Basically, the feathers are growing too fast and weigh too much for all the connective tissue in the wings to support the weight. It causes the wings to jut out instead of laying flat on their backs. This only occurs as the ducks are growing and if untreated it is a permanent deformity. It is treated by taping down the wings to the body to force them to grow in the right position. Once fully grown, a treated bird is no longer at risk of getting angel wing.
There's a couple schools of thought on what causes angel wing.
1. too much protein in the diet. Protein is needed to form feathers so too much protein allows the feathers to grow faster
2. too much carbs in the diet. in the wild ducks eat a high protein diet in the spring because that's what's available. the relatively high carb diet they get in captivity is not normally available in nature
I have a slightly differing opinion that overlaps with both theories (it's purely conjecture on my part).
In the wild, ducks have to work hard for their food so they do not get as calories in their diet as domesticated ducks. Also, domestic breeds have been selected for over centuries to maximize their growth rate. In addition, nearly all of the nutrition and diet research for ducks are for meat ducks that need to grow fast on the least amount of feed possible - not for pets or shows.
We generally raise ducks under the following conditions:
- providing the ducks with an optimal amount of calories to fastest growth
- genetics that promote faster growth
- enough carbs that protein is not used for energy - only growth. this frees up the protein for growth
- enough protein to allow maximum growth speed
- sometimes with medication to allow the ducks to spend all their efforts into growing, not fighting off infections or parasites
I don't have the research to support my theory, but if it's true we'd expect to see the following:
1. breeds that grow faster are more prone to angel wing. Also, wild mallards fed the same feed for optimal growth would be less like ot develop angel wing than domestic breeds.
2. ducks that only free range with no additional feed are less likely to develop angel wing
3. ducks without a balanced amino acid (protein) diet would not get angel wing (not enough essential amino acids to maximize growth. People that only feed ducks corn should almost never see angel wing)
4. Ducks given feed for optimal growth but not enough total calories would not develop angel wing
5. Ducks fed antibiotics as "growth hormone" would be more likely to develop angel wing.