Northern Mockingbird (Mimus polyglottos). North Mole, Gibraltar. Perhaps from 15.2 until 24.2 at least (Charles Pérez & Keith Bensusan/GONHS).
News via Rare Bird Spain Facebook Page
EDIT Sunday 26th: Just got back from a couple of days away with no internet to find this bird has turned into a TROPICAL MOCKINGBIRD - a first for the western palearctic!! Pretty happy on a ship I guess! Still not seen any pictures in a brief search though.
UPDATE: PHOTOS HERE