We have 15-5 month old hens (Barred Rocks, Rhode Island Reds, and Golden-laced Wyandottes) that started laying earlier in Dec. They live in a 300 sq ft enclosed yard plus an attached coop. At least one has a milky/watery diarrhea (not brown at all) for the past couple of days. They are drinking water and eating well, seem healthy otherwise. We have not changed their food, but have recently given them iceburg lettuce and wheat bread as a treat. We have given them lettuce and bread before and never had this problem so I don't think it is that. They have never been dewormed. What are possible causes of this type of diarrhea? Please help.