Broody hens can sometimes break eggs when moving them and getting on and off the nest. There is no way to know if that duckling hatched or the shell was broken some how. It is part of letting any bird sit and hatch eggs. Some are great at it, some are ok and some are awful at it and shouldn't be left with eggs.
Great= lays, sits, hatches and raises successfully
OK = lays, steals others eggs, sits, sorta raises, but not completely attentive
Awful = collects everyone's eggs or her own clutch. Sits then abandons at the end, OR sits and starts kicking out perfectly fine on the verge of pipping eggs, OR sits and as soon as they start hatching or are hatched walks off and leaves them to chill and die. OR breaks every eggs in the clutch because she ends up being an egg eater.
Great= lays, sits, hatches and raises successfully
OK = lays, steals others eggs, sits, sorta raises, but not completely attentive
Awful = collects everyone's eggs or her own clutch. Sits then abandons at the end, OR sits and starts kicking out perfectly fine on the verge of pipping eggs, OR sits and as soon as they start hatching or are hatched walks off and leaves them to chill and die. OR breaks every eggs in the clutch because she ends up being an egg eater.