I don't think so, I've never seen large numbers of birds in my yard

It happened between noon and 2:00 pm

When I was in the USAF we tried those and many other things
 

with pretty limited success. It seems that the birds always get used to what ever you try. At first they are scared away but eventually they figure out that it doesn't actually hurt them. Of course anything that makes lots of noise is out of the question anyway (the neighbors are pretty tough here). I'm just wondering if maybe the stuff on my car was something other than bird crap. I noticed that the half of my car that wasn't under a tree had nothing on it so it seems that it came from the tree but nothing is ever in that ree when I look at it.

I'd still guess birds. They like to sit & s_it in trees.
 

The cannon might work mounted on a car since it would move around a lot. Except, of course, for the neighbors issue. I found the pointer to this one from an article about a trailer park that moved in next to a farmer & were accusing him of using the cannons because he didn't like the trailer park. The courts, as usual, will settle it.

- Tom