This is straight from Dave Holderread's Storey's Guide to Raising Ducks:
Feather Eating
Cannabalism of this type, where birds pull out and eat one another's feathers, is most prevalent among ducklings (especially Muscovies, Mallards, and wild species) that are brooded artificially.
Causes
Feather eating is usually the result of boredom, but can also be triggered by excessively high brooding temperature, intense light, overcrowding, an unbalanced diet, or the lack of green feed.
Treatment and Prevention
At the first sign of feather eating, check brooder temperature, reduce light intensity (using blue or red bulbs often helps), and provide ducklings with sufficient space, a balanced diet, and adequate quantities of feed. As long as ducklings are kept inside, put tender green foods such as grass, lettuce, and dandelions in front of them as much as possible. As soon as it is safe, allow ducklings access to grassy pens during the day.