Domestic cats that regularly go outside have rather large territories they’ll patrol. The transition from housecat to stray to feral comes with pretty distinct physiological changes too, cats are only ‘cute’ when theyre inside and figuratively defanged, killer instincts relegated to attacking hair bands and odd socks.

The story might be different elsewhere, but aus straight up doesn’t have a lot of predators, so cats can readily escape and find themselves comfortably at the top of the food chain, to the horror of our marsupial brethren

Are cats killing that many birds?

This article says they’re “likely the single greatest source of anthropogenic mortality for US birds and mammals” (which I personally doubt, I’d say modern agricultural practices is much higher, but still, it shows it’s significant). Yet at the same time the same article also says the majority comes from un-owned cats, not pets.

the neighbor put little bells on all the cats so they can’t sneak up on shit anymore

It works, but only decreases about 41% of killings (the little bastards learn to hunt by silencing the bell, lowering their jaw to stop it tingling).