We may need to watch where we put our feet on this
First, some published regional lists define their terms.
This is a good idea, and should reduce confusion, even if the reader is not
happy with the given definitions.
Secondly, I would agree with Mark that 'vagrant' should not be
a synonym for 'rare' or 'very rare'. The reason is that most people have
some kind of shared understanding of what is meant by 'rare' or 'very rare' -
although, again, a given definition would aid precision.
On the other hand, we lack a short term for 'Has been recorded
in the relevant area, but was (well?) outside its normal
Here we lose sight of our feet in murky waters.
What is a 'normal range', or even a 'range'? Many descriptions of range
merely indicate where a bird usually occurs, even if it has been regularly if
very infrequently recorded elsewhere, perhaps as a result of unusual (long-term
cyclical?) climatic conditions. And just when does a 'former range' become
'former', or for that matter, a first record in a new place create ipso facto a
I fear we may be whistling in the wind seeking consensus
on what such words connote or denote.
To avoid the need for it, better to follow the approach in
'Rare Birds of Britain and Europe', thus:
A = 1-10 individuals recorded (ie in the area of
B = 11-50
C = 51-100
D = over 100
It is only necessary to designate as such birds
transported by human agency, if this is known.