Anyone seeking online information on the classification of birds will be confronted with the fact that several global checklists are available:
& TiF (link)
The primary focus of the first four checklists is on species and subspecies recognition, but the numbers of recognised species differ significantly (Neate-Clegg et al., 2021). Pairwise comparisons between selectable checklists are available on the Avibase website (link).
Hierarchical ranks above species-level are largely identical in the typical checklists, but Howard & Moore waived "orders", and IOC waived both "subfamilies" and "tribes". No checklist recognises "superfamilies".
To overcome the current polyphony, proposals to work towards a single world birdlist were debated in 2018 at the International Ornithology Congress in Vancouver. As a result, the International Ornithologist's Union (IOU) established the Working Group Avian Checklists (WGAC), chaired by Leslie Christidis from Australia. (link)
All presently available checklists share the same shortcomings, i.e. lack of defining:
Neate-Clegg MHC, Blount JD, and Sekercioglu ÇH (2021), Ecological and biogeographical predictors of taxonomic discord across the world‘s birds, Glob. Ecol. Biogeogr. 30(6), 1258-1270. (abstract)