Bird-urine is white, their poo is brown/dark.

Washing-soda or baking-soda ( & if kids are going to be playing on it, or someone’s going to be picnicking on it ) in a weak HE-laundry-detergent solution.

( urine is acidic: the alkali in the washing-soda or baking-soda neutralizes it )

scrub/soak

then rinse.

hth.