I'd be curious to know what others have found works or doesn't work.

I mounted bird spikes on a 1x4 and lay it on the main sail cover, attached with a bungee -- that works.

I also find that reflective mylar streamers hung from the shrouds also work, so long as there is a wind.

Monofilament strung taught between the mast and stays, about 2 or 3" above the spreaders also works.

Fake predators don't work, unless they move now and then.