Sharon, I showed the picture to Jim Pichner, Avian Supervisor here at the MN Zoo and he's guessing it's a Hooded merganser egg. If you still have the egg, you can test it by rubbing your fingers on surface of the egg. If the green rubs off and it starts to turn to an ivory white, it's a hoodie egg.
The other option could be a Goldeneye, but we thought it looked too small. Steve Estebo Avian Zoologist Minnesota Zoo
