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Message: <blockquote data-quote="Mysticete" data-source="post: 1644543" data-attributes="member: 67784">I actually really don't understand the category system that BOU uses.  The ABA pretty much has only three categories, Checklist, Origin Hypothetical, and Extirpated exotics.  It seems that if you want to create a checklist of countable mammals, than just place an (I) by the name to recognize if it is non native or not.  And I would probably only count as (I) if the animal never occurred anywhere in the British Isles naturally.As for bats, I think that if migratory warblers can reach the UK, it isn't a stretch to imagine migratory bats as arriving without ship assistance.  Hoary Bats colonized Hawaii on their own wing power, so my guess is it is a natural vagrant.  Little Brown is more problematic, as that species (and I believe Silver-haired) hibernate, so are less likely to undergo long distance movement.  I guess ship assistance at any rate is not an issue for British Birders (ABA does not count ship assists).As for Dead stranded marine mammals, I don't see why they shouldn't be on the list.  Marine mammals are very difficult to census, so odds are Fraser's dolphin does occasionally reach the area, and is simply unreported.  Of course, seeing one, now that would be the challenge!</blockquote>