Duckling cannibalism

[melanie33]

I have 8 week old Muscovy ducklings. 11 of em. I moved them to a bigger pen and outside. I noticed them pulling each other's wing feathers out. (Bloody mess) I covered them all with Pick-No-More from TSC and it stopped. But, why and how can I prevent this? I've read a few forums and mites are the only thing I can't rule out. I'm heading to get mite spray in the am.