Honestly, the best approach is to soak, soften, and gently lift the mess rather than scrub it off dry. Bird droppings are acidic and gritty, so if you attack them with force or harsh tools, you can easily scratch painted surfaces, dull a powder-coated finish, or grind debris into wood.
Start with lukewarm water and a few drops of mild dish soap or a pH-neutral cleaner. Soak a microfiber cloth or paper towel in the solution, lay it over the droppings, and let it sit for 5 to 10 minutes. That rehydrates the mess and breaks its bond with the bench finish. After soaking, wipe in one direction with the cloth. If a stubborn patch remains, use a plastic scraper or an old credit card held at a low angle. Never use a metal blade, steel wool, or anything abrasive.
For dried-on white stains, mix one part white vinegar with four parts water. Dampen a cloth with the mixture, place it on the stain for a few minutes, then wipe gently. Test this on a hidden area first, especially on wood or older paint. Once the droppings are gone, rinse the area with clean water and dry it with a towel so moisture does not sit in joints, bolts, or unpainted edges.
Avoid bleach, ammonia, strong bathroom or kitchen sprays, stiff brushes, and pressure washers at close range. These can strip paint, cloud clear coats, force water into wood, or etch the surface.
If the bench is powder-coated steel or painted metal, you can apply a thin coat of automotive paste wax or a polymer spray protectant after cleaning. That makes the next cleanup easier and adds a little extra protection against sun and moisture. For wooden slats, use a breathable wood protectant rather than a heavy film.
If the droppings are fresh, plain water and a microfiber cloth are often enough. If they are old and baked on, be patient and repeat the soak-and-wipe cycle. The key is to dissolve and lift the mess away, not grind it into the finish.