I recently purchased the Mighty Eagle in Angry Birds for iPhone. To use him, you toss out a can of sardines and the Eagle swoops down to retrieve it, destroying everything in it's path, including all the pigs. If the Eagle destroys everything, you get a feather (you're rated on a percentage scale, e.g. 85% destroyed, 93% destroyed, etc.). Get enough feathers and you earn some achievements. I've been able to earn a few feathers, but haven't figured out what has to happen to get to 100%. Everytime I've earned 100%, there are usually some wood/glass/stone pieces leftover.
What criteria does the game use to measure the amount of destruction the Mighty Eagle has wreaked?