What do we do with an abandoned duck egg?

whatever you do, don’t touch it. There are animal control specialists that can handle this depending on your locale.

No one has touched it. Will animal control concern themselves with an unborn duck? I guess that’s a question for animal control.

Update: I called animal control, they said that they are “positive that the mother is going to come back for the egg” so they’re not going to do anything about it.

Fry it in butter.

Yep. Even abandoned duck eggs are part of the wheel of life, and ducks are one animal that isn’t in any immediate danger.

Also, you might end up as mommy duck.

With wildlife in trouble, you might want to find (e.g. via web search or asking animal control or vets) who your nearest licensed wildlife rehabilitators are.