I always make a strong tea using the ingredients for 1-1,5 litre as follows:
4 tablespoons or teabags of oregano
4 tablespoons or teabags of thyme
2 tablespoons or teabags of sage
1 tablespoon or teabag of aniseed or
2 tablespoons or teabags of fennel
Instead of aniseed or fennel you can use 1...
You can brew some tea for your chickens which will help to alleviate the symptoms and speed up the recovery process: oregano, thyme, sage and some aniseed. Let the tea steep for at least 3-4 hours before serving instead of water. You may add some poultry vitamins to it as well.
Edit to add...