Data from: Among-species variation in six decades of changing migration timings explained through ecology, life-history and abundance

Species utilising seasonal environments must now alter timings of key life-history events in response to large-scale climatic changes, thereby maintaining trophic synchronies. Yet substantial among-species variation in cross-decadal phenological changes is observed. Transitioning from basic description of such variation towards prediction of future phenological responses now requires standardised studies that rigorously quantify and explain variation in the direction, magnitude and form of changing timings across diverse species in relation to key ecological and life-history variables. Accordingly, we fitted multi-quantile regressions to 59 years of high-quality multi-species data on spring and autumn bird migration timings through northern Scotland. We demonstrate substantial variation in cross-decadal changes in timings among 72 species, and quantify the degree to which variation can be explained through differences in species ecology, life-history and population trajectories. Consistent with predictions, species with seasonal diets, narrower breeding habitat breadths, shorter generation lengths and capability to produce multiple offspring broods per year advanced their migration timing in one or both seasons. In contrast, species with less seasonal diets, and that produce single annual offspring broods, showed no change. Meanwhile, contrary to prediction, long-distance migrants advanced their migration timings as much as short-distance migrants. Changes in migration timing also varied with changes in local migratory abundance, such that species with increasing seasonal abundance apparently altered their migration timing, whilst species with decreasing abundance did not. These patterns concur with expectation if changing migration timing is adaptive. However, we demonstrate that similar patterns can be generated through numerical sampling processes given changing abundances, implying that apparent phenology-abundance relationships should be carefully validated and interpreted. Overall, our results show that migrant bird species with differing ecologies and life-histories have shown systematically differing phenological changes over six decades contextualised by large-scale environmental changes, potentially facilitating future predictions and altering temporal dynamics of seasonal species co-occurrences.