I believe that our alien overlords killed the birds as an experiment and to strike fear into our heats.
In all seriousness, I'd tend to believe this.
Now I have to BS ya. I've seen them in upstate NY in the winter, and they have zero problem running other birds out of my parents' feeders. (Though, the bird type that is the fiercest, that
nothing will mess with, is a male cardinal. I've seen one male cardinal run four blue jays off before. It's a beautiful thing.)