Anyone seeking online information on the classification of birds will be confronted with the fact that several global checklists are available:
The primary focus of at least the first four checklists is on species and subspecies recognition, but the numbers of recognised species differ significantly among the lists (Neate-Clegg et al., 2021). Pairwise comparisons between selectable checklists are available on the Avibase website (link).
Categorial ranks above species-level are largely identical in the checklists, but Howard & Moore waived "orders", while IOC waived "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)
It should be noted, however, that all presently available checklists share the same shortcoming, i.e. the lack of temporal thresholds for defining categorial ranks, including species. Under these conditions, it is highly unlikely that a common global bird checklist will ever be agreed upon.
Collar NJ (2018), Taxonomy as tyranny, Ibis 160, 481-484. (pdf)
Garnett ST, Christidis L, Conix S, Costello MJ, Zachos FE, Bánki OS, Bao Y, Barik SK, Buckeridge JS, Hobern D, Lien A, Montgomery N, Nikolaeva S, Pyle RL, Thomson SA, van Dijk PP, Whalen A, Zhang ZQ, and Thiele KR (2020), Principles for creating a single authoritative list of the world’s species, PLOS Biol. 18, e:3000736. (pdf)
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), 1258-70. (abstract)