Of all the low hanging fruit on the "stop destroying the planet for no reason", keeping your cats inside or on a leash requires basically 0 effort.
https://www.smithsonianmag.com/science-nature/moral-cost-of-...
Cats are predators. And they need freedom. If you're concerned for birds et al. don't get a cat.
Don't be cruel to cats to satisfy your need for a pet by forcing your view of how nature should work onto them.
Regarding birds, I'm not sure. It can't catch magpies and other crows. We don't see or hear other birds (we live outside Paris and don't have that many birds). I don't think it would make a lot of difference to keep our cat indoors since all other cats in the neighborhood also live outside -- except upsetting and depressing ours.
While cats do kill a lot animals, including endangered species, many articles that claim your outdoor cat is irreversible destroying wildlife seem quite sensationalist. Many articles make it sound like pets are the real threat but only mention on the side that feral cats are responsible for the majority of killed animals instead. It would be far more beneficial (for wildlife, pets and feral cats) if people would support projects for controlling feral cat population than confining their pets.