Hi JJ
What kind of cocks are they??. I would use alot of game cocks in my tying and dying but the only capes I would dye are the badger because the other shades are generally too strong and would involve some sort of bleaching.
You would be safer running your fingers through the feathers to see if they can be used.
If you decide to use them I would pluck first the feathers in a line from the base of the neck, along the wing, to the tail and back up again. Then get a sharp blade and skin, basically cutting the saddle off him. Place the saddle in between a newspaper for a few weeks and store inside. Wash the saddle using conditioner, dry fully, and to get rid of the mites freeze for a few days.
Heres a few I done a while back the quality of the photo is poor but believe me the colours are all natural. Proper Irish traditional colours