All splits adopted by any of the following have been implemented:
A AOU: American Ornithologists' Union
B/b BirdLife International
C Clements (2007 + updates)
D DBA: Dutch Birding Association
E AERC: Association of European Records and Rarities Committees
F ABC: African Bird Club
G BOU: British Ornithologists' Union
H Howard & Moore [Dickinson (2003 + corrigenda)]
I/i IOC: International Ornithological Congress [Gill, Wright & Donsker]
M/m OSME: Ornithological Society of the Middle East, Caucasus and Central Asia
O/o OBC: Oriental Bird Club
S COS: China Ornithological Society
W HBW: Handbook of the Birds of the World
Taking Voous (1977) as a baseline, all subsequent splits are annotated in the 'Split by' column using the above alphabetic codes. In a small number of instances where lumping has been widely adopted cf Voous (eg the Phylloscopus trochiloides Greenish Warbler complex), the authorities retaining a split are indicated. (Hover the pointer over the individual 'Split by' cell to reveal the taxon from which the split occurred.)
Hopefully the selection of authorities referenced (ranging from worldwide to regional, and conservative to progressive) provides a good illustration of the state of recognition of each split. [The choice of European authorities should not be taken to suggest that I do not equally value the work of CAF, STC etc - I believed that a useful cross-section of recent taxonomic views would be provided by including the positions of one rather conservative national committee (BOURC), one rather progressive national committee (DBA/CSNA), together with the closest thing to a consolidated European view (AERC).]
In the cases of BirdLife International, IOC and OSME, lower case 'b/i/m' indicates a potential split that has been included in the checklist concerned, but where the taxonomic treatment is still under review; in the case of OBC, lower case 'o' indicates a split that has been noted and/or described (but not necessarily adopted) by OBC, since the OBC Checklist was last updated (2001), but has not yet been reviewed by BirdLife International.
Where an elevated form occurs within the Holarctic region, the elevated form is annotated. But where the elevated form is extralimital (or extinct), the parent (nominate) form is annotated - in such cases, the 'Split by' code and the explanatory comment are placed within square brackets. Note that splits wherein only the parent form occurs within the region, and the English name is unchanged by the split, have been ignored, except where a formerly-polytypic parent species is rendered monotypic as a result of the split.
Implementation of splits does not necessarily imply that I endorse them. They are included for information only - it is for individual birders to decide which taxonomy to follow. [It is always important to consider which relevant authorities do not recognise a split.]
For other (both historical and potential) splits, each form that occurs within the region is identified by the trinomial scientific name, with the specific name in parentheses. Note that any historical (ie subsequently lumped) and potential future splits that I have noted have been included. These represent a complete spectrum of circumstances, from clearly invalid (eg Chen (caerulescens) 'hyperborea' white-morph Snow Goose), to widely recognised but not yet adopted by at least one of the above authorities.
The intention is both to highlight possible future splits, and to provide as complete a list as possible of those interesting forms which any keen birder will wish to record (irrespective of the validity of the split).
