The mystery of the broken egg

It was Topsie who found the broken egg.  Her sharp sense of smell led us straight to it.  We gathered around and looked at it ... a white broken eggshell a bit larger than a chicken egg.  "A duck egg," I surmised.  It was still nesting season and the ducks from the pond nest in the area.  Someone or something had raided a nest and stolen an egg.  But who or what?  We looked at each other in somber silence as our questions hung unanswered in the air. 

A white broken eggshell