Seychelles warblers with silver spoons: juvenile body mass is a lifelong predictor of annual survival, but not annual reproduction or senescence

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.