Keep birds from nesting in my grotto?

I have a swimming pool with a waterfall over a grotto. Several birds have made nests in the grotto, so the grotto is unusable and we get bird waste in the pool. Once the birds have moved on, I want to make sure they don't come back and nest next year. Any suggestions? Would one of those plastic owls nearby scare them off, or do I have to put up some sort of barrier?