Land birds on atolls: diversity, origin, and function

Oceanic islands are global hotspots of avian biodiversity and endemism. Their isolation and distinct environmental conditions have uniquely shaped the evolutionary trajectories of island birds, while, in turn, land bird communities have become critical providers of ecological functions to island ecosystems. However, these processes and dynamics have been studied almost exclusively on large volcanic islands. Coral atolls - the other principal type of oceanic island - remain largely absent from ecological research on island bird diversity, evolution, and functioning. This omission constitutes a major gap because atoll islands are systems with distinct geo-evolutionary histories and dynamics. Recurrent cycles of emergence and submergence of atolls with glacial sea-level fluctuations have created unique natural experiments that offer novel insights into island assembly and evolution, different to those on volcanic islands. Here, we present the first synthesis of land bird assemblages on atoll islands across the Indo-Pacific. Our quantitative review of land bird records across Indo-Pacific atolls reveals that land birds are widely occurring on atolls throughout this region, often represented by endemic species or subspecies. Building on this macroecological data, we then synthesise the evolutionary pathways of land birds in the context of the atolls' geo-evolutionary dynamics, including refugial persistence on uplifted atolls, rapid divergence following recent colonisation, and relictual persistence of bird lineages extirpated from volcanic islands. Together, this synthesis suggests that atoll islands act as unique evolutionary arenas in which avian colonisation and diversification are repeatedly 'replayed' across successive emergence-submergence cycles. In addition to the unique conditions that atolls provide for land birds, we also review the ecological functions that land birds provide to atoll island ecosystems. Despite typically low species richness, atoll land bird assemblages provide ecological functions including pollination, seed dispersal, seed predation, and insectivory. Extinctions and range contractions have eroded much of this functional spectrum from many atolls but the consequences for atoll ecosystem integrity remain virtually unstudied. By synthesising land bird diversity, evolutionary history, and functional ecology on atolls, we highlight a reciprocal relationship: atolls provide unique evolutionary arenas for land birds, while land birds support ecological functioning on atoll island ecosystems. Together, the generated insights are relevant for domain-specific knowledge of atoll ecosystems, for a more holistic theoretical understanding of island evolution processes in dynamic and young island systems, as well as for conservation and restoration actions aimed at safeguarding both Indo-Pacific avian diversity and the integrity of atoll ecosystems.