Life cycles consist of linked stages, such as reproduction and overwintering. Carry-over effects (COEs) via body condition can postpone the impacts of environmental change to later stages and seasons. This complicates identifying drivers of population change, as these occur in other seasons, and for migratory species also in other places, than their impacts. Furthermore, COEs are rarely studied at a landscape scale or rigorously quantified using path analysis, so their role in population change remains poorly understood. We investigated COEs in a declining shorebird (Haematopus ostralegus) by linking poor environmental conditions at communal wintering grounds to low breeding success across habitats. We advanced beyond the traditional focus on mass, applying a multifaceted approach by measuring multiple physiological aspects of body condition in 1574 individuals. We followed them across their annual life cycle using a 20-year nationwide citizen science effort and applied an analytical framework integrating structural equation modelling and capture-recapture analysis to quantify the COE and its mechanism. Winter body condition predicted reproductive success, indicating a COE. Offspring from parents with high winter body condition survived longer the next summer, but not due to earlier breeding. Winter body condition was higher where grassland cover was greater, while individuals with different diet specialisations responded differently to temperature and food availability. Grassland decline, estimated at 40% over recent decades, can explain a 13.5% reduction in offspring, supporting our hypothesis that reproductive decline throughout the Netherlands is partly due a COE from a deteriorating winter environment. Causes thus do not necessarily lie in the breeding environment. Though grasslands are often seen as marginal habitat, our findings suggest they are vital for maintaining these shorebirds. We recommend preserving and restoring grasslands around major winter roosts. Our multifaceted, landscape-wide approach offers a template for overcoming challenges in studying COEs and identifying drivers of population decline.