Grackles. What a joy. We didn't have much trouble the first couple of years in town. Then word got out that we had a water source and we've been bombarded with poop packages. It is an instinct. They intentionally clean their nests and dispose of the packages into or next to a water source. Mostly it doesn't affect chemistry other than the need to shock more frequently when they are nesting. Keep your eyes open though, because if they have babies die in the nest, they will clean that too and deposit it into the pool. That definitely gets me to shock the water.

Last year we had three nestings because of a long spring. sigh...