The cause of "angel wing", a deformity found in waterfowl such as ducks, geese and swans has been uncovered. Sadly, it is often caused by well-intended people feeding birds foods that are too high in proteins or carbohydrates (bread, crackers and popcorn anyone?). Not surprisingly, this condition mainly impacts birds that live in public areas. Because the deformed birds are unable to fly, thousands succumb every year to predators or inclement weather.

Conservationists suggest instead feeding the birds waterfowl feed, duck pellets, sliced seedless grapes, shredded swiss chard and romaine lettuce, or grains.

Source:

LiveScience