Similar to the CO2 footprint, it's another way to individualize social issues and distract from large corporate sources of environmental destruction.
https://www.theguardian.com/environment/2022/aug/14/cats-kil...
"A study published in April estimated that UK cats kill 160 to 270 million animals annually, a quarter of them birds. The real figure is likely to be even higher, as the study used the 2011 pet cat population of 9.5 million; it is now closer to 12 million, boosted by the pandemic pet craze"
The study didn't include animals that cats killed and abandoned in the wild, so it is likely higher still.
The UK’s largest bird charity, the Royal Society for the Protection of Birds (RSPB), is not particularly concerned about the impact of cats on the British mainland. Instead it focuses on what it says is driving UK bird declines: global warming, intensive agriculture and expanding towns and cities leading to habitat and food loss. “While we know that cats do kill large numbers of birds in UK gardens, there’s no evidence this is affecting decline in the same way that these other issues are,” said a spokesperson.
But even taking the lower end of the estimate of the number killed by cats, 40 million birds each year is a felt loss. The RSPB may opine as they wish, but that's the data.
Anecdotally, my neighbour's cat is known to bring home as many as three birds on some weeks. I've seen them stalk my bird table using the bushes as cover. Data + eyewitness observation makes a compelling case.
Though 40 million dead birds sounds like a lot, in regions where wildlife had millenia to adapt to cats' existence – in continental Europe cats have existed for tenthousands of years – it’s not a significant issue.
And it's sad that so much time and energy is wasted on this discussion when people are still spraying their own backyards with pesticides and destroying "weeds", which are actually necessary for insects, and as result birds, to thrive.
The Audubon society lists domestic outdooor cats as one of the biggest threats to birds.
The issue in the report you linked isn’t cats living outside, the issue here is european flora and fauna existing at all in the US, where it doesn’t belong. This issue isn’t limited to cats, it even includes things like different species of lawn grass being invasive in regions it doesn’t belong.
They definitely do stalk feeders and add to the stressors already in place for bird species. More than one thing can contribute to the reduction of bird populations.
If you don't want to let your cat move freely, I suggest just not having one.
Have you known many people who own outdoor cats? It's a rare story that doesn't end with "then an asshole in a truck ran him down", "I think a coyote got him", "I think a hawk got him", "I never saw him again", "the neighborhood kids were chasing him with sticks", "found him nearly starved and bleeding", "lost a fight to a dog", or one of many other terrible ways to go. Every one of those is a true story from someone I know. The hawks, coyotes, and cars having happened to innumerable outdoor cats whose owners I've been acquainted with.
So, in my opinion, if you think a cat can only live a decent life outdoors, you shouldn't get one.
That, and if cats are our friends and we care about them, we should be sheltering them from the hostilities of an outdoor environment. I have had enough cats I knew in the past die from injuries from cars, or diseases/injuries they acquired outside to feel that it's just best to keep ours inside. We live rural, on 6 acres, and she loves it outside, naturally, but so do birds we love. Would she be "happier" outside? Maybe. But considering all trade-offs, I'll keep her pampered inside, and let the birds nest on my deck.
It is of course a good argument to consider the study generally valid on e.g. the British isles and North America where the cat was more recently introduced into the fauna.
The other issue is destruction of ecosystem in the EU, where cats are native and primarily used as scapegoat.
Both of these need entirely different solutions, but "don't let cats outside" is very oversimplified and just serves to divide people.