Birds use gravel to help digest their food so grit and sand are some of the major components, which can cause scratching. Insect honeydew (sometimes mistaken for tree resin) excreted by aphids or scale insects that infest the leaves and branches is also acidic. Residue should be removed (see below) as soon as possible as it could compromise the clear coat.
Take your time and be careful with its removal, these acids should be removed and/or neutralized as quickly as possible. Using an IPA (isopropyl alcohol) no ammonia based (ammonia acts as an acid accelerant) glass cleaner. Or use a gel alcohol hand cleaner (Purell®) place a small amount on to a first aid gauze pad and apply to effected area this will help it stay in place during the necessary dwell time. The alcohol content acts as a drying agent, which will extract the acid, then thoroughly flush with water to rinse away any grit or sand, etc or use detailers clay to encapsulate it, re-spray the area let it dwell for 2-3 minutes and wipe dry.
Avoid using a QD spray to flush residue, as they contain either carnauba wax or polymer sealants, which will seal the acid and not allow it to dissipate. Using an alkaline rinse of baking soda and water (to neutralize the acid) as soon as is practical is highly recommended, if the affected paintwork is not neutralized any remaining acid residue will be reactivated.
If the paint or glass surface has been etched the surface can be levelled with a suitable polish (Zaino PC Fusion Dual Action Paint Cleanser and then apply a paint protection).