Lots of neighbors really love birdwatching, and have feeders and watering places set up so they can watch the birds. So if your cat goes into their yard and murders birds right before their eyes, it probably detracts from the serenity and enjoyment of the bird watching experience, for sure!
And cats roam anywhere they please when they're out, so that's a concern.
I have read, though, that in many built-up areas the bird population is very "un-natural", with high populations of non-native species who only live there because of the changes created by humans to the natural ecosystems that were there before towns and cities sprang up.
Around here, the area would be a high plains desert with sagebrush and prairie grasses, but no trees or open water. But since we have a town, there are watered lawns, gardens, and trees galore. And bird baths, bird feeders, etc., which have created an unnatural oasis for all manner of birds who would never stop, much less inhabit the area.
So adding domestic cats to this artificial "zoo" really doesn't affect anything natural. It just changes the character of the zoo.
But for the sake of domestic tranquility, I think keeping our cats to ourselves is a very good idea.
If you want to protect endangered bird species, you need to actively oppose wind power projects. Those monstrosities are responsible for huge losses of Raptors and other dwindling species in vast areas of otherwise undisturbed land. It's an amazing mess here in Wyoming, with huge areas blighted by those ugly things. And all in the bogus name of "green" energy.
But I digress!