I got several responses with ideas of everything from little blue herons to screech owls. Here was one interesting post from the Minnesota Zoo:
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
The other option could be a Goldeneye, but we thought it looked too
I tried that and nothing came off the egg. Then I got this email with an interesting article link from Anne Hanley about nest parasitism in wood duck boxes followed by this:
I forwarded your original email, with the link to the photos, to Mike Zicus,
a good friend of mine who is a waterfowl research biologist for the MN DNR
at the Wetlands Wildlife Research Group in Bemidji. He has done extensive
research on cavity-nesting ducks and has handled countless numbers of eggs
from Wood Ducks, Hooded Mergansers and Goldeneyes.
Here is his response:
I'm certain that the "wood duck" egg is a hooded merganser egg. The key is
the almost spherical shape and whitish color. They look a lot like cueballs.
Wood duck eggs would be more elliptical or ovate and smaller too (assuming
the "chicken" egg is "medium to large" in size), and they are beige in
color. Their shells are much thinner than hoodies too. Hoody eggs have a
very solid "sound" to them when knocked together because of the thick
shells. Of course, that's not evident from the photo.
I'm almost as certain that the "mystery" egg is a goldy, but it's hard to
tell from the photo. Was it collected where goldies occur? The color in the
photo looks a bit off (goldy eggs are decribed variously as "pale green,
blue-green, olive green, or malachite green"), but goldy eggs often discolor
as they sit around putrifying. Often they get darker and can get kind of
blotchy. The shape and size are OK for a goldy (assuming the "chicken" egg
is "medium to large" in size). Common mergs have much larger eggs that are
buffy to beige in color.
Hope this is helpful.
It was also recommended that I get A Guide to the Nests, Eggs, and Nestlings of North American Birds by Paul Baicich and Colin J. O. Harrison and after reading that book and photo comparisons I agree that the mystery egg is a common goldeneye egg...but this leads to another mystery. I thought the egg came from a wood duck box in the customers backyard in the west metro of the Twin Cities--not a known breeding area for goldeneye. I will have to confirm with him the next time he comes in if this was from the metro area or a cabin up north. Also, after reading the book and the emails posted above I agree that the egg is a hooded merganser egg and not a wood duck egg. What a fun winter mystery this was! Thanks for all the responses.