In the Checklist of Birds of Ireland (1998) the IRBC referred to the prospect of the Association of European Rarities Committees (AERC) Taxonomic Subcommittee playing an important role in establishing a European consensus on taxonomic matters.
In December 2003 the AERC published a very thorough 'progress report' dealing with virtually all of the major taxonomic questions being considered at this time. On the basis that four of the five European taxonomic committees recommend the splitting of 'Herring Gull 'into six species (Larus argentatus, L. smithsonianus, L. vega, L. cachinnans, L. michahellis and L. armenicus) the IRBC has decided to adopt these splits in so far as they concern taxa recorded in Ireland. Consequently, Yellow-legged Gull Larus michahellis, Caspian Gull Larus cachinnans and American Herring Gull Larus smithsonianus are now elevated to the rank of full species and are added to Category A of the Irish List.
Irish Rare Birds Committee 1998. Checklist of Birds of Ireland. BirdWatch Ireland, Dublin.