I'd also cut out the corn for a while to up their protein - keeping them on starter.
Get a can of mackerel and feed a bit of that every day or two.
I had a bad case of cannibalism a few years ago with birds about the same age. They were outside in a large coop at night and large forage area during the day.
I went out one morning and one was dead and all the meat had been eaten off the back. I thought I had a rat problem.
The next morning - another partially eaten.
That night I went out after dusk looked in and 2 or 3 chickens were picking feathers and eating another's back. I saved that bird and another they had started on.
This followed when I dropped the protein.
At the time, I was using an organic 16% protein grower for all birds and adjusted the protein by adding fishmeal. For chicks, I added 60% fishmeal at a 10:1 ratio which raised the protein to 20%. At about 10 weeks I eliminated the fishmeal.
That's when the problem started.
I then started adding fishmeal back and the problem stopped.
I know this is anecdotal evidence but what you described in your first post was a classic example of a desire for more animal protein.
Edited by ChickenCanoe - 6/7/16 at 5:26am