I know this problem intimately.
Bought our house 4 years ago, there is a spruce hedge that runs down both sides of the property, in summer 2 I trimmed back the tops so it stays low and I actually pulled out 21 NESTS!!!
It's true that the adult birds carry the droppings out of the nest and I swear the pool acts as a target, I've found that a solar cover then acts like a bird bath.
So:
- it's true that they will move out when the baby birds learn to fly, keep the solar cover off during the day so they won't land on it
- scoop out droppings from pool floor, I can confirm that I do this everyday and my FC/CC levels are still good
- clean the solar cover with a hose regularly
- try to get rid of the nests if at all possible and break the reproduction cycle, some people may not like that but grackles are aggressive birds, they move into other birds nests, they will kill other species of chicks and they will swarm/kill other birds.
My trees are about 18' off the ground and they nest relatively high, they build their nests where a branch meets the stem and they build them down from that point, so they are below the limb. I have a modified pole about 15' with a 2-3" lag bolt thru one end, I get on a ladder and use it to pull the nests down and they are tricky to get at. I will admit their nests are pretty impressive, watch their behaviour and you'll tell where they are nesting, if they swarm you your getting close.
One thing about the BB/pellet gun solution (which really is the best solution), I believe they are part of the Migratory Bird Act which (unfortunately and irresponsibly) protects them in Canada and the US, so whatever you do make sure you keep it to yourself
