A friend of mine had a similar situation happen. Her daughter was crawling after the bird on the floor and the bird stopped and she didn't and landed on her tail. EVERY SINGLE tail feather was GONE!
She was so sad but after a few weeks they started coming in.
Give it time and your birds' will grow back too!
One thing I do recommend is giving Tweety some moulting and conditioning formula along with his regular food.
You can find it at any Petsmart or Petco.
You only need to sprinkle 1/4 tsp. in with the regular seed.