How far can frugivorous birds disperse seeds during migration?

Long-distance seed dispersal plays a crucial role in shaping plant distributions. The annual migration of millions of frugivorous birds holds substantial potential for the long-distance dispersal of fleshy-fruited plants, which are a major component of forests and woodlands globally. However, quantitative estimations of seed-dispersal distances mediated by migrant frugivorous birds are still lacking. Here, we uniquely combine GPS-tracked movement data of thrushes migrating across Europe (n = 38 individuals) with empirical data on fruit consumption, flight departure time and flight speed, and seed retention times in the birds' gut to mechanistically simulate dispersal events. Our results reveal that, during migratory flights, frugivorous birds can disperse seeds over tens to hundreds of kilometres, depending on the plant species (range across species: mean = 9-45 km; percentile P_99.9 = 42-221 km; max. = 58-521 km). In contrast, most dispersal distances (P_99.9) by non-migratory movements were within 1 km. Smaller-seeded species disperse farther owing to their longer seed retention times. Our study fills an important knowledge gap on the large-scale dispersal of fleshy-fruited plants by migratory birds and uncovers interspecific, trait-based differences in their long-distance dispersal capacity.