A bird's gizzard is a strong muscular organ which can not grind a bird's food solely by itself. It requires the bird seeking someplace where it can find grit or gravel to fill the gizzard. This is where the lead shot comes from... not necessarily from mud but from sandy or gravelly areas in somewhat shallow water. The lead shot will not sink away as readily in these areas as it would in mud.
I believe lead shot is a far more serious concern to waterfowl than lead ingestion is for any other creature, including man.
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