I have this sickening problem for many moons before I found a method to chase those away. I've been looking for the chemical and hardware repellents at every chances I saw a DIY shop and hardware shops. To my surprise it wasn't really easy to find bird repellent at all.
Damn, the birds still stop and drop at my windows. I still continue to wash up once every couple of weeks, it depends on their like.
And then I thought of this......CATS. Bird scare of cats, yeah right. So I gave them this to scare the shit out of them.
Just paste them at your windows of problem, you'll have peace of mind.
Its not that I do not like birds, just that I do not like their dropping, it may one day toxicate you.