Ecological correlates of extinction vulnerability in terrestrial birds in the Zhoushan Archipelago, China

Understanding why some insular bird species are especially vulnerable to local extinction under prolonged isolation is important for clarifying extinction processes and setting conservation priorities. However, the species traits linked to such vulnerability remain poorly understood in long-isolated island systems. Here, we examined how species traits influence local extinction vulnerability of terrestrial birds in the Zhoushan Archipelago, China, a land-bridge island system isolated from the mainland for approximately 7000–9000 years following Holocene sea-level rise. We surveyed birds on 40 islands and at 16 sites on the adjacent mainland. Using data on 10 ecological and life-history traits for 110 species, we applied phylogenetic generalized least squares models to test their associations with two vulnerability metrics: species restrictedness and species nestedness ranking. We further assessed island-level correlates of species occurrence using AICc-ranked generalized linear mixed models that included island area, isolation, habitat richness and human population density. Natural abundance on the adjacent mainland was the strongest predictor of both vulnerability metrics across breeding birds, wintering birds, resident birds and migratory birds. Species with lower natural abundance had more restricted island distributions, while other traits showed weaker and less consistent associations. Species occurrence probability increased with island area and habitat richness but showed no clear association with isolation or human population density. Conservation in this system should therefore prioritize species with low natural abundance and restricted island distributions, together with large, habitat-rich islands.