Mystery Egg

One of my job perks are the bird mysteries people bring to us. I love trying to id birds from grainy photos or pieces of feathers, beak and legs. We've had guy come in and tell us that this past fall when cleaning out his wood duck box. He found one white egg--presumably wood duck and one bright blue egg about the size of a chicken egg. We couldn't find any cavity nesting bird that laid a bright blue egg the size of a chicken's egg so thought maybe his neighbor pranked him. Yesterday he brought in the eggs. The egg isn't blue but more of a greenish color with light brown speckling. The thought he remembered the color incorrectly but egg color can fade. I'm not sure what the egg is since I'm not an oologist, my best guess would be some type of merganser but I'm not sure. If anyone has any idea, please email!

The mystery egg next to a wood duck egg.

Chicken egg, mystery egg and wood duck egg