Angel wing (also sometimes called crooked wing, drooped wing or slipped wing) is a deformity which affects ducks, geese and swans as they grow. The wing starts to grow downwards and then outwards so that the wing feathers point outwards instead of laying flat against the body.
It can affect one or both wings, but when only one wing is affected it's nearly always the left one. Also it's more commonly seen in male birds.
It's thought the cause is either genetic or due to a diet too rich in protein. Angel wing is not life-threatening in captive birds, but when it occurs in migratory geese it's usually fatal.