Angel wing or slipped wing is wing deformity, where the last joint of the wing is twisted outward. Commonly found in ducks and geese, the causes of angel wing is genetics and too high concentrations of protein and/or too low levels of Vit D, Vit E, calcium and manganese in growing waterfowl's diets. Due to this incorrect diet, the wrist joints of the wings of the affected bird develops incorrectly, twisting outwards, giving the birds' wing the appearance or aeroplane wings, as seen in the above pic. When spread out, the wing may appear relatively normal, but the affected bird will find flying near impossible.
In adult birds, the disease is incurable and usually leads to an early death for wild birds, as affected birds are rendered effectively or totally flightless. In domestic waterfowl, clipping the affected bird's wings is recommended, to prevent possible injury and should be repeated after each molt.
In young birds, wrapping the wing and binding it against the bird's flank, together with feeding the bird a more nutritionally balanced diet, can reverse the damage.
Pic by @PlumTuckered
For illustrated guide on wrapping for angel wing, please see here: https://www.backyardchickens.com/t/644239/angel-wing-in-ducks-how-we-wrap-to-mend-it-pictorial/0_30
To prevent angel wing in waterfowl make sure your goslings and ducklings are fed a proper diet that meets their nutritional requirements and restrict feeding of extras and treats to the bare minimum.
If you already have/had a case of angel wing in your flock, do not use the affected bird for breeding as the condition can be passed on to offspring.
Further and recommended reading: