Well, there goes another first for Britain that the rest of us can no longer dream about finding. On Friday 7th December 2007, a juvenile Great Blue Heron was discovered at Lower Moors on St Mary's, Scilly, where it stayed for the rest of the day. However, this may turn out to be a major blocker in future as by the following morning it was gone, denying it a place on many keen listers' lists, and denying the Isles of Scilly travel operators a large amount of unseasonal revenue! It seems reasonably likely that it may yet get relocated, however, lurking elsewhere in Scilly or west Cornwall.
Great Blue Heron has long been anticipated as an addition to the British list, with a number of records from the Azores but also one as close as Ouessant, north-west France, in 1996. Moreover, a number have been documented crossing the Atlantic on board ships. For an excellent summary of the identification of Great Blue Heron in a British context, see Birding World 11(1).
Given that it was well-watched by multiple observers and that perfectly convincing pictures of the bird have been posted online (e.g. see Birdguides) Great Blue Heron has been added as a provisional new species for BUBO lists using the BOU, UK400, Birdwatch and 'Official British & Irish' authorities. In the event of it being rejected in future, however, it would then be removed from all such lists.