You could do what I've been doing for the past few years (same problem as you) that is buy a good silicone based polish I use autoglym but Mer is probably just as good and give the car a really good going over every three months It doesn't stop the birds crapping but it makes it easier to get off and stops the acids from eating into your paintwork.
I was told there is actually a polish that is made for this kind of problem but as my autoglym is half full I haven't investigated.
An alternative - you could run the barrell of a gun along the wire with a photo-electric cell that detects the bird landing then pulls the trigger, complicated, possibly illegal, but there's nothing like retribution to put a smile on you face.....although perhaps disposing of dead birds that have landed on your car might be a problem!