Dave,
I think Kenc's concern was if it was cleaned before it was added to the feed as when he said,
"If so, do they clean the feathers before it is made into feed?"
Sounds like you're right but all the feed manufacturers add feather meal don't they? I see one supplier of feather meal advertises it as 80 percent protein. I'm thinking that ground up feathers, but could be wrong.
I've got a friend who researches fish feed at Purdue University, Dr. Paul Brown. I'd be curious to see what he thinks of adding feather meal. I'll post it here.