If you had ducklings hatch out, you will see small crushed up egg pieces and membranes halves, almost perfectly equal in size. You can count the membrane pairs as a way to determine how many ducklings hatched in that box. You would also find a lot of down in there, several inches thick. This is typically layered on top of the nesting material you had placed in the box when putting it up or doing annual cleaning/refilling of nesting material.
Any egg shells outside an intact box means predator activity. Large egg pieces, especially with membranes still inside the shell pieces, also suggests predation occurred.
