You don't know this, but instead assuming parent lives in some suburban area with lots of other cats. They could be living outside in the woods, 5km to any close settlement, with minimal side-effects of having a cat outside (besides the side-effect of having a few less rodents around).
But no, lets have a knee-jerk reaction to anyone who has an outside cat, without understanding any of the context.
Besides, many people put bells on their cats, and then they're unlikely to catch anything at all in the wilderness.
It seems that in the modern era of social media campaigns for everything there can hardly exist a perfectly chill, normal activity that someone hasn't somehow contrived into a type of moral and ethical sin. It's tedious, sad and ultimately, kneejerk stupid. I strongly doubt that the world's domesticated cat population is generally creating an ecological apocalypse and the studies I have seen around it are far from anything you could call solid. Either way, believe it or not, you can actually also use very practical solutions like bell collars to easily fix most of these situations. How about getting off your moral pedestal about such a silly "issue".
Maybe I've just had stupid cats, but they never managed to catch any birds, even when they were without bells. Plenty of mouse offerings though, but seems the bells help with that too.
Yeah, my opinion is a bit more pragmatic and attached to reality, where context, environment and your actions matter, not some "empirical" study done by universities.
Personally, I love my cats so I let them roam outside instead of keeping them inside like a prison. Then I also care about other animals so naturally they have a bell so they cannot (successfully) hunt other animals. But again, pragmatic approaches aren't for everyone, some people love books and/or data instead :)
The alternative to empiricism (science) is rationalism (wish-casting), not pragmatism (least harm).
I often let my dog off-leash. Weighing the risks & rewards, I pragmatically choose to break the law, knowing full well that I'm in the wrong, not some special case. I eat the tickets and social scorn without complaint. My dog has pretty good recall and is super gentle (esp w/ kids). But the big bad govt (and other parents) didn't write the laws with my special pooch in mind.
You're confident your cat doesn't harm birds. Terrific. It's still wrong, in the general case. So take your lumps.
A (huge) point in your favor is that 2/3rd of (domesticated) cats are feral. So keeping cats indoors in order to better protect birds seems quixotic.
In these parts, owners keep their cats indoors to protect them. Recently, my SO's cat escaped her "catio" and was swiftly caught by a coyote. (A neighbor saw it happen. Horrifying.) Maybe your locale doesn't have coyotes.
Edit: Another exception (that I can think of) is farm/barn cats. Pretty much a necessity. Alas, coyotes. And probably hawks.
Could be worth considering that outdoor cats in the US may actually be a positive because so many of our country's natural predators of rodents and birds have been wiped out.
