Seagulls, like all birds, will crap on ANYTHING. But heres the main story. When I was stationed at Shemya AFB in the Aleutian Islands, seagulls would go down to the beach and pick up Sea Urchens. They would then fly over the end of the runway nearest the beach and drop them onto the concrete thereby breaking them up and then would congregate in the 100's and eat the juicy insides. The shells (the birds mostly) would be so thick that at times when an aircraft would take off or land it would Fod out engines.