The environment experienced during development, and its impact on intrinsic condition, can have lasting outcomes for individual phenotypes and could contribute to variation in adult senescence trajectories. However, the nature of this relationship in wild populations remains uncertain, owing to the difficulties in summarizing natal condition and in long-term monitoring of individuals from free-roaming long-lived species. Utilizing a closely monitored, closed population of Seychelles warblers (Acrocephalus sechellensis), we determine whether juvenile body mass is associated with natal socio-environmental factors, specific genetic traits linked to fitness in this system, survival to adulthood and senescence-related traits. Juveniles born in seasons with higher food availability and into smaller natal groups (i.e. fewer competitors) were heavier. In contrast, there were no associations between juvenile body mass and the genetic traits. Furthermore, size-corrected mass - but not separate measures of natal food availability, group size or genetic traits - was positively associated with survival to adulthood, suggesting juvenile body mass is indicative of natal condition. Heavier juveniles had greater body mass and had higher rates of annual survival as adults, independent of age. In contrast, there was no association between juvenile mass and adult telomere length attrition (a measure of somatic stress) nor annual reproduction. These results indicate that juvenile body mass, while not associated with senescence trajectories, can influence the likelihood of surviving to old age, potentially due to silver-spoon effects. This study shows that measures of intrinsic condition in juveniles can provide important insights into long-term fitness of individuals in wild populations.

Please refer to the methods section of the associated manuscript for data collection/processing details