Spatiotemporal dynamics of migratory birds reshape seed dispersal and conservation implications on fragmented islands

Seed dispersal by frugivorous birds underpins forest regeneration and biodiversity conservation in fragmented landscapes. Migratory frugivores, through their seasonal movements, connect distant plant populations, but their roles in plant–bird interaction networks remain poorly quantified amid global declines in migratory bird populations. How these birds mediate plant–animal interactions across space and time, and how they functionally differ from residents, remains unclear. Using arboreal camera trapping over three fruiting seasons (2019–2022), we recorded 10,992 interactions (343 unique links) between 31 fleshy-fruited plants and 48 bird species (15 migratory, 33 resident) across 13 reservoir islands in the Thousand Island Lake of China. Migratory birds accounted for 14 % of all interactions, with 99.3 % occurring during the autumn/winter fruiting peaks (October–January), and interacted with 67.7 % of the plant species. Despite their lower overall richness, smaller islands (<10 ha) hosted 43 % of migratory interactions, highlighting their importance as stopover sites. In contrast, larger islands (>30 ha) supported more frugivore richness but lower migratory proportions, acting as refugia for residents. Network analyses showed no significant differences in species roles (degree, species strength, specialization d’) between migratory and resident birds though migrants expanded the spatial and temporal scope of seed dispersal. These findings challenge area-centric conservation priorities by revealing the complementary roles of small and large islands. We advocate conserving island networks through seasonal management aligned with fruiting phenology, restoring native fruiting plants with staggered phenologies to buffer climate-driven mismatches, and enhancing habitat connectivity to sustain seed dispersal and ecosystem resilience in fragmented landscapes.