I was in on Whibbey Island last week and noted something I have seen before, but it took awhile to dawn on me as my attention was focused on work related observations. The large vacant paved area was littered with sea shells. Gulls are resourceful creatures that have adopted well to the tools we have given them - large flat hard surfaces.
I noted that the parking area immediately joining the vacant lot I was at had no shells. Perhaps the lot is swept clean since it is in use or the gulls prefer not being interrupted by car when they drop their shells.