alt Hacker NewsI believe the Loss et al. 2013a numbers from [2] come from [1] Scott R. Loss (2013).
And, sure, you can look up some other studies [6] that will make you question the accuracy of the numbers but, even if you decrease the estimate by 70%, cats are still killing a lot of birds. Instead of #1 on the [2] list maybe they're #2 (behind buildings).
It's very easy to give a cursory search and see overall North American bird populations are decreasing. Heck, even flying insect biomass is significantly down.
We don't care about people killing flies in their house. We do care about flies dying on a mass scale. Flies are important pollinators! Ecosystems need them.
A few cats killing a few birds is no big deal. Millions of cats killing hundreds of millions of birds, in an ecosystem that shouldn't have cats, is a big deal.
If we armed every American human with flyswatters and sent them outside every day with orders to kill every flying insect they saw then it would probably be very bad (though I think this imagery is also hilarious).
I don't really want to get into my full opinion on the ethics and morality of pet ownership. Stealing other people's pets is wrong. I think if one lives in North America and feels their cat needs outdoor time then it should be supervised on their own property or train it and walk it with a harness and leash. Catios are neat too.