Even though i never have seen what a snake does to an egg i believe this is snake sign. I have caught 2 ratsnakes in my nest boxes last month or so. Today i found one egg in one nest box and this in another. Its flattened and softened. I know snakes reguritate food items when caught in the act and disturbed. I thought they digested the shell but they may not. Fat chance my wife will collect eggs while im at work!