Do they work? And is there and danger of them actually hurting a cat?
We have a nest box, put up before we got a cat, and never used - until now! I'm very excited but also worried as our cat (and occasional random cats) like to sit in top of it. Obviously he can't reach in but it would be terrifying for the poor birds I'm sure.
Should I set up spikes in the fence around it to keep them safe?