Today, I saw a pair of Herring Gulls standing very close together on wet grassland in the Valley of Rocks, Lynton. One would drum its feet fast on the ground for about 20 seconds, then stop and both would feed - presumably from worms or small prey emerging from the ground.
I watched for 10 minutes, and this behaviour continued on the same patch of ground. Presumably they were simulating heavy rain.
It is called foot trembling and is used most frequently by small plovers such as lapwing, often after a little run forward. It, presumably, startles small creatures to the surface. (You think the worms would have learned by now)