Long-distance migrants are particularly susceptible to climate change because of their multi-stage life-cycle, but understanding how climatic conditions at each of these stages influence population dynamics remains a key challenge. Here, we use long-term data from a UK population of Sand Martins Riparia riparia, a declining Afro-Palaearctic migrant, to investigate how weather on the wintering grounds and at passage sites impacts population size and arrival date. General linear models revealed that population size increased and arrival date advanced over the study period, and both were predicted by regional climatic variables in the previous winter and on passage. These results add to a growing body of evidence showing that population change in migrant birds is influenced by climatic conditions at all stages of the life cycle.