BIRDBASE is a global avian trait database with data on 78 traits of all bird species (11,589), genera (2398), families (254), and orders (44) based on the most recent avian taxonomies published in October 2024 (eBird/Clements v2024), January 2025 (HBW/BirdLife 9.1), March 2025 (IOC v15.1), and including all the species in the first unified checklist of the birds of the world published in June 2025 (AviList). We compiled species-level ecological data from a variety of published datasets, ornithological publications and the first author's field observations of over 9300 bird species. Data were primarily compiled from 367 ornithological sources, including regional books, field guides, focused datasets, and primary literature. Most of these data were collated from the Handbook of the Birds of the World/Handbook of the Birds of the World Alive, now Birds of the World, augmented with other sources including BirdLife International, the ornithological literature and our unpublished field observations. The dataset is provided in an Excel file, with separate worksheets for trait values, trait definitions, nest details and data sources. The dataset is available in the Figshare repository for public use.The database contains one row per species for all 11,589 species. Additional explanations for the database columns are provided in the Legend worksheet of the database Excel spreadsheet, which also contains both the database content and the source list as separate worksheets. We included 78 traits in 10 major categories: conservation, geographic distribution, morphology, elevational distribution, habitat, diet, social behavior, reproductive behavior, demography, and mobility. Traits include conservation status (2024 IUCN Red List16), latitudinal distribution, biogeographical realm, island endemism, body mass, elevational limits, primary habitat, habitat breadth, primary diet, diet breadth, ecological specialization, sociality, clutch size, nest type and placement, and movement (Figure 2).