Ducks and other birds suffer from a severe shortage of teeth.
Food they ingest goes directly to the gizzard which is a very
muscular organ(deliscious too, btw) where it is ground by stones
that the bird has previously swallowed. These stones wear down
and need constant replacement and the shot is ingested along
with the stones.
This method of grinding food was also used by dinosaurs and the
smoothed gizzard stones found with their fossil bones are called
gastroliths. Maybe they all died from lead poisoning??? 8^)