Why do seagulls hit the ground with their feet?

In several occasions I saw some seagulls standing still and pretend as if walking that is they hit the ground with their feet. It always happens on the soil and not for example on the sidewalk. I am wondering why they do this. Sometimes, from a distance, I see they pick something up from the ground and eat it. Do they do this to simulate the sound of raining and encourage the earth worms to come to the surface?!