For each taxon listed, its regular occurrence (other than as a vagrant) in the WP, Eastern Palaearctic and Nearctic is indicated by ‘X’ (or ‘I’ for introduced/reintroduced taxa) in the appropriate column. Species subtotals are given at the end of the checklist for each of these regions and for the Palearctic, plus a grand total for the entire Holarctic region.
For the purposes of the distributional summary, the Western-Eastern Palaearctic boundary follows BWPC (Snow & Perrins (1998)), except that Iran, the whole of the Arabian Peninsula, and the Socotra archipelago are additionally considered to lie within the WP. Taxa restricted to the Socotra archipelago are indicated by 'S'.
[Note that all BOURC British List Category A (recorded in an apparently natural state since 1 Jan 1950) species which are not BBRC rarities are treated as regularly occurring within the WP; ABA Checklist Code-3 (Rare) [but not Code-4 (Casual) or Code-5 (Accidental)] species are treated as regularly occurring within the Nearctic.]