I'm pretty convinced it has nothing to do with protein levels in feed. So many ducks and geese in park areas supposedly develop it due to people feeding bread that kind of blows the too much protein theory to me. If that is the case one would think it would be a lack of protein. Plus my experience with it was one out of three developed it and they all had the exact same diet. I feel like there is probably more of a genetic predisposition to it than protein levels.
Here is a video that helped me.
And a thread where we've been talking about Angel wing with a different wrap method.
https://www.backyardchickens.com/threads/angel-wing-treatment.1252014/