Vicki deserves the credit, as she was the one who read on the Internet that fishing line might keep them away. Willing to try anything at this point, we set up several garden supports on either side of the pool and then strung lines between them, crisscrossing the entire area.
It's probably hard to see in the photo but if you can click on it to make it bigger that should bring our handiwork to light. We've received so little bird poop in the time since we put them up that we honestly thought "poopy bird season" might not have even started yet. However, one of our friends who lives nearby and always has the same problem confirmed for me last week that the filthy spring ritual is in full swing already. The baby grackles must've been born earlier this month and now the parents are dropping their offspring's crap near a convenient water source (so that it'll be washed away from the nest and not give away their location, a practice that evolution has instilled over countless past generations).
I'll admit to being skeptical of this solution when Vicki suggested it, but not any more! It definitely seems like a winner, with the only downside being that the pool is less accessible for swimming and skimming while the lines are up. However, I think we can live with that for a couple weeks every spring if it means not having to clean up hundreds of little piles of shit, as we've had to do for the past eight or nine years now. Yay for Vicki and the Internet!