The Boomerang Birds' main ability is to swap directions in mid air. When the screen is tapped, the Boomerang Bird will open its large beak and proceed to head opposite the direction they are headed and strike areas that the other birds can't reach. Besides this, the Boomerang Birds are fired much differently than the other species. When they launch, they're constantly spinning, and they're launched at a farther distance.
http://angrybirds.wikia.com/wiki/Boomerang_Bird
