The debate over steel versus Bismuth (tungsten matrix now being extremely expensive) rages, but assuming you have a gun which could use either (steel is not recommended for older English guns or for barrels with fixed choke tighter than ½ [0.02 inches of constriction] we always recommend at least one shot size if not two shot sizes larger than the equivalent lead cartridges, as well as a heavier load. For instance, where we used to use 30 gram of 6 shot for duck with lead shot, 32 grams of 4 shot of steel or Bismuth should be considered.
Ducks (and geese) have much denser, downy feathers for the shot to penetrate and they require much greater hitting power found in the kinetic energy of the larger shot.
For the record we would never use anything less than No. 2 shot for wild geese or American BB’s when flighting them on the foreshore.