Non-native bird invaders
In the past, birds were imported from other continents have escaped or were intentionally released. In fact, even birds from a different part of our continent, like these house finches, can become problematic for our own native birds.
Pigeons are another non-native bird, but we seldom see them in our yard, even though we frequently see them just a few blocks from our home. The main invaders in our yard are house sparrows and starlings.
European house sparrows are the worst animal invaders in my yard, especially because they're here year-round. Large flocks of these birds eat huge amounts of food, and they aggressively crowd out other birds. Even worse, they compete for nesting spaces and are usually the victors.
Interestingly, they do have one parasite: cowbirds.
One interesting fact is that they actually aren't sparrows at all—they're weaver finches.
True sparrows, even though they tend to also be what are called "LBJs" (little brown jobs), are quite charming and often very beautiful once you learn to distinguish one from the other.
Fortunately we haven't had as many starlings, though even one is more than enough. They compete for nesting areas with our native birds, and generally are unpleasant to have around.
This starling is eating the winterberries that our robins and Northern mockingbird rely on to help them get through the long winter. Starlings are pretty aggressive and chase away our other birds. Even worse, they tend to travel in large flocks and can strip the bushes bare, leaving little for our native birds.