The common tern, Sterna hirundo, fast moving and almost totally white (apart from the black skull cap). Very difficult to photograph on a dull day. According to RSPB web site, migratory species, breeds on the coast where there are shingle beaches and rocky islands or on rivers with shingle bars, also on inland gravel pits and reservoirsrivers and over freshwater. Migrating birds can be seen offshore in autumn. This one of a pair snapped at Bottisham Lock, Waterbeach in South Cambridgeshire, on the river Cam, a few miles north of the city of Cambridge itself.
I need to get a second shot of a tern, of course…one good tern deserves another, after all.