I know it's not nice for cats to kill birds and mice etc, mine don't as they are indoor and enclosure only, but the cats outside that I feed do hunt, howver it's dangerous what with all the trees and snagging bushes and bastards who would use the collars in a bad way. I'd rather keep them well fed and indoors at dawn and dusk (when they kill most birds) as those bells sound much much louder to a cat and can you imagine having a dingly bell on you every time you moved!!! (only much much louder! )
It would drive me insane