Leftover meat, probably legs, neck, etc. Who knows. I've heard a lot of disgusting things. Once a woman literally found a chicken's HEAD in her meal. Their food is nasty and full of fat, why would you eat it? I saw their video and I vow never to eat there in my life, again.
I did a project on this, and several resources said that while they actually contain chicken, they also have ground-up duck feathers and sand in them because of a protein or something found in the sand and feathers that they want in the chicken. Believe it or not, your call.