I have a copy of the latest electronic birdlist from Sibley. In that he
has resolved many of the cases that were marked as uncertain in Sibley
and Monroe: in most cases, the two forms are split, but in a minority
they are lumped. I have incorporated all these changes into my program.
But there are still quite a few uncertain cases.
What I object to is that half the time, a taxon Y is listed as a species,
with a footnote 'This may be a subspecies of X', and the other half the
time a taxon Y is listed as a subpecies of X, with a note 'Y may be a
good species'. It is totally arbitrary which method is chosen. For that
reason, I prefer to be totally consistent and put all such forms in the
species list, but using the Genus(?X)Y notation. I think it is better
until the matter is resolved to alert birders that a form may be a good
species, so they should look for it rather than ignoring it, as they are
likely to do if it is listed as a subspecies.
Back to Brown Skua(c.lonnbergi). I have'nt got the electronic Sibley
with me at the moment, but since I list it as a full species in my
birdlist, I assume that the electronic Sibley Birds of the World treats
it as a full species. I'll confirm this on Monday.