Hi Steve I had this problem since I got my mooring. Tried plastic bags, CDs etc ( the owls are a joke - only today a big gull sat next to one here....quite funny the gull was bigger than the owl)
I have however put this on mine this year;
one continious length from bow round the A frame to 'spread' it sideways & back to the bow. I have tied a loop in each end and use cheap plastic clips at the bow. I put it back on the spool when I go out to stop it tangling.
So far its ace ! Pull it quite tight and only in nil wind does it not twang away.
The birds do seem to hate it !
( I have had some mullett eaten on the deck
but no crapping worth noticing ! )
Cheap as you can get too !