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 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 directional within-individual change between years. Furthermore, subsequent reproductive success varied substantially among strategies, and relationships differed between years; residents and late migrants had highest success in the 2 years, respectively, matching the years in which these strategies were most frequently expressed. These results imply that migratory strategies can experience fluctuating reproductive selection, and that flexible expression of migration can be partially aligned with reproductive outcomes. Plastic seasonal migration could then potentially contribute to adaptive population responses to currently changing forms of environmental seasonality.

Small, declining populations can face simultaneous, interacting, ecological and genetic threats to viability. Conservation management strategies designed to tackle such threats independently may then prove ineffective. Population viability analyses that evaluate the efficacy of management strategies implemented independently versus simultaneously are then essential to the design of effective management plans, yet such quantitative evaluations are typically lacking. We used stochastic individual‐based models, parameterised with high‐quality multi‐year demographic and genetic data, to evaluate the efficacy of independent or simultaneous ecological (supplementary feeding) and genetic (translocations to alleviate inbreeding) management strategies for a red‐billed chough (Pyrrhocorax pyrrhocorax) population of major conservation concern. This population is experiencing ecological threats from food limitation and genetic threats from escalating inbreeding. Conservation managers therefore face a dilemma: supplementary feeding may be ineffective if inbreeding is limiting stochastic population growth rate (λs), while translocations may be ineffective if food is limiting. Model simulations suggested that the focal population will decline to extinction relatively rapidly with no conservation management (mean λs ≈ 0.86) and with genetic management alone (λs ≈ 0.90). Ecological management alone reduced, but did not halt the population decline (λs ≈ 0.93). However, simultaneous genetic and ecological management yielded population stability (λs ≈ 1), with genetic rescue lasting ~25 years. These outcomes arose because the capacity for translocations to alleviate inbreeding depression is limited by food availability, while supplementary feeding cannot achieve population viability in the presence of accumulating inbreeding. However, supplementary feeding improved environmental quality enough to allow expression of variance in fitness and thus inbreeding depression, meaning that reductions in inbreeding following translocations can increase λs. Synthesis and applications. Our analyses suggest that simultaneous management of ecological and genetic threats will be critical to ensuring viability of Scotland's chough population; neither strategy independently is likely to achieve population persistence and may consequently waste conservation resources. Managers of other resource‐limited, inbred populations should consider that the efficacy of strategies designed to alleviate ecological and genetic threats may be interdependent, such that holistic management is essential to ensure population viability.

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1. Effective evidence-based conservation requires full quantification of the impacts of targeted management interventions on focal populations. Such impacts may extend beyond target individuals to also affect demographic rates of non-target conspecifics (e.g. different age classes). However, such collateral (i.e. unplanned) impacts are rarely evaluated despite their potential to substantially alter conservation outcomes. Subsequent management decisions may then be poorly informed or erroneous. 2. We used 15 years of individual-based demographic data in a 'before-after controlimpact' (BACI) analysis to quantify collateral demographic impacts of a targeted multi-year supplementary feeding programme designed to increase sub-adult survival and hence viability of a small, threatened red-billed chough Pyrrhocorax pyrrhocorax population. Specifically, we assessed whether the intervention also affected adult survival and reproductive success, and whether such collateral effects were themselves sufficient to stabilize population size and hence achieve short-term conservation aims. 3. The probabilities of adult survival and successful reproduction increased substantially between the 'before-feeding' and 'during-feeding' periods in those choughs associated with supplementary feeding, but not otherwise. Overall breeding success (i.e. number of chicks fledged per occupied territory) also tended to increase, even though brood sizes did not increase. These relationships, which were detectible only through BACI analyses, suggest that supplementary feeding targeted at sub-adults had unplanned positive impacts on adult demographic rates. 4. Deterministic matrix models designed to project population growth demonstrate that these estimated collateral effects were sufficient to make a substantial contribution to increasing population growth rate and achieving short-term population stability.

This is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.