Apple cider vinegar is good for over all health, increases appetite, works as maintenance for killing off bacteria and fungus, it also keeps the crop to a low ph value. The crop is basically a storage for food until it moves into the stomach and it can grow bacteria if it is not kept acidic. 2 tablespoons for maintenance, 3 tablespoons for sick birds. Once a week for one day is a good technique.
It is best to look into what you have growing in the chicken yard and make sure they are not poisonous to your chickens. If they aren't, chances are the chickens will eat them. LOL My chickens turned my garden into a barren moonscape in a few months!