causes, and heritable components, of individual variation in senescence in the wild, using the Seychelles warblers (Acrocephalus sechellensis) as model system. The warblers have been intensively studies on the islands of Cousin (1985-2007) and Denis (since the translocation in 2004 ? 2007). Importantly, the absence of predation means that individuals live long enough to senesce. This, and the lack of migration on or off the islands, allows us to monitor individuals throughout their lives, including beyond being deposed from a breeding position. In this study we aim to investigate the causes of individual variation in reproductive and survival senescence. New data acquired over four seasons will be combined with the extensive data and results already collated. Using these data, in conjunction with molecular screening and field experiments, we will investigate: (1) The effects of within- and between-individual variation on age-specific fitness performance. (2) The environmental and genetic effects on individual variation in reproductive and survival senescence (cross-fostering experiment). (3) Specifically, we will experimentally examine the effect of early-life reproductive effort on senescence, which has not been conducted in any wild species so far (translocaton of warblers to the island of D?Arros). (4) Our final aim is to investigate whether phenotypic variation in senescence has a heritable component, using the extensive pedigree. The mechanisms involved in senescence in the Seychelles warbler will probably mirror many of those that would affect other vertebrates including humans. The results of our proposed study will help us understand the biological basis of senescence in natural populations.