Dancing Gulls

I went for a long walk the other day and passed next to some grassy sport grounds. Several Herring Gulls and a Woodpigeon were on it. The Woodpigeon and some of the gulls were lazily dozing on the ground, enjoying the early morning sun. Two of the gulls were doing something more interesting. They were patting the ground moving their feet rapidly, as if running on the spot. After a little while, they stopped and looked into the grass intently, only to start their tap dance-and-watch sequence all over again. Their sustained interest indicated this behaviour was related to obtaining food. Occasionally they pecked the ground, but if they caught something it was small and I coudn't see it. In any case they did not seem to catch any good size earthworms. I took some photos and a very poor quality video. Yesterday I saw the same thing in the local park grass, but this time it was a Common Gull. I got a few answers in The Last Word, the Question and Answer section in the New Scientist. I also found some better quality than mine:

Here, the gull is seen actually catching a couple of worms. It is quite an intriguing behaviour reminiscent of the facts behind the World Worm Charming Championships, which take place in the village of Willaston (Cheshire, U.K.) since 1980. Participants must encourage worms to the surface of the ground by vibration means only (no digging allowed!). The current record is impressive: 567 worms in 30 min from a 3 sqm plot. I was also wondering if the gull behaviour arose when they feed in puddles in the beach and when adopting it on grass, found by chance it actually encourages worms to surface. I can only say, Blackbirds, learn!

Common Gull checking for the effect of its dance