All species which regularly occur in the Holarctic region, including established introduced (or reintroduced) species, have been included. A considerable number of additional marginally occurring species are included cf Voous (1977), reflecting the availability of more recent distributional data, particularly in Mexico and Himalaya. In the case of Himalaya, Beaman (1994) has already considerably refined Voous's list (both adding and removing species), but later distributional information has allowed this process to be continued.
The following have been excluded:
There is considerable inconsistency in the definition of established introduced and reintroduced taxa by different local recording authorities (even where such exist), and few reports of introduced taxa in many areas due to understandable birder indifference. Nevertheless, the following are included in the list:
An inclusive approach has generally been followed.
It is intended that the list should be consistent in coverage. This is most easily achieved by including only those taxa which can be expected to regularly occur within the Holarctic region (or are largely restricted to the region), thus excluding extreme vagrants to the region. Wide variation in the depth of observer coverage across the region results in great disparity in the level of knowledge of vagrants. The inclusion of extreme vagrants in checklists can easily give an unbalanced and distorted picture of the regional avifauna.
Vagrancy from the tropics to the WP is both well-studied and anyway very limited (partly because most tropical species do not undertake large movements, and partly because the principal southern boundary of the WP, the Sahara, is a large physical barrier with few species present). It would therefore have been possible to include a relatively complete list of tropical vagrants to the WP.
However in both the Nearctic and the Eastern Palaearctic the situation is quite the opposite; on the southern boundaries there is comparatively little observer coverage, the boundaries pass through species-rich zones, the boundaries themselves are rather arbitrary, and there are few obstacles to local movement. Therefore, actual records of vagrants in these regions can only give an incomplete picture; there are many hypothetical species which could be expected to occasionally wander across the notional boundaries.
In the case of species on the ABA Checklist, those categorised as Code-5 (Accidental) or Code-4 (Casual) are considered to be extreme vagrants to the ABA Checklist area for the purposes of this list.