Okay, thank you.
By far the most common cause of diarrhea in chickens is intestinal parasites. Most flock owners worm their birds on a quarterly basis. There are many other possible causes of loose droppings, but since worming is so easily done, and necessary in most cases anyway, this is the place to start. Flubenvet, available from many feed and farm supply stores, or online is an excellent and very easy to use product with a broad spectrum of activity.
Other common causes of loose droppings that should be considered if the worming doesn't resolve the problem are the following:
Infectious bursal disease
Ulcerative enteritis
Coccidiosis (the second most likely suspect, see article on this parasite here: http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/200800.htm
Thrush
Campylobacteriosis
Chronic respiratory disease
If the droppings don't improve after worming, treat them for coccidia. If the problem still persists, Google each of the conditions I've listed, and read about them to learn other symptoms to look for and available treatments.
I'll be very surprised if worming and treating them for coccidiosis does not solve the problem for you. Unfortunately there is no "one per customer rule" with poultry diseases and more than one problem is very common, especially worms and coccidia together.
If you should have further questions, please let me know.
Best regards,
Dr. Bob