Ongoing collapse of avifauna in temperate oceanic islands close to the mainland in the Anthropocene.

Oceanic island ecosystems are highly vulnerable to human activities. Oceanic islands near the mainland offer insights into natural processes, including overseas dispersal from the mainland. However, most studies examining community changes on oceanic islands focus on human activities, while natural colonisation from the mainland has received limited attention. To clarify the mechanisms of insular community changes due to human activities and natural colonisation, we investigated bird assemblages on 10 islands during 2016 and 2021 and compared them to the assemblages during 1970 and 1973 on the Izu Islands, Japan. We explored how the species traits and mainland distribution changes of species are associated with distribution changes of each species on the 10 islands using phylogenetic generalised least squares (PGLS). Furthermore, bird community structures on each island were examined based on species richness and functional and phylogenetic structures. A null modelling approach was used to examine functional and phylogenetic cluster/overdispersion structures; clustering and overdispersion imply environmental filtering and limiting similarity, respectively. We analysed the effects of geographical and topographical characteristics of the islands, introduced predators and human-induced landscape transformations on changes in community structures using linear models (LMs). Species that expanded their mainland distribution and with large clutches colonised many islands. Raptors disappeared from many islands. Species richness decreased, and functional and phylogenetic structures became clustered on almost all islands, which were only statistically associated with geographical and topographical characteristics. The distribution changes in species on islands were associated with their mainland distribution changes, which indicates that mainland community dynamics influence insular communities via overseas colonisation. Despite the colonisations, the net loss of species richness, clustered phylogenetic and functional structures, and disappearance of top predators on many islands suggest that bird communities across the archipelago degraded. Previous studies have demonstrated that introduced weasels negatively affect birds through competition for food resources and direct predation on the introduced islands. These degraded bird communities due to introduced weasels would have transboundary biodiversity effects on communities across the archipelago via inter-island movements of birds, which suggests the need for comprehensive conservation strategies across the archipelago to effectively conserve insular biodiversity.