Much as I love them, outdoor cats can be a major threat to wildlife. Extreme examples are places like Hawaii, where feral cats have devastated the native bird population. In most areas cats compete with native predators and can do significant damage to native bird and animal species. Outdoor domestic cats usually have humans feeding them, and thus are not controlled by the population pressures that keep wild predators in check. Contrary to what many people think, bells don't work.
Cats are smart animals and even a mediocre hunter can easily learn to stalk silently with a bell on.
The bell will ring for their final charge, but by then it's too late. Several organizations are supporting campaigns to keep cats indoors and control the feral cat population, including the Humane Society, American Bird Conservancy and Seattle Audubon Society. However, I have to admit I'm troubled by keeping my cats inside, since I know they want to go outside. It seems wrong to keep an animal locked up like that. I would be saddened if all the friendly neighborhood cats were to disappear, but I am also saddened by the loss of native birds and animals.