Weather at the winter and stopover areas determines spring migration onset, progress, and advancements in Afro-Palearctic migrant birds

Climate change causes changes in the timing of life cycle events across all trophic groups. Spring phenology has mostly advanced, but large, unexplained, variations are present between and within species. Each spring, migratory birds travel tens to tens of thousands of kilometers from their wintering to their breeding grounds. For most populations, large uncertainties remain on their exact locations outside the breeding area, and the time spent there or during migration. Assessing climate (change) effects on avian migration phenology has consequently been difficult due to spatial and temporal uncertainties in the weather potentially affecting migration timing. Here, we show for six trans-Saharan long-distance migrants that weather at the wintering and stopover grounds almost entirely (similar to 80%) explains interannual variation in spring migration phenology. Importantly, our spatiotemporal ap-proach also allows for the systematic exclusion of influences at other locations and times. While increased spring temperatures did contribute strongly to the observed spring migration advance-ments over the 55-y study period, improvements in wind conditions, especially in the Maghreb and Mediterranean, have allowed even stronger advancements. Flexibility in spring migration timing of long-distance migrants to exogenous factors has been consistently underestimated due to mismatches in space, scale, time, and weather variable type.