Keeping your cat indoors

Lately I've been thinking about outdoor cats for several reasons. In New Zealand, they have a huge problem with feral cats that prey on native wildlife. There aren't many natural predators in NZ, so cats can easily kill lots of birds that aren't used to be hunted. On continents, feral and outside cats also kill birds and small mammals. I love cats, but I think it's unnecessary for cats to hunt, especially when wildlife also have to contend with natural predators, diseases, cars, and habitat loss. This morning I was woken up at 4 am to the screams of two cats fighting in my backyard- which points out that outside cats have a much shorter lifespan than indoor cats. Even putting a bell on a cat collar doesn't solve the problem, because a study has shown that cat can learn to stalk so quietly that the bell doesn't jingle.
I've found a website that discuss the issues of feral/outdoor cats: http://www.njaudubon.org/Conservation/CatsIndoors/FAQ.html#three
One man in Texas shot a feral cat who was killing endangered shorebirds, and it brings up an interesting ethical question of killing a cat who kills birds: http://www.nytimes.com/2007/12/02/magazine/02cats-v--birds-t.html?pagewanted=1&_r=2