How to protect cars from bird and squirrel sh1t?

Cut the tree down simples

Get small realistic looking stuffed toy squirels and site them in the tree. And have some hanging upside down on bits of rope, with red paint for effect. The real squirels will either see them as claimers of teritory not to do battle with, or get the hint of what may happen to them if they venture a bit too near.

Or more seriously stick a particular good likeness of a creature of prey that birds and squirrels don't get on with. A hawk perhaps?

Run stainless steel clotheslines across the drive and put a cover over them.

are fake and ignore them.

Sadly, there's a TPO

Strangely, that's exactly what I thought of last night after writing this plea. I might even be anchor one end on the front of the garage so only 2 poles would be needed plus either some tent fabric or sail cloth. The tricky bit might be making it look acceptable.

I have cheap (polypropylene?) tarpaulins. Some last well in the sun. You'd be best to pull it tight so it doesn't flap in the wind and annoy the neighbours.

Pick and pickle all the walnuts. No food for critters they'll go elsewhere (in more ways than one).

Another good suggestion, but the tree has been around for a few hundred years and is rather large - picking them would require a sky hook.

After some further thought I can do this without any poles. If I run a s/s wire between 2 trees (parallel to the front of the garage) I can then run 2 wires back from this first wire to the front of the garage. The cover would be stretched between the 2 wires.

Question: how can I attach the wires to the trees in a semi-permanent without damaging the trees? Some sort of strap?

Vine eyes.

One tree has a TPO on it so I think a strap of some form would be preferable.

Or a long stick but you normally pickle green walnuts so they might not knock off that easily.

Get a high powered air rifle, dual job of getting rid of vermin and keeping car clean, with added bonus of fun.

Then when you get really pleased about shooting things with fur & feathers you can get one of the BFSA stickers for the back of your car to let everyone know what you do for fun.

collect all the s*it and process it to make saltpetre and blow up some terrorist?