Basically, I’m with Ace here.
I think we can all agree that you’re on the hook, ethically speaking, for your cat’s actions if you let him outdoors. If he kills a neighbor’s pet bird, or if he somehow kills a significant portion of the ecosystem’s wildlife, then that’s your fault.
. . . but how does morality attach to a cat killing a single wild bird in the first place? That’s what cats do.
To put it another way: who was wronged?
– It’s not the woodpecker. He was certianly inconvenienced, but by his very nature (and by the fact that he himself kills poor defenseless insects for sustenance) he must “accept” the possibility that one of his natural predators will kill him.
– It’s not the neighborhood. A few birds and/or rodents here and there isn’t going to affect significantly much of anything. No ecosystem of any size is that fragile. Hell, it’s just as likely that the bird population needed thinning as it is that it was too thin already.
If no one was wronged, then there’s simply no “wrong” in the situation that can be attached to Ace, his cat, or anyone else.
The question of whether you should keep your cat indoors for its own sake, though valid, is quite beside the point.