temporary emergency food ... oatmeal is good as QuakerJack mentioned, and while I hate to use just cornmeal or rice I have on occasion had to when the owner of my local feed store changed up his delivery days and I ran out before he got new feed.
I try to just give my ducks greens from the yard to tide them over, they think they are getting a treat anyway, but I will try to make a cooked mash of things like what I mentioned above along with some sort of protein boost like a egg beaten in or a splash of olive oil. Cooked potatoes or just the potato skins (always cook them first!) mixed in is good too.
There are a lot of foods out there that they will not like, but they will eat if they have no other choice. They might be spoiled, but when they are hungry my ducks will try anything.
