Citizen science rapidly delivers extensive distribution data for birds in a key tropical biodiversity area

Abstract Citizen science projects remain rare in biodiverse yet data-poor countries, contributing to a shortfall in data for biodiversity monitoring and promoting public stewardship of nature. We document and analyse BigMonth2020, a month-long birdwatching event across Java and Bali, publicised through social media and incentivised with grants and competitions. Over 20,000 lists containing 100,000 bird records were submitted to the ‘Burungnesia’ phone application. Spatial coverage extended to 71% of the islands’ 3408 atlas grid squares (6.9 × 6.9 km), including 1613 previously undocumented squares, with 353 bird species recorded, representing 74% of Java and Bali’s avifauna excluding vagrants; 27 threatened species were recorded, with new records for 204 grid squares. Almost 25% of contributors were female, 72% were under 30 years old, and most were graduates and members of birdwatching clubs. The project cost less than US$10,000 to run, and serves as a model for rapidly establishing a distributional baseline for monitoring biodiversity trajectories in the tropics.