The role of viability selection in shaping short- and long-term consequences of early-life conditions: An experiment in the Alpine swift.

Environmental conditions experienced early in life can shape growth, morphology and survival, and can have lasting effects on individuals that survive to adulthood. Understanding how early-life conditions affect fitness components across the life cycle is therefore essential for predicting variation in life-history trajectories and their ecological and evolutionary consequences. We experimentally tested for short- and long-term effects of early growth conditions by manipulating brood size in a wild population of the long-lived Alpine swift Tachymarptis melba. Enlarging or reducing brood size alters sibling competition and per capita resource availability, with potential consequences on resource allocation between growth and self-maintenance. We manipulated 45 pairs of broods between 2003 and 2006 and followed the offspring throughout their lives until 2019. Brood enlargement reduced average growth and survival, while increasing phenotypic variability among nestlings. Nestlings from enlarged broods were smaller, fledged later, and had lower survival to fledging and recruitment. Recruitment probability increased with nestling size, indicating strong selection on developmental phenotype. Consequently, nestlings from enlarged broods that recruited were a non-random subset of bigger, higher-quality individuals, and morphological differences between treatments disappeared among adults. Despite lower recruitment, individuals from enlarged broods showed equal or higher lifetime reproductive output, primarily through more frequent breeding attempts. Age-specific analyses showed that reproductive performance patterns reflected both within-individual changes and selective disappearance among individuals. We discuss how early-life conditions shape life-history trajectories through both developmental effects and selection acting before adulthood, arguing that apparent advantages in adult performance among individuals from harsher developmental environments arise from viability selection rather than beneficial effects of early-life stress. Our findings show how missing-fraction problems can bias inferences about fitness when early-life mortality is ignored, highlighting the need to integrate early survival when evaluating the lifelong consequences of developmental environments.