Purina puts out --
Game Bird Startena, (30% protein)
Game Bird Flight Conditioner, (19% protein) You can either feed this by itself or in a mix like the one below.
If you don't mind mixing feed you could try this --
1/3 (by weight) Game Bird Flight Conditioner
1/3 (by weight) 32% Game Fish...
I don't know who said anything about 15% protein but a lot has to do with what type of chicken you are feeding.
A 15% protein is just fine for most Heavy Breed and Gamefowl that are still growing but wouldn't be the best feed for a laying or breeding hen.
Also the quality of the protein that...
If it is a commercial feed check and see if it has animal protein in it.
Most hens will do well on a 16 to 17 percent with out feather pecking/eating as long as it has animal protein in it.
You are correct in that feathers have protein in them, in fact they have a lot of protein in them...
There are a number of thing's that can/will cause feather picking.
Lice and or Mites,
Boredom,
Low Protein Feed,
Poultry being fed a "vegetarian feed" with no animal proteins in it.
The Cheese, Egg and Tuna probably help with there need for animal proteins and so the feather picking/eating...