hmmmmm…. someone dosent know ducks very good. there is a difference. only time i have ever seen ducks eating shad, is VERY late in the year. the shad die, and float down the river, and the ducks eat them. probably 90 percnt of the rest of they year, they eat grain, or vegetation. divers on the other hand, mainly eat vegetation and some snails and whatnot. mergansers are the ones that are the fish eaters, they specifially target fish year round, why do you think cans were almost market hunted to death? because they taste good.
i believe mallards are oppurtunistic feeders. i have shot them with zebra mussles in their crop as well. whatever is available
if i could bet any money, i would guess that is a hen mallard. the bill on a black is either yellow for a drake or green for a hel. never seen a black duck with a black bill. i think it is just the quality of the photo