Pine tar. Get it in the horse section of your local feed store. Comes in a can like a small paint can. Get rubber gloves and a cheap paint brush. Get a helper, or do it at night when roo is roosting. Paint affected areas with pine tar. Tastes vile, at least to birds! Mine killed a flockmate last winter, pecked her to death while I was away, in the space of about two hours. Two more birds had blood, I did the pine tar thing, one taste and no more pecking. It is sticky, shavings will stick to it, but who cares. It eventually does wear off, reapply as needed. I never had to separate any birds after using pine tar.
Good luck, and thanks to all others for the alternate suggestions, as I do have occasional picking without blood. I will try the cat food and see if it stops. But the pine tar worked wonders for keeping them from doing serious damage to each other.
