Never spank or hit your ducks. Please it will only teach fear. and they could be injured. Eating feather usually means protein deficient, try adding some high quality cat kibble to their feed, and yes adding more ducks could help but remember to let them all adjust slowly over several day to a week[where they can all see each other]  before putting them all together. My drake was pulling on my ducks flight feathers and causing them to bleed I sprayed Blue Kote on her and the pulling stopped. It's great for disguising wounds and other things But if you use it make sure to use gloves and wear old clothes it stains.