All splits adopted by any of the following have been implemented:
A AOU: American Ornithologists' Union
B BirdLife International
b (proposed split, under review)
C Clements Checklist
c (distinctive subspecies group)
D DBA: Dutch Birding Association
d (distinctive subspecies group, sometimes considered specifically distinct)
E AERC: Association of European Records and Rarities Committees
F ABC: African Bird Club
G BOU: British Ornithologists' Union
H Howard & Moore Checklist
I IOC: International Ornithological Union
i (proposed split, under consideration)
J OSJ: Ornithological Society of Japan
M OSME: Ornithological Society of the Middle East, Caucasus and Central Asia
m (possible split)
O OBC: Oriental Bird Club
o (proposed split, reported by OBC)
R Checklist of the Birds of the Russian Federation
r (preliminary split)
S CBR: China Bird Report
W HBW: Handbook of the Birds of the World
w (distinctive subspecies group)
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.
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).