This is also a feature in winter when say there can be 1, or 2 at a stretch at Musselburgh and just 1 at Seton Sands with none or very few elsewhere in East Lothian, but at Barnbougle counts ranging from 20-70 are the norm in winter.
This is I am sure due to this species wintering habits within the Central belt of Scotland, with large numbers wintering at Strathclyde Park and elsewhere in winter, perhaps Barnbougle is on the edge of this wintering range!
However, this does not explain the fact that in early spring a large build up not seen elsewhere in Lothian occurrs at this site!
You're right it is a very curious phenomenon - but during winter I guess it's partly that east of Musselburgh there is no reason for LBB to be present, with no major artificial food sources beyond Edinburgh, whilst the Barnbougle crew must presumably have some good feeding somewhere inland. I wonder if any go to the landfill by the M8 at Polmont, in Forth - only about 10 miles west, not far for a gull commute? Never been there specifically to look for gulls but have heard of large numbers in that general area, and around Winchburgh.
The spring build-up may be a separate factor. I saw a report of 200 LBB high N over Penines last wk, mentioned on blog, so some must come directly north overland whilst others clearly pass along the coast. Maybe they congregate initially where they find others, or maybe many are en route to NW Scotland or beyond where they breed?
Good gull numbers at Seton tonight - 2k small gulls on beach, 80% BHG and nearly half with full hoods now; another c. 3.5k small gulls on sea, with c. 100 Herring Gulls and a single LBB; could not really check those on the sea properly due to fading light by the time I arrived. Suffice to say no rarities were apparent...