There are several iconic species on the British list with just one or two records to their names, usually doused in legend, mystique and folklore. And while there hasn't been a Northern Hawk Owl or Egyptian Nightjar in a few decades, they and other extreme megas have still occurred in many a birder's lifetime – thus the chance of a future record seems at least possible. Even Macqueen's Bustard – which hasn't reached Britain since 1962 – is remotely plausible, not least because of a Belgian record from 2003 and a Swiss bird in 2008. Tengmalm's Owl is fresh proof that the ultimate of unblockings can happen!
However, in this the year 2020, there is a small group of mystical birds that haven't been recorded in Britain for more than a century. All of them are stored in a dusty and long-unopened box, labelled Category B of the BOU's British list: for species that were recorded in an apparently natural state at least once between 1 January 1800 and 31 December 1949, but have not been recorded subsequently.