Nothing you can do, puss - I won't ever put a collar on a cat again, and while a bell might warn birds, it makes no difference to creeping things. It's that time of year, too - No.1 Son brought in a mouse no bigger than my thumb last week, Rover had a mouse the four times as big. I found a pigeon's wing on the bedroom floor - no other parts of the bird, just one wing. It's their nature, and don't forget these things are treats for you, not done for no reason :-)