Temporal changes to migratory fuel-loading in migratory birds across Europe

Climate change is a major threat to biodiversity, with migratory birds particularly vulnerable. We used long term ringing data from 286 sites and 33 species across Europe to demonstrate a large-scale decrease in migratory fuel-loads during autumn over the last 40-years, but not in spring. Declines were strongest across southern Europe and were related to rising temperatures. The timing of autumn fuel-loading has also shifted, occurring earlier in the north and later in the south. These relationships varied depending on diet and breeding cycle length, with obligate insectivores more constrained by temperature in the timing and magnitude of fuel-loading compared to frugivores and species with short breeding cycles more likely to extend their breeding season in the south in warmer years. Altogether, these latitudinally varying findings suggest a trade-off between maximising productivity or maximising adult-survival as climate drives changing constraints on breeding season length and resource availability.