maybe use a little potassium carbonate to convert the uric acid in the bird shit to dipotassium urate, thus increasing its water-solubility 300×, and follow up with a strong buffer that's mildly acidic like monopotassium phosphate to prevent any stray residues from the solution from causing alkali corrosion of the glass when it dries

probably there are a lot of possibilities you haven't tried on your windshield yet