A'ha, in a way it's a kind of rain dance. the pitter patter of the birds feet on the ground sound like the pitter patter of rain drops to any worm close enough to pick up the vibrations and sends them instinctivly to the surfice to avoid possible drowning.
Blackbirds do it too.
Thanks folks-- I wasn't aware that worms were part of a seagull's diet though. Thinking about it they eat most other things, dab hands at Ice cream pinching and cleaning up after fish and chip litter! Curry sauce and mushy peas are more than interesting when splashed down your washing second hand!