I am a little worried about your cats. You said they were outside cats and wore bells. I guess that means they also wear collars to attach the bells onto. I don't like to see outside cats wearing collars because climbing trees and wandering in the bushes they can get caught and possible die as a result.
Our neighbors cat got caught in my yard, by her collar, and almost died until I heard her crying and released her.
The law of unintended consequences is what I think it is called. There is no shortage of birds, I'd rather keep my cat safe.
We used to have birds hit our picture windows. I put post-it notes on the windows and have not had one hit since then.