[magduf]

>Is it ethical for me to have a cat, when cats are obligate carnivores that not only have to eat meat but also independently kill billions of birds a year?

Properly-kept housecats stay inside and don't kill any birds. I suspect the vast majority of those bird deaths are caused by feral cats, not pets (though of course, most ferals probably are or descend from pets that were abandoned or escaped). Our society could be doing a better job with dealing with feral animals like this, because they are bad for ecosystems; they're basically invasive predators.

Bird-killing aside, the environmental impact of pets is actually pretty staggering. It's probably worse for dogs too, since they're much larger animals on average.

>The megafauna that existed on planet Earth just 200, 500, and 12,000 years ago were far more numerous than the number of livestock cows today.

Citation needed. Yes, there were millions of buffalo on the American plains 1000+ years ago, but a quick Google search shows there's over 94 million cows in America today. Also, from what I've read, there are more plains now than in the past, because forests were destroyed by humans to make grasslands.