I feel your pain -- been there more than once. Also on a mooring.
They especially seem to like perching on the mainsail cover, where they tear their fish apart while eating, strewing scales, guts, eyeballs, bones, etc all over the deck.
Over the years, we've found that the key is to make your boat slightly less convenient than your neighbor's as a roosting/feeding spot.
In your case, I would string a line over the boom, from the mast back to the topping lift, maybe a foot or so above the boom. This makes it very difficult for them to land on/takeoff from the boom, so they don't perch there. Same with your halyards -- set them up so as to interfere with the "airspace" around your boat as much as possible, which makes it difficult for them to fly up to any part of your boat.
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT