As a cat lover this only strengthens my opinion that they should stay inside the house, humans cause enough damage to the ecosystem without needlessly unleashing pretty efficient predators for no gain.
Most cats do tend to stay inside the house, or briefly go outside before coming back in again. Some animal-protection org here once published a story about the number of birds killed by cats which worked out to each cat in the country killing something like three birds a day, including all the cats who never go outdoors or who are way to comfortable to ever chase anything more mobile than a food bowl. After that I stopped paying attention to claims about bird numbers.
Not to mention that the cats who aren't too lazy to chase things are far more likely to go for ground-based animals like rats and mice than birds. As in, the rats that would be raiding bird nests if the cats didn't keep their numbers down.