What's the most difficult bird to see on a particular list? We have dramatically improved the performance of the "blockers" facility in BUBO Listing so that is now possible to see, in a matter of seconds, those species that have been seen by the fewest listers.
For example, there are currently five species on British lists that have only been recorded by single observers. Interestingly these are all American vagrants and include such megas as American Kestrel, Yellow-bellied Sapsucker and Magnolia Warbler.
To view blockers for any list, just go to View All Lists as normal but ensure that you check 'Blockers' rather than 'Rankings'. Clicking on a blocker species will show you who has recorded it and where.