Among-individual and within-individual variation in seasonal migration covaries with subsequent reproductive success in a partially-migratory bird

Within-individual and among-individual variation in expression of key environmentally-sensitive traits, and associated variation in fitness components occurring within and between years, determine the extents of phenotypic plasticity and selection and shape population responses to changing environments. Reversible seasonal migration is one key trait that directly mediates spatial escape from seasonally-deteriorating environments, causing spatio-seasonal population dynamics. Yet, within-individual and among-individual variation in seasonal migration versus year-round residence, and dynamic associations with subsequent reproductive success, have not been fully quantified. We used novel capture-mark-recapture mixture models to assign individual European shags (Phalacrocorax aristotelis) to ‘resident, ‘early migrant’ or ‘late migrant’ strategies in two consecutive years, using year-round local resightings. We demonstrate substantial among-individual variation in strategy within years, and d...