Hissy, that was a fascinating link to the Liberator collar. I never knew there was such a thing. It gives a great explanation about why bells on collars don't work to alert birds,since the bell CAN'T ring while the cat is in the smooth flight of a split-second attack. I also like how they mention the annoyance of the bell ringing all the time during a cat's ordinary movements. It would drive me crazy to have a bell attached to my neck, ringing every single time I moved. I'm sure it irritates the heck out of cats, who have more sensitive hearing than humans.
I still think, though, an outdoor enclosure for cats is better than letting them roam free. I've seen way too many dead cats in the road and sure don't want to ever see any of mine there, in that state.