I would part them from the old birds if you have the space , put them on a treatment for Canker (Ridzol) and respiratory , use the canker for 5 days then vitamins for 2 days , then treat for respiratory for 5 days afterwards then 2 days of Vitamins , always feed them a heavy mix whilst treating , but not too much , only really feed once a day , then put them on half barley half all round , and add cider vinegar in the water , 1 teaspoon to 1 litre of water .
Hope this has helped.