We had that problem for several years on our mooring. I made up strips of fiberglass with nails protruding that I tape to the tops of the spreaders. Antennas on top of the main keep the birds off. I run a pig stick with a burgee up the mizzen. Like Tom Young I run lines a foot and a half above the booms. Those precautions keep "most", but not all of the birds off. Once they pick your boat, they will keep coming back. I suggest you buy a good scrub brush and make sure your wash down pump is working well