Everyone poos, including birds, who tend to poo wherever the hell they happen to be, like, for example, sitting on the edge of your carport directly over your car. Or on the overhang over your porch. There’s really not much you can do to stop them from dumping where they’re comfortable, but you can keep them from getting comfortable in places that you’ll have to clean. And you can do it humanely, too — with bird spikes.
Yeah, I know. They don’t sound very humane, calling to mind visions of bad head-on-a-spike horror movies and medieval torture devices. But bird spikes don’t affect birds at all. The spikes just make normal landing places look like lousy landing places, so birds choose somewhere else to hang out. And poo.
Bird spikes generally come in strips sold by the foot, usually arranged in V-shaped patterns to maximize the area they cover. Expect to pay around $1-3/foot, plus a little more for glue or screws to attach ’em. I’ve listed a couple of sources below, but just Google “bird spike” to find lots of others.
(Thanks, josh.ev9, for the great CC-licensed photo.)