I've got lots of them here. They come partly because I feed the local finches, sparrows, towhees, dove and quail, and there's food available. I wouldn't care so much, but there's also a couple of pairs of nesting Dusky Flycatchers, and life is hard enough for them, so I do end up chasing the cowbirds off a lot.
It does bring up the question of what naturally "belongs" someplace and what doesn't. Humans are not the only element of change in the makeup of species to any given area, as most birds can and do fly, or sometimes get aided by the wind to end up somewhere their species has never been—but we definitely speed up the process.
Point is, I think, that WE, as humans, are also animals, and our interaction with our environment is part of the natural course of life on this planet. I'm not saying we shouldn't use our considerable abilities of self-awareness to regulate these effects—I'm just saying they aren't altogether "unnatural."
It's just a thought.
photo courtesy of National Geographic Birds of North America