S20004C-Man!

I'm sure many of you have gotten them when you park the S out in the open. Those yellow spots that come off of the paint fine with a good wash or a damp cloth. BUT, I had one get on the rear plastic window and it left a "burn" mark. A dark brown spot that looks like it got burned.

First, does anyone know where these spots come from? Birds? If so, what is it? Bird crap? Bird piss?

Second, does anyone know how I can remove that dark brown spot that was left behind? I've used the usual cleaning supplies for a rear plastic window and it's still there. Thankfully there's only 1 and it's relatively small. But, I'm afraid of it happening again in a more noticeable area.

Thanks in advance for any help...