My Finches are losing their feathers

Question
Dear Expert,

Two years ago, I gave two zebra finches to my 6 year-old daughter as a birthday gift. We were amazed to see them lay eggs, and happily welcome new birds into the cage. We have 9 birds since, a bigger cage, and more work for me.

Last year, I noticed that 6 of the birds are losing feathers on top of their backs, under their body, and tail. My local petstore advised me to buy extra vitamins and minerals but I don't seem to see any improvement. I change their water and feed every day. They have various toys to play with.  

Is there something else I should be doing, or could it be an internal disease?  Please advise.
Thanks a lot.
Cemond.  

Answer
Are they rubbing their beaks on perches? Or sitting around ot in the food? Is anything out of the ordinary besides the feathers?
If so, it sounds like their bored. Try changing their toys or turning the tv on for them. Bored birds pick at their feathers and sooner or later it turns into a habit.
Keep the temperature up in their cage so they don't get sick because of their lack of feathers.