We have four ex-battery chickens. Three of them have developed a cough over the last week. None of them has any sort of discharge from nose or eyes and they are breathing normally. They are all feeding well and are very active, but egg production has decreased from 2-3 eggs per day to 1-2 per day. They aren't showing any signs of ill-health that I can see other than the cough. Does anyone have any suggestions as to what could be causing this and what I should do?