As it turns out, I know a lot about cats.
I can't remember a time my family didn't have a cat as a child, and I've had cats as pets for 27 years since leaving home. My current cat is an indoors cat.
You otoh seem to know diddly squat about birds. They don't generally take and kill the young of other birds for starters. Yes, raptors will do that, but they are uncommon. The rest are not carnivorous, or they focus on bugs and worms, not the young of other birds. Cats, like raptors, are carnivores and wreak havoc on bird populations.
There's a lot of truly gobsmacking ignorance on this thread when it comes to cats, based in parochialism - the 'reasoning' goes that cats are allowed to roam freely in the UK, so therefore letting them roam Is absolutely and unquestionably the best thing a cat owner could do for a cat. Yet millions and millions of cats live long and healthy and happy lives in apartments all over the world (and even in the UK, where quite a few flat dwellers own indoor cats). Cats live in apartments in Japan, China, Russia, all over Europe, Australia, and North and South America... There are probably more indoor cats in New York City than the entire UK.