Angel Wing
Angel wing can be caused genetically, but is usually the result of too high a protein level in the feed, causing the feathers to grow faster than the skeleton can support.
Here's a downloadable film on taping the growing wing. Note that the link doesn't seem to work on my Mac but was available on a PC.
A Muscovy Duck showing angel wing
Photo courtesy of Julie Hendricks
Direct questions and comments to Barry at FeatherSite -- questions and comments