These birds are so angry, even a stuffed bird will set them off. Well, not so much angry as aggressive.
Researchers at Duke University studied the territorial habits of swamp sparrows with a taxidermied pal of theirs implanted with a computer and mechanics. The stuffed bird could turn from side to side, but more importantly it could lift its wing a very significant gesture. To male swamp sparrows, flipping your wing is kind of like flipping the bird.
When males saw the decoy flipping its wing they responded in kind. After a few moments of offering this rude gesture, the male bird would violently attack the decoy, going so far as to decapitate it in one instance. Males also attacked the decoy when it was turning and not flipping its wing, but less aggressively.
According to study leader Rindy Anderson, the violence of the attack points to another reason for the wing-flipping motion. Since most of the attacks following the gesture are meant to seriously injure or even kill, she suspects the wing flipping is a warning or a way of settling the conflict before it comes to blows.
Check out the birdy-beat-down for yourself.