Apple cider vinegar is kind of a placebo. There are some studies that show benefits in humans, but the data aren't conclusive, and from what I've read, you would need to consume more than you'd want to in order to reap the benefits.
I think I'd be looking for reasons the birds don't have feathers and correct that before I started adding ACV or hippie herbs to them. They have nutritional needs and aren't a stew after all.