Birds are infamous for getting used to any moving or dangling stuff on the life-lines and generally will return...again and again. Try this, it has worked for over three years for me as I went to war with a huge number of swallows. I used plastic clothes pins clipped to the life lines and down the length of the boom. I thread a heavy monofilament line (i.e. 50 lb. fishing line) through the holes at the tip of the clothes pins and attached a small carabiner clip at each end of the line to attach to the boat. Once you have the appropriate length (It can stretch a bit to fit tightly) it only takes a very few minutes to put the lines on at the end of the sailing trip / or take them off before you sail. I used an old fashioned technique of rolling up the line and attached clothes pins on a rolled up magazine (like you would do a string of Christmas lights). The birds hate to run into the line and learn it is best and easiest to rest and poop on your neighbor's boat. A small investment of time compared to the scrubbing of the bird droppings. Good luck.
