If you have a sailboat I assume their roosting on your spreaders ? ( the Cormorants do that up here in NE) We connect stainless rigging wire from o/s shroud into the mast, about 3-4 inches above the spreader. This prevents them from landing and roosting. If you search the internet there is a co. in Chicago that sells a metal strip with sharp points that you attach to the spreaders to prevent landings. We also hoist up a fake owl up the mast - it helps.