Global gaps in citizen-science data reveal the world's “lost” birds

Biodiversity knowledge gaps, which limit scientific research and conservation planning, are especially acute for the most poorly known organisms. Citizen science offers a powerful and effective means to fill these gaps. The recent growth of citizen-science platforms has resulted in near-complete coverage of global avian diversity (~11,849 species). Because shrinking knowledge gaps increasingly reveal meaningful absences, we evaluated the potential of citizen-science data to establish “lost” bird taxa: those without documentation for more than 10 years. Collating more than 42 million photographic, audio, and video records returned 144 bird species (1.2%) as lost, the majority of which (62%) are in danger of extinction. The higher the coverage by citizen scientists and the longer the interval since their last documented record, the more likely that lost birds are to be imperiled. Our approach provides a data-driven and reproducible method to identify lost species and elucidates high-priority knowledge gaps to inform future conservation action.