Pigeons

Pigeons shit like other animals breathe. The second a pigeon lands on your balcony, there is already a mark.

Not after.
During.
Possibly before.

One morning, after a summit of feathered rats on your balcony, the balcony is gone. Everything covered in layers of shit. A geological record of every feathery bastard who ever visited.

Pigeons shit.
Humans think.
That's why humans invented balconies.