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BirdLife’s Taxonomic Approach
BirdLife International, as the IUCN Red List Authority for birds, carries out Red List Assessments at the species level for birds globally. The taxonomy used needs to be globally consistent and as accurate as possible. To satisfy this need, BirdLife uses the taxonomy published in the two volumes of the HBW and BirdLife International Illustrated Checklist of the Birds of the World (del Hoyo and Collar 2014, 2016), the Illustrated Checklist, as a new baseline in 2016. The Illustrated Checklist made extensive use of systematic criteria by which species rank can be consistently assessed where necessary (e.g. for newly-described species or proposed splits). These criteria (Tobias et al. 2010) involve weighting morphological and acoustic differences as compared with the nearest believed relative, and are particularly intended to help make decisions involving allopatric taxa (as opposed to those in sympatric, parapatric or hybrid zone situations, where the situation is generally clearer).
Further details on the basis of the Illustrated Checklist, the application of these criteria and the incorporation of molecular data are given in the Introductions to the two published volumes;
Updates and taxonomic revisions
Since the publication of the Illustrated Checklist, the BirdLife Taxonomic Working Group has published annual updates as the Handbook of the World and BirdLife International Digital Checklist of the Birds of the World.