Abstract Population dynamic and eco‐evolutionary responses to environmental variation and change fundamentally depend on combinations of within‐ and among‐cohort variation in the phenotypic expression of key life‐history traits, and on corresponding variation in selection on those traits. Specifically, in partially migratory populations, spatio‐seasonal dynamics depend on the degree of adaptive phenotypic expression of seasonal migration versus residence, where more individuals migrate when selection favours migration. Opportunity for adaptive (or, conversely, maladaptive) expression could be particularly substantial in early life, through the initial development of migration versus residence. However, within‐ and among‐cohort dynamics of early‐life migration, and of associated survival selection, have not been quantified in any system, preventing any inference on adaptive early‐life expression. Such analyses have been precluded because data on seasonal movements and survival of sufficient young individuals, across multiple cohorts, have not been collected. We undertook extensive year‐round field resightings of 9359 colour‐ringed juvenile European shags Gulosus aristotelis from 11 successive cohorts in a partially migratory population. We fitted Bayesian multi‐state capture‐mark‐recapture models to quantify early‐life variation in migration versus residence and associated survival across short temporal occasions through each cohort's first year from fledging, thereby quantifying the degree of adaptive phenotypic expression of migration within and across years. All cohorts were substantially partially migratory, but the degree and timing of migration varied considerably within and among cohorts. Episodes of strong survival selection on migration versus residence occurred both on short timeframes within years, and cumulatively across entire first years, generating instances of instantaneous and cumulative net selection that would be obscured at coarser temporal resolutions. Further, the magnitude and direction of selection varied among years, generating strong fluctuating survival selection on early‐life migration across cohorts, as rarely evidenced in nature. Yet, the degree of migration did not strongly covary with the direction of selection, indicating limited early‐life adaptive phenotypic expression. These results reveal how dynamic early‐life expression of and selection on a key life‐history trait, seasonal migration, can emerge across seasonal, annual, and multi‐year timeframes, yet be substantially decoupled. This restricts the potential for adaptive phenotypic, microevolutionary, and population dynamic responses to changing seasonal environments.