Well, the standard defence against alien, sorry, avian attack is of course by the judicious use of netting. But judging by your description of the monster, I'm gonna assume that's not a practicable solution for you. The only other viable defence (which I use for my figs) is this:
Yes, it's time-consuming, and occasionally slightly fiddly too, but if ya wanna save any of those luscious plums for your table, it's (almost) the only way to go. Unless you feel comfortable using a ladder, you can sacrifice the plums that are out of reach to predation. If you go down that route next year, make sure you 'bag' those plums before they show any signs of ripening. As you harvest the ripe plums, the bags can be reused on those plums that are next in ripening sequence.
No need to tie any knots either, tho a single knot around the branch will stop the fruit falling to the ground, and is easily undone. If there is no danger of fruit fall, simply pulling the drawstrings over the stalk area tightens the bag to provide a protective shield, and is effective against all pests except for 'squerls'. They will eat straight thru the bags to get to the fruit, no prob. But if you are ever troubled by squerls, let me know. I have a special deterrent for them that works a treat.
Just look at that will ya - gardening and theological advice all rolled into one package. Is there no end to my talents I ask myself?