Yes, cats kill lots of birds every year. They also kill a lot of mice and rats and other undesirables, but no one complains about those! But, consider the alternative. All those birds don't get caught or eaten by cats. We will be over run by birds and those cute little birdies will "birds plop" all over cars, property, and heads! Mother Nature does her job with ecosystems to regulate populations. When I taught grade 7, we did many simulations on what would happen if you removed a predator from an area. We learned about actual experiments doing this. In every case, the "prey" was able to rapidly reproduce until, lol and behold, there were too many and the food source became depleted. Then they starved...the predators returned for "easy pickings" and things balanced out again.
TNR manages to cut back on the undesirable parts of feral cats....noise, spray, and poop. But the feral cats still have a role in the ecosystem. No, we will never get all the freaks TNRd, but those we do mean that we are helping.n