Species Traits and Island Biogeography: Wing Metrics Linked to Avian Dispersal Ability Predict Species Occurrence on Remote Islands Worldwide

ABSTRACT Aim We assessed the influence of island isolation on the composition of insular bird assemblages with a particular focus on species traits associated with dispersal. To do so, we tested whether ecomorphological metrics of dispersal ability, namely hand‐wing index and Kipp's distance, increase with increasing island isolation. Location Global. Taxon Birds. Methods We integrated global datasets of island characteristics with distribution and ecomorphological trait information of birds; our final dataset comprised information for 2034 native, resident and terrestrial species inhabiting 2399 islands. Species restricted to islands were removed to avoid potentially confounding effects of speciation, such as the evolution of flightlessness or poor flight on islands. Using the generalised additive models, we tested for the relationship between hand‐wing index or Kipp's distance and island isolation, accounting for the effects of island area and spatial autocorrelation. We performed the analyses separately for (i) continental and oceanic islands and (ii) for all terrestrial birds and for passerine birds only. Results Hand‐wing index and Kipp's distance were positively related to island isolation on oceanic islands, that is bird communities on more isolated oceanic islands were composed of species with wings that had a higher aspect ratio and were more elongated. However, this relationship did not hold for continental islands. We found these patterns to be consistent for all terrestrial birds as well as for passerine birds only. Main Conclusion Our study provides strong evidence that island isolation influences the trait composition of island bird assemblages at a global scale. Our results highlight the variation of dispersal‐related ecomorphological traits among bird assemblages on islands, suggesting that these traits play an important role in mediating the influence of island isolation on community assembly processes on islands.